JP4258170B2 - Positive electrode for secondary battery and secondary battery using the same - Google Patents
Positive electrode for secondary battery and secondary battery using the same Download PDFInfo
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- JP4258170B2 JP4258170B2 JP2002135183A JP2002135183A JP4258170B2 JP 4258170 B2 JP4258170 B2 JP 4258170B2 JP 2002135183 A JP2002135183 A JP 2002135183A JP 2002135183 A JP2002135183 A JP 2002135183A JP 4258170 B2 JP4258170 B2 JP 4258170B2
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- positive electrode
- secondary battery
- active material
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- 239000011572 manganese Substances 0.000 claims description 75
- 239000007774 positive electrode material Substances 0.000 claims description 48
- 239000002131 composite material Substances 0.000 claims description 30
- 229910052596 spinel Inorganic materials 0.000 claims description 24
- 239000011029 spinel Substances 0.000 claims description 24
- 229910015645 LiMn Inorganic materials 0.000 claims description 21
- 229910052744 lithium Inorganic materials 0.000 claims description 16
- WHXSMMKQMYFTQS-UHFFFAOYSA-N Lithium Chemical compound [Li] WHXSMMKQMYFTQS-UHFFFAOYSA-N 0.000 claims description 15
- 239000007773 negative electrode material Substances 0.000 claims description 13
- KLARSDUHONHPRF-UHFFFAOYSA-N [Li].[Mn] Chemical compound [Li].[Mn] KLARSDUHONHPRF-UHFFFAOYSA-N 0.000 claims description 11
- 229910001416 lithium ion Inorganic materials 0.000 claims description 11
- 239000008151 electrolyte solution Substances 0.000 claims description 10
- HBBGRARXTFLTSG-UHFFFAOYSA-N Lithium ion Chemical compound [Li+] HBBGRARXTFLTSG-UHFFFAOYSA-N 0.000 claims description 8
- 239000010419 fine particle Substances 0.000 claims description 7
- 238000007600 charging Methods 0.000 claims description 6
- 229910021385 hard carbon Inorganic materials 0.000 claims description 6
- 229910021445 lithium manganese complex oxide Inorganic materials 0.000 claims 4
- 229910008163 Li1+x Mn2-x O4 Inorganic materials 0.000 claims 1
- 239000000463 material Substances 0.000 description 53
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- 239000011230 binding agent Substances 0.000 description 9
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- WPBNNNQJVZRUHP-UHFFFAOYSA-L manganese(2+);methyl n-[[2-(methoxycarbonylcarbamothioylamino)phenyl]carbamothioyl]carbamate;n-[2-(sulfidocarbothioylamino)ethyl]carbamodithioate Chemical compound [Mn+2].[S-]C(=S)NCCNC([S-])=S.COC(=O)NC(=S)NC1=CC=CC=C1NC(=S)NC(=O)OC WPBNNNQJVZRUHP-UHFFFAOYSA-L 0.000 description 1
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Images
Classifications
-
- Y—GENERAL 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
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
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- Secondary Cells (AREA)
- Battery Electrode And Active Subsutance (AREA)
Description
【０００１】
【発明の属する技術分野】
本発明は、リチウムイオン二次電池用正極およびそれを用いた二次電池に関する。
【０００２】
【従来の技術】
リチウム金属やリチウム化合物を負極として用いる二次電池は、正極活物質としてコバルト酸リチウムを用いる場合、４Ｖを越える起電力が得られることから、近年、精力的に研究が行われている。このコバルト酸リチウムは電位平坦性、容量、放電電位、サイクル特性などトータルな性能で良好な特性を示すため、今日のリチウムイオン二次電池の正極活物質として広く利用されている。
【０００３】
しかしながら、コバルトは可採埋蔵量が少なく高価な材料である。またコバルト酸リチウムは層状岩塩構造（α−ＮａＦｅＯ２構造）を有しているため、充電時のリチウム離脱により、電気陰性度の大きな酸素層が隣接することとなる。そのため、過充電状態など、リチウムの引き抜き量が多すぎる場合、酸素層間の静電反発力による構造変化を起こし発熱することから、安全性の確保という点で課題を有していた。このような背景からコバルト酸リチウムの代替材料が求められている。
【０００４】
コバルト酸リチウム以外の４Ｖ級二次電池の正極活物質としてはニッケル酸リチウム、スピネル型マンガン酸リチウムなどが考えられている。しかしながら、ニッケル酸リチウムはコバルト酸リチウム以上の容量を有しているものの、結晶構造はコバルト酸リチウムと同じ層状岩塩構造で、充電時のＮｉ４＋の不安定性に起因し、コバルト酸リチウムよりも酸素脱離温度が低く安全性確保はより困難な材料である。さらに、放電電位がコバルト酸リチウムよりも低いこと、Ｎｉの高環境負荷を考慮すると、コバルト酸リチウムの代替材料としては魅力が薄い。
【０００５】
一方、スピネル型マンガン酸リチウムは、安価なマンガンを原料としていること、安定なスピネル型結晶であり、過充電時にのみ使用される余分なリチウムをほとんど含んでいないため、コバルト酸リチウムと比較して高い安全性を示す。このことから、非常に有望視されている材料であり、既に一部で実用化されている。さらにスピネル型マンガン酸リチウムは、資源供給や環境負荷の面でコバルトやニッケルと比較して有利であるため、前述の安価であるという利点を加味し、ロードレベリング用の電力貯蔵や将来の電気自動車用電源として高いポテンシャルを有している。
【０００６】
【発明が解決しようとする課題】
このように、スピネル型マンガン酸リチウムは、コスト・安全性・資源の安定供給・環境負荷の種々の面で優位性を示す材料であり、電力貯蔵あるいは電気自動車用電源として大きな期待を集めている。しかしながら、これらの用途の場合、小型携帯機器向けとは異なり、高エネルギー密度という側面よりは、むしろ充放電サイクル寿命や容量保存特性といった長期信頼性に関するファクターや、パワー特性の方が重要となると考えられている。
【０００７】
パワー特性は活物質自体の特性以外にも、電極設計・集電デザインなど構成・構造面の影響が大きいが、幸い、スピネル型マンガン酸リチウムを正極活物質として用いた電池のパワー特性は、同じ構成で他の正極活物質を用いた場合と比較し、同等以上の性能が得られており、この面では有望と思われる。
【０００８】
一方、スピネル型マンガン酸リチウムは、これまでの様々な報告に見られるように、高温における充放電サイクル寿命や容量保存特性が満足できる水準にまで到達していなかった。換言すると、スピネル型マンガン酸リチウムの高温サイクル特性ならびに保存特性を改善することは、電力貯蔵あるいは電気自動車用の電源の実用化への道を開くことであり、工業・産業面かつ環境保全面で非常に重要な意味を有している。
【０００９】
上記事情を鑑み、本発明は、スピネル型マンガン酸リチウムを正極活物質として用い、かつ高温サイクル特性ならびに保存特性に優れた二次電池を提供することを目的とする。
【００１０】
【課題を解決するための手段】
上記課題を解決する本発明によれば、集電体と、該集電体上に備えられた正極活物質層とを含む二次電池用正極であって、該正極活物質層は、リチウムイオンを吸蔵・放出可能であり、正極活性物質としてＬｉ １＋ｘ Ｍｎ ２−ｘ Ｏ ４ からなる微粒子を含み、該微粒子の表面が、ＬｉＭｎ １．４ Ｔｉ ０．１ Ｎｉ ０．５ Ｏ ４ またはＬｉＭｎ １．４ Ｓｉ ０．１ Ｎｉ ０．５ Ｏ ４ で表されるスピネル型リチウムマンガン複合酸化物によって被覆されていることを特徴とする二次電池用正極が提供される。
【００１２】
Ｍｎを含む正極活物質を用いた二次電池の場合、充放電サイクル劣化の原因の一つとして、正極活物質からのＭｎ溶出が挙げられる。このＭｎ溶出現象は、電解液中の酸濃度と密接な関係があるが、Ｍｎの価数に着目すると、３価の方が４価よりも比較的溶け易い。これは、３価のＭｎが不均化を起こし、２価のＭｎを生じさせるためであると考えられている。本発明では、スピネル型リチウムマンガン複合酸化物が正極活物質の表面を部分的あるいは全面を覆う構成を採るため、電解液中へ溶出するＭｎイオン量が低減することができる。
【００１４】
このような構成を採用することにより、正極活物質の表面をスピネル型リチウムマンガン複合酸化物により効果的に覆うことができる。その結果、上記したＭｎ溶出現象を効果的に抑制することができる。
【００１６】
正極活物質表面がスピネル型リチウムマンガン複合酸化物によって被覆されることにより、正極活物質から電解液へのマンガンイオンの溶出をさらに安定的に抑制することが可能となる。
【００１７】
上記の二次電池用正極において、上記スピネル型リチウムマンガン複合酸化物はＬｉＭｎ２−ａＮｉａＭｂＯ４（０．４５≦ａ≦０．５５，０≦ｂ≦０．３，ＭはＳｉまたはＴｉの少なくとも１種）であることが好ましい。この化合物中には３価のＭｎがほとんど存在しないため、電池の通常使用範囲である３．０〜４．２Ｖ（あるいは４．３Ｖ）のような電圧領域では充放電容量を示さない。すなわち、この電圧領域においては、結晶中の遷移金属の価数変化を伴うようなＬｉの出入りは行われないため、格子長の増減もなく非常に安定に存在する。
【００１８】
また、上記化合物はスピネル構造を有しており、その結晶中に空サイトを含んでいる。そのため、安定に存在しながら、同時にＬｉイオン伝導性をも有している。さらに、正極活物質とほぼ近い格子定数を有するため、正極活物質との親和性も高い。
【００１９】
このように安定であり、かつＬｉイオン伝導性を有する物質が正極に含まれることにより、正極活物質と電解液との副反応が抑制され、正極活物質の容量劣化ならびに負極表面の抵抗増加が抑制される。その結果、高温環境下での充放電サイクル特性や容量保存特性が改善される。
【００２０】
さらに、上記化合物は金属Ｌｉ対極電位で４．５Ｖ以上に充放電容量を有するため、電池が過充電状態になった場合に、バッファー材料として働く。従って、電解液中に過充電防止剤などを添加した際には、その働きを確実に機能させる補助になり、電池の安全性確保の面からも極めて有用である。
【００２１】
また、上記の二次電池用正極において、上記正極活物質はＬｉ１＋ｘＭｎ２−ｘＯ４を含む構成とすることができる。このとき、０≦ｘ≦０．２０であることが好ましく、０．０６≦ｘ≦０．２０であることがより好ましい。
【００２２】
また、上記の二次電池用正極において、上記スピネル型リチウムマンガン複合酸化物中におけるＭｎの原子価は３．９〜４．０の範囲であることが好ましい。このようにすることにより、上記スピネル型リチウムマンガン複合酸化物中における３価のＭｎの絶対量を抑えることができる。ここで本明細書において、Ｍｎの原子価は、ＪＩＳ Ｋ１４６７に準じて分析したＭｎＯ２％の数値ならびに滴定法により分析したＭｎ％の数値を用いて算出される値である。
【００２３】
また、上記の二次電池用正極において、上記スピネル型リチウムマンガン複合酸化物の、リチウム基準電位に対する平均充電電圧は４．５Ｖ以上であることが好ましい。このようにすることにより上記スピネル型リチウムマンガン複合酸化物は、電池の通常使用範囲である電圧領域（３．０〜４．２Ｖ（あるいは４．３Ｖ））では充放電容量を示さなくなる。すなわち、この電圧領域においては、結晶中の遷移金属の価数変化を伴うようなＬｉの出入りは行われないため、格子長の増減もなく非常に安定に存在する。
【００２４】
また、上記の二次電池用正極において、上記スピネル型リチウムマンガン複合酸化物の格子定数ａ１と、上記正極活物質の格子定数ａ２は、互いに近似することが両物質の親和性の観点から好ましい。具体的には０．９９１２≦ａ１／ａ２≦０．９９８２を満たすことが好ましく、０．９９３３≦ａ１／ａ２≦０．９９８２を満たすことがさらに好ましい。
【００２５】
また、上記の二次電池用正極において、ＬｉＮｉ複合酸化物をさらに含む構成としてもよい。ＬｉＮｉ複合酸化物は電解液中の酸生成を抑制する効果を有するため、上記したＭｎ溶出現象をさらに抑制することができる。ＬｉＮｉ複合酸化物としては、たとえばＣｏを含むＬｉＮｉ０．８Ｃｏ０．２Ｏ２が挙げられる。
【００２６】
さらに本発明によれば、上記の二次電池用正極と、電解液を介して該二次電池用正極と対向配置された負極とを備えたことを特徴とする二次電池が提供される。この二次電池は正極活物質からのＭｎ溶出現象が抑制されることから、長期信頼性に優れる。
【００２７】
上記二次電池において、上記負極中の負極活物質はハードカーボンとすることができる。これにより、さらに、サイクル特性など長期信頼性に優れる電池を得ることができる。
【００２８】
本発明者らは、Ｍｎを含む正極活物質を用いた二次電池において、高温環境化におけるサイクル特性、容量保存特性を改善することを目指して鋭意検討した結果、本発明に至ったものである。
【００２９】
正極活物質として、立方晶スピネル型構造を有する、一般式ＬｉＭｎ２Ｏ４で表されるＬｉ含有複合酸化物を用いることは、特開昭５５−１００２２４号公報や特開昭５８−２２０３６２号公報に既に開示されているように、従来から良く知られた技術である。
【００３０】
しかしながら、この材料系は、現在、主流であるＬｉＣｏＯ２と比較すると充放電サイクルに劣っていたため、特開平２−２７０２６８号公報や特開平６−１８７９９３号公報で開示されているようなＬｉ過剰組成、すなわちＬｉ１＋ｘＭｎ２−ｘＯ４とする技術、あるいは特開平３−２１９５７１号公報、特開平４−１６０７６９号公報、さらには特開平５−３６４１２号公報に開示されたようなＭｎサイトへの他元素置換、特にＣr置換などの技術が派生してきた。これらの技術による充放電サイクル特性の改善は実験的にも認められるものであるが、電力貯蔵や電気自動車向け電源に要求されるほどの改善幅は得られず、加えて、結晶中の３価Ｍｎイオン量の低減を招くため、充放電容量が低下するというデメリットを伴っていた。
【００３１】
一方、Ｌｉや他遷移金属元素による置換というアプローチとは別に、ＬｉＭｎ２Ｏ４の表面を他の物質で被覆する方法も、種々、検討されてきた。例えば、特開平９−３５７１５号公報、特開平１０−１７２５７１号公報、特開平１１−７１１１４号公報、特開２００２−６８７４５号公報などに記載の技術が挙げられる。
【００３２】
特開平９−３５７１５号公報では、ＬｉＭｎ２Ｏ４の表面をＬｉＣｏＯ２で被覆する技術が開示されている。
【００３３】
また特開平１０−１７２５７１号公報では、ＬｉＭｎ２Ｏ４の表面におけるＭｎサイトが他元素により置換された結果、Ｍｎ価数が中心材とは異なるＬｉｘＭｎｙＯｚで被覆する技術が開示されている。
【００３４】
また特開平１１−７１１１４号公報では、ＬｉＭｎ２Ｏ４の表面をＣｏ置換型のＬｉｘＭｎ２−ｙＣｏｙＯ４で被覆する技術が開示されている。
【００３５】
さらに特開２００２−６８７４５号公報では、Ａｌ置換型のＬｉＭｎ２−ｙＡｌｙＯ４の表面を、Ｌｉ過剰型のＬｉ１＋ｘＭｎ２−ｘＯ４で被覆する技術が開示されている。
【００３６】
正極活物質であるスピネル構造Ｌｉ含有複合酸化物の表面を、別種のスピネル構造Ｌｉ含有複合酸化物で被覆する、という構成を有するという観点からは、本発明と特開平１０−１７２５７１号公報、特開平１１−７１１１４号公報、特開２００２−６８７４５号公報とは類似するが、その技術的思想や材料組成は、以下の理由から全く異なる。
【００３７】
第一に、上記公報で開示された技術では、被覆材も充放電容量を示すのに対し、本発明において被覆材に相当するスピネル型リチウムマンガン複合酸化物は電池の通常使用電圧範囲においては、充放電に全く寄与しない。
【００３８】
第二に、上記３公報で開示された技術では、被覆材中のＭｎ価数が３.５〜３.６５程度であるのに対し、本発明におけるスピネル型リチウムマンガン複合酸化物中のＭｎ価数は実質的に４価である。たとえばスピネル型リチウムマンガン複合酸化物として、ＬｉＭｎ２−ａＮｉａＯ４を採用する場合、０．４５≦ａ≦０．５５とすることによりＭｎ価数を実質的に４価とすることができる。
【００３９】
第三に、上記第二の理由から、本発明におけるスピネル型リチウムマンガン複合酸化物は、金属Ｌｉ対極電位で４．５Ｖ以上という、通常、使用しない電圧領域で充放電容量を有する。一方、上記公報で開示された技術における被覆材は、この電圧領域では充放電容量を示さない。
【００４０】
以上の理由から、本発明の技術は上記従来技術と比較して、特に電力貯蔵や電気自動車向けの大型電池に好適であり、高温環境下における充放電サイクル特性ならびに容量保存特性の改善幅も従来の表面被覆よりも大きい。
【００４１】
【発明の実施の形態】
本発明に用いられる正極活物質（以下、実施形態において適宜、正極活物質を中心材と称する。）は、金属Ｌｉ対極で４Ｖ付近にプラトーを有するＬｉ含有酸化物から選ばれたものである。特にＬｉ１＋ｘＭｎ２−ｘＯ４（ｘは０．０６≦ｘ≦０．２０の範囲）が好ましく、目的とする電池の諸特性の優先順位によっては、適宜、更にＭｎサイトを他のカチオンで置換してもよい。
【００４２】
正極活物質の粒子形状は塊状・球状・板状その他、特に限定されず、粒径・比表面積も正極膜厚・正極の電極密度・バインダー種などを考慮して適宜選択することができるが、エネルギー密度を高く保つ観点からは、集電体金属箔を除いた部分の正極電極密度が２．８ｇ／ｃｃ以上となるような粒子形状・粒度分布・平均粒径・比表面積・真密度が望ましい。また、正極活物質、バインダー、導電性付与剤などにより構成される正極合剤のうち、正極活物質が占める重量比率が８５％以上となるような粒子形状・粒度分布・平均粒径・比表面積・真密度が望ましい。
【００４３】
Ｌｉ１＋ｘＭｎ２−ｘＯ４（ｘは０．０６≦ｘ≦０．２０の範囲）の合成に用いる出発原料としては、Ｌｉ原料としてＬｉ２ＣＯ３、ＬｉＯＨ、Ｌｉ２Ｏ、Ｌｉ２ＳＯ４などを、Ｍｎ原料としてＭｎＯ２、Ｍｎ２Ｏ３、Ｍｎ３Ｏ４、ＭｎＯＯＨ、ＭｎＣＯ３、Ｍｎ(ＮＯ３)２などを用いることができる。以上の中で、Ｌｉ原料としてＬｉ２ＣＯ３が、Ｍｎ原料としてはＭｎＯ２またはＭｎ２Ｏ３が特に好ましい。
【００４４】
以下、合成方法について説明する。上記の出発原料を適宜選択し、所定の金属組成比となるように秤量・混合する。この際、Ｍｎ２Ｏ３異相の残留を避けるために各試薬の粒径は１０μｍ以下が好ましい。混合はボールミル、ジェットミル、ピンミルなどを用いて行うが、選択試薬の粒径・硬さなどにより適宜、装置を選択することができる。得られた混合紛は６００℃〜９５０℃の温度範囲で、空気中または酸素中で焼成する。
【００４５】
得られたＬｉ含有酸化物の比表面積は３ｍ２／ｇ以下であることが望ましく、更に１ｍ２／ｇ以下が特に好ましい。３ｍ２／ｇ以上の比表面積の場合、バインダーの必要量が多くなり、正極の容量密度の点で不利となるためである。
【００４６】
本発明では、結晶中のＭｎ価数が実質的に４価であり、金属Ｌｉ対極電位において４．５Ｖ以上で充放電容量を有するＬｉ含有複合酸化物を中心材に対して被覆材として被覆する。具体的には、ＬｉＭｎ２−ａＮｉａＯ４が挙げられる。このとき、０．４５≦ａ≦０．５５とすることにより、結晶中において３価のマンガンを充分に減ずることができるため、Ｍｎ価数を実質的に４価に保つことができる。また、同時に当該化合物は４．５Ｖ以上で充放電容量を有するため、本発明には好適である。
【００４７】
これらの材料は、金属Ｌｉ対極電位における４．５Ｖ以上での充放電をＭｎ以外の遷移金属の酸化還元で行うため、Ｍｎサイトを別種のカチオンで置換しても構わない。ただし、Ｍｎ価数を４価のままとするために、置換種はＳｉやＴｉなど、Ｍｎよりも軽量の４価のカチオンが好ましい。これにより、電池の軽量化・エネルギー密度の向上に寄与することができる。
【００４８】
Ｌｉ１＋ｘＭｎ２−ｘＯ４への被覆は、被覆材の微小粉の吹きつけや混合に続く加熱焼成、あるいは各元素の所定比溶液への浸漬やスプレー噴霧など、粒径や被膜厚により適宜、選択することができるが、全面塗布の場合は液相経由が好ましい。
【００４９】
被覆材の合成は、粉体形状を選択の場合、Ｌｉ１＋ｘＭｎ２−ｘＯ４の合成と同様で行うことが可能である。ただし、あらかじめＭｎとＮｉの複合酸化物を用いるのが望ましい。更に別種のカチオンを導入する場合には、その元素種も含む複合酸化物を用いることもできる。液相経由の場合は、複合アルコキシドを用いても良い。
【００５０】
得られた複合正極活物質は、レート特性・低温放電特性・パルス放電特性・エネルギー密度・軽量化・小型化などの、電池として重視する特性に応じて適宜選択したバインダー種と導電性付与剤を混合し電極とする。バインダーは通常、用いられている樹脂系結着剤を用いることができ、たとえばポリフッ化ビニリデン（ＰＶｄＦ）、ポリテトラフルオロエチレン（ＰＴＦＥ）等が用いることができる。集電体金属箔としてはＡｌなどを主体とする金属薄膜を用いる。
【００５１】
正極における上記中心材と被覆材との比率（重量比）は、９８：２乃至９０：１０の範囲であることが好ましい。本発明における被覆材は充放電容量に関与しないため、被覆材の比率が大きすぎると電池のエネルギー密度の観点からは不利となる。一方、被覆材の比率が小さすぎる場合、中心材から電解液へのマンガンイオンの溶出を効果的に抑制できない。
【００５２】
本発明で用いられる負極は、Ｌｉイオンを吸蔵・放出可能なＬｉ金属、Ｌｉ合金、カーボン材料から選ばれるものが望ましいが、Ｌｉと合金化する金属、金属酸化物あるいはそれらとカーボン材料の複合材料、遷移金属窒化物を選択することもできる。負極材料の選択は、容量・電圧・重量・サイズならびにレート特性・低温放電特性・パルス放電特性などの電池の使用目的に応じて適宜行うことができる。
【００５３】
負極活物質は、レート特性・低温放電特性・パルス放電特性・エネルギー密度・軽量化・小型化などの電池として重視する特性に応じて適宜選択したバインダー種と混合し電極とする。バインダーは通常、用いられているポリフッ化ビニリデン（ＰＶｄＦ）、ポリテトラフルオロエチレン（ＰＴＦＥ）等を用いることができる他、ゴム系バインダーを用いることもできる。集電体金属箔としてはＡｌ、Ｃｕなどを主体とする金属薄膜を用いる。
【００５４】
セパレータは特に限定されないが、織布、硝子繊維、多孔性合成樹脂膜等を用いることができる。例えば、ポリプロピレン、ポリエチレン系の多孔膜が薄膜でかつ大面積化、膜強度や膜抵抗の面で適当である。
【００５５】
本発明における電解液としては、例えばカーボネート類、塩素化炭化水素、エーテル類、ケトン類、ニトリル類等を用いることができる。好ましくは高誘電率溶媒としてエチレンカーボネート（ＥＣ）、プロピレンカーボネート（ＰＣ）、γ−ブチロラクトン（ＧＢＬ）等から少なくとも１種類、低粘度溶媒としてジエチルカーボネート（ＤＥＣ）、ジメチルカーボネート（ＤＭＣ）、エチルメチルカーボネート（ＥＭＣ）、エステル類等から少なくとも１種類選択し、その混合液を用いる。ＥＣ＋ＤＥＣ、ＰＣ＋ＤＭＣ、ＰＣ＋ＥＭＤ、ＰＣ＋ＥＣ＋ＤＥＣなどが好ましいが、溶媒の純度が低い場合や含有水分量が多い場合などは、電位窓が高電位側に広い溶媒種の混合比率を高めると良い。さらに水分消費や耐酸化性向上、安全性向上等の目的で微量の添加剤を加えても良い。
【００５６】
支持塩としては、ＬｉＢＦ４、ＬｉＰＦ６、ＬｉＣｌＯ４、ＬｉＡｓＦ６、ＬｉＳＢＦ６、ＬｉＣＦ３ＳＯ３、Ｌｉ(ＣＦ３ＳＯ２)２Ｎ、ＬｉＣ４Ｆ９ＳＯ３、Ｌｉ(ＣＦ３ＳＯ２)３Ｃ、Ｌｉ(Ｃ２Ｆ５ＳＯ２)２Ｎなどから少なくとも１種類を用いるが、ＬｉＰＦ６を含む系が好ましい。支持塩の濃度は０．８Ｍ〜１．５Ｍが好ましく、さらに０．９Ｍ〜１．２Ｍがより好ましい。支持塩の濃度が低すぎると電気伝導率が低下することがあり、高すぎるとエネルギー密度の観点から不利となるためである。
【００５７】
電池の構成としては、角形、ペーパー型、積層型、円筒型、コイン型など種々の形状を採用することができる。外装材料その他の構成部材は特に限定されるものではなく、電池形状に応じて選定することができる。
【００５８】
ここでは一例として図３のような構造を有する電池について説明する。正極集電体３上に、中心材および被覆材を含む正極活物質層１が形成され、正極を構成している。また、負極集電体４上に負極活物質層２が形成され、負極を構成している。これらの正極と負極は、電解液に浸漬した状態の多孔質セパレータ５を介して対向配置されている。正極を収容する正極外装缶６と、負極を収容する負極外装缶７とが、絶縁パッキング部８を介して接合した構成となっている。
【００５９】
正極と負極に電圧を印加することにより正極活物質からリチウムイオンが脱離し、負極活物質にリチウムイオンが吸蔵され、充電状態となる。また、正極と負極の電気的接触を電池外部で起こすことにより、充電時と逆に、負極活物質からリチウムイオンが放出され、正極活物質にリチウムイオンが吸蔵されることにより、放電が起こる。
【００６０】
【実施例】
以下、本発明を実施例によりさらに説明するが、本発明はこれらに限定されるものではない。
【００６１】
[Ｌｉ１＋ｘＭｎ２−ｘＯ４の合成]
Ｌｉ１＋ｘＭｎ２−ｘＯ４の合成には、出発原料としてＬｉ２ＣＯ３と電解二酸化マンガン（ＥＭＤ）を用いた。
【００６２】
上記の出発原料の混合の前段階として、反応性の向上と目的粒径を有するＬｉ１＋ｘＭｎ２−ｘＯ４を得ることを目的に、Ｌｉ２ＣＯ３の粉砕およびＥＭＤの分級を行った。Ｌｉ１＋ｘＭｎ２−ｘＯ４は電池の正極活物質として用いる場合、充放電反応の均一性確保、スラリー作製の容易さ、安全性等の兼ね合いにより、５〜３０μｍの重量平均粒径が好ましいので、ＥＭＤの粒径もＬｉ１＋ｘＭｎ２−ｘＯ４の目的粒径と同じ５〜３０μｍとした。
【００６３】
一方、Ｌｉ２ＣＯ３は均一反応の確保のためには５μｍ以下の粒径が望ましいので、粒径加積曲線で５０％通過する粒径であるＤ５０粒径が１．４μｍとなるように粉砕を行った。
【００６４】
このように所定の粒径に揃えたＥＭＤおよびＬｉ２ＣＯ３を、０≦ｘ≦０．２０の範囲の所定比となるように混合した。
【００６５】
この混合粉を乾燥空気フローの雰囲気下、６００〜８００℃で一次焼成した後、再度、乾燥空気フローの雰囲気下、６００〜８００℃で二次焼成した。
【００６６】
次いで、得られたＬｉ１＋ｘＭｎ２−ｘＯ４の粒子中の粒径１μｍ以下の微小粒子を空気分級機により除去した。この時、得られたＬｉ１＋ｘＭｎ２−ｘＯ４の比表面積は０．６〜０．９ｍ２／ｇであった。
【００６７】
またタップ密度は２.１４〜２.２２ｇ／ｃｃ、真密度は３.９８〜４．１３ｇ／ｃｃ、Ｄ５０粒径は１１〜１６μｍという粉体特性であった。
【００６８】
なお、得られたＬｉ１＋ｘＭｎ２−ｘＯ４は、過剰Ｌｉ量ｘの値（仕込み組成比から算出）に対応して、ｘ＝０．０をＡ−１、ｘ＝０．０３をＡ−２、ｘ＝０．０６をＡ−３、ｘ＝０．０８をＡ−４、ｘ＝０．１０をＡ−５、ｘ＝０．１５をＡ−６、ｘ＝０．１８をＡ−７、ｘ＝０．２０をＡ−８と表記する。
【００６９】
[ＬｉＭｎ１．５Ｎｉ０．５Ｏ４による被覆]
ＬｉＭｎ１．５Ｎｉ０．５Ｏ４の合成には、出発原料としてＬｉ２ＣＯ３と(Ｍｎ０．７５Ｎｉ０．２５)３Ｏ４を用いた。
【００７０】
混合の均一性を確保するため、上記の出発原料を所定比で秤量し、それらを湿式ボールミルにて、２４時間混合した。その後、酸素フロー雰囲気中、７５０℃で８時間の焼成を２回繰り返した。
【００７１】
得られた粉末は粉末Ｘ線回折より立方晶スピネル構造の単一相であることが確認され、格子定数は８．１７７Åであった。また化学分析によりＭｎとＮｉがほぼ３対1の比であることも確認した。さらに、この粉体のＭｎ価数を分析したところ、３.９９と算出された。
【００７２】
次いで、得られた粉末を湿式遊星ボールミルにて４８時間粉砕し、Ｄ５０粒径が０．７μｍのＬｉＭｎ１．５Ｎｉ０．５Ｏ４を得た。
【００７３】
続いて、中心材であるＬｉ１＋ｘＭｎ２−ｘＯ４と上記の工程を経て得られたＬｉＭｎ１．５Ｎｉ０．５Ｏ４を、重量比で９５：５の比でエタノール中に混合することにより、中心材の周囲にＬｉＭｎ１．５Ｎｉ０．５Ｏ４を付着させて複合粒子とし、１００℃で１２時間乾燥させた。その後、乾燥空気フロー雰囲気の環境下、５００℃で加熱処理を施した。なお、Ｘ線粉末回折により、中心材およびＬｉＭｎ１．５Ｎｉ０．５Ｏ４の微粒子の各々の格子定数が変化していないことが確認された。このことにより、中心材とＬｉＭｎ１．５Ｎｉ０．５Ｏ４の微粒子とは反応・固溶していないことが判明した。また、エネルギー分散型Ｘ線分光（ＥＤＸ）分析により、複合粒子の表面にのみＮｉが検出されたことから、ＬｉＭｎ１．５Ｎｉ０．５Ｏ４の微粒子は中心材の表面に付着していると考えられた。
【００７４】
一例として、中心材にｘ＝０．１０のマンガン酸リチウム、すなわちＬｉ１．１０Ｍｎ１．９０Ｏ４を用いた場合の粉体の粉末Ｘ線回折パターンを図１に、金属Ｌｉを負極として試作したコイン型電池の充電カーブを図２に示す。
【００７５】
図１中、三角形でマークされたピークがＬｉＭｎ１．５Ｎｉ０．５Ｏ４のピークであり、それ以外はＬｉ１．１０Ｍｎ１．９０Ｏ４のピークである。Ｌｉ１．１０Ｍｎ１．９０Ｏ４による強いピークの高角度側に、ＬｉＭｎ１．５Ｎｉ０．５Ｏ４の弱いピークが観測されることがわかる。また図２より、金属Ｌｉ対極電位で４．５Ｖ以上の領域にプラトーの存在が確認できた。
【００７６】
[比較例１]
正極活物質として、Ａ−１〜Ａ−８をそれぞれ用いて、８種類の１８６５０円筒電池を作製した。
【００７７】
正極は、活物質：カーボンブラック：ＰＶｄＦ＝９０：６：４（重量％）の混合比で作製し、負極はハードカーボン：カーボンブラック：ＰＶｄＦ＝９０：２：８（重量％）の混合比で作製した。
【００７８】
電解液は１ＭのＬｉＰＦ６を支持塩とし、プロピレンカーボネート（ＰＣ）とエチルメチルカーボネート（ＥＭＣ）の混合溶液（３０：７０／体積％）を溶媒とした。
【００７９】
上記８種類の電池は、用いた正極Ａ−１〜Ａ−８に対応して、それぞれＢ−１〜Ｂ−８とする。
【００８０】
[実施例１]
正極活物質として、Ａ−１〜Ａ−８をそれぞれ中心材とし、上記の方法でＬｉＭｎ１．５Ｎｉ０．５Ｏ４を中心材の粉末の表面に付着させた複合材料を用いた以外は、比較例１と同様にして８種類の１８６５０円筒電池を作製した。
【００８１】
得られた１８６５０円筒電池は、用いた活物質の中心材Ａ−１〜Ａ−８に対応して、それぞれＣ−１〜Ｃ−８とする。
【００８２】
[評価試験例１]
比較例１および参考例ａで作製した１８６５０円筒電池を用いて容量保存特性を評価した。
【００８３】
まず最初に各円筒電池は室温において充電および放電を1回ずつ行った。このときの充電電流および放電電流はともに２００ｍＡであり、この際の放電容量を初期容量とした。なお、全ての電池において、放電側のカットオフ電位は２．５Ｖ、充電側のカットオフ電位は４．３Ｖとした。
【００８４】
その後、各電池を２００ｍＡで４．３Ｖまで充電し、さらに３時間の定電位充電後、５０℃の恒温槽中で４週間放置した。放置後に室温で再度、２００ｍＡで放電操作を行い、続いて同じく２００ｍＡで充電・放電操作をもう１度繰り返し、その時の放電容量を回復容量とした。
【００８５】
[評価試験例２]
同じく比較例１および参考例ａで作製した１８６５０円筒電池を用いて、充放電サイクル試験を行った。
【００８６】
サイクル評価は５００ｍＡで４．３Ｖまで充電し、その後、２時間の定電位充電を行い、５００ｍＡで２．５Ｖまで放電させる、という操作を繰り返すことによって行った。なお、試験は５０℃の温度で行った。
【００８７】
表１に各電池の正極活物質の中心材、中心材の過剰Ｌｉ組成ｘ、被覆材、被覆材の格子定数ａ１と中心材の格子定数ａ２の比、５０℃、４週間放置後の回復容量率（＝１００×［回復容量］／［初期容量］）、５０℃、３００サイクル後における容量維持率（＝１００×［３００サイクル目の放電容量］／［１０サイクル目の放電容量］）を示す。比較例１のＢ−１〜Ｂ−８の各電池と比較し、参考例ａのＣ−１〜Ｃ−８の各電池では、５０℃、４週間放置後の回復容量率ならびに５０℃、３００サイクル後の容量維持率の両方が改善されていることが分かった。Ｃ−１〜Ｃ−８の電池は、回復容量率と容量維持率が共に８０％以上の良好な特性を示した。このときのａ１／ａ２比は０．９９３３以上であった。
【００８８】
【表１】
すなわち、Ｍｎ価数が３.９９で、金属Ｌｉ対極電位で４．５Ｖ以上に充放電容量を有するＬｉＭｎ１．５Ｎｉ０．５Ｏ４によって、部分的に表面を被覆されたＬｉ１＋ｘＭｎ２−ｘＯ４は、その容量保存特性ならびにサイクル特性が大幅に改善されることが明らかとなった。また、このときのＬｉＭｎ１．５Ｎｉ０．５Ｏ４の格子定数ａ１とＬｉ１＋ｘＭｎ２−ｘＯ４の格子定数ａ２の関係は、特にａ１／ａ２≧０．９９３３であることが好ましいことが判明した。
【００９０】
[実施例２]
続いて、ＬｉＭｎ１．５Ｎｉ０．５Ｏ４のＭｎサイトの一部をＴｉで置換したＬｉＭｎ１．４Ｔｉ０．１Ｎｉ０．５Ｏ４を合成した。出発原料として、Ｌｉ２ＣＯ３と(Ｍｎ０．７Ｔｉ０．０５Ｎｉ０．２５)３Ｏ４を用いたこと以外は、ＬｉＭｎ１．５Ｎｉ０．５Ｏ４と同様の手順で合成し、粉砕した。
【００９１】
得られたＬｉＭｎ１．４Ｔｉ０．１Ｎｉ０．５Ｏ４は、粉末Ｘ線回折にて単一相であることが確認され、そのＭｎ価数は４．００と算出された。また、Ｄ５０粒径は０．８μｍであった。
【００９２】
続いて、中心材にはＡ−５（Ｌｉ１．１Ｍｎ１．９Ｏ４）を用い、上記の工程を経て得られたＬｉＭｎ１．４Ｔｉ０．１Ｎｉ０．５Ｏ４を、重量比で９５：５の比でエタノール中に混合し、１００℃で１２時間乾燥させた後に、乾燥空気フロー雰囲気の環境下、４００℃で加熱処理を施した。
【００９３】
このようにして得られた複合材を正極として、比較例１と同様に１８６５０円筒電池を作製した。この電池をＤ−１とする。
【００９４】
[実施例３]
同様に、ＬｉＭｎ１．５Ｎｉ０．５Ｏ４のＭｎサイトの一部をＳｉで置換したＬｉＭｎ１．４Ｓｉ０．１Ｎｉ０．５Ｏ４を合成した。出発原料として、Ｌｉ２ＣＯ３と(Ｍｎ０．７５Ｎｉ０．２５)３Ｏ４、ＳｉＯを用いたこと以外は、ＬｉＭｎ１．５Ｎｉ０．５Ｏ４と同様の手順で合成し、粉砕した。
【００９５】
得られたＬｉＭｎ１．４Ｓｉ０．１Ｎｉ０．５Ｏ４は、粉末Ｘ線回折にて単一相であることが確認され、そのＭｎ価数は３.９７と算出された。また、Ｄ５０粒径は０．６μｍであった。
【００９６】
続いて、中心材にはＡ−５（Ｌｉ１．１Ｍｎ１．９Ｏ４）を用い、上記の工程を経て得られたＬｉＭｎ１．４Ｓｉ０．１Ｎｉ０．５Ｏ４を、重量比で９５：５の比でエタノール中に混合し、１００℃で１２時間乾燥させた後に、乾燥空気フロー雰囲気の環境下、５５０℃で加熱処理を施した。
【００９７】
このようにして得られた複合材を正極として、比較例１と同様に１８６５０円筒電池を作製した。この電池をＤ−２とする。
【００９８】
[参考例ｂ]
以上は、微粉状の被覆材による部分的な被覆であったが、液相経由による全面被覆も試みた。Ｌｉ原料としては、リチウムエトキシドを、Ｍｎ原料としては酢酸マンガン水和物を、Ｎｉ原料としてはニッケルアセチルアセトナートを用い、それらの金属比を１：１．５：０．５（モル比）となるように２−エトキシエタノール中で反応させ（窒素雰囲気下）、ジエチルアミンによって安定化させた溶液を用意した。
【００９９】
この溶液を乾燥・固化させた物質を酸素フロー雰囲気下、６００℃で焼成したところ、粉末Ｘ線回折では、僅かに各ピークがブロードではあるものの、ＬｉＭｎ１．５Ｎｉ０．５Ｏ４の単一相が得られた。この材料のＭｎ価数は３.９８と算出された。
【０１００】
続いて、上記と同様の溶液を用意し、その溶液と中心材であるＡ−５（Ｌｉ１．１Ｍｎ１．９Ｏ４）を体積比で１：１となるように浸漬・混合させた。そのスラリーを１５０℃で乾燥後、酸素フロー雰囲気下、６００℃で焼成した。この「スラリー作製およびそれに続く焼成」を３回繰り返した後の試料を粉末Ｘ線回折で分析したところ、Ｌｉ１．１Ｍｎ１．９Ｏ４とＬｉＭｎ１．５Ｎｉ０．５Ｏ４の２つのスピネル相のピークが観測された。
【０１０１】
上記のような手法で得られた複合材を正極活物質として、比較例１と同様に１８６５０円筒電池を作製した。この電池をＥ−１とする。
[参考例ｃ]
参考例ｂと同様に複合材を作製した。ただし、「スラリー作製およびそれに続く焼成」工程を６回繰り返した。すなわち、被覆材の厚さは参考例ｂの複合材の約２倍と推測された。この複合材を用いて、比較例１と同様に１８６５０円筒電池を作製した。この電池をＥ−２とする。
【０１０２】
[比較例２]
負極活物質をグラファイトとした以外は、比較例１のＢ−５と同様に１８６５０円筒電池を作製した。この電池をＦ−１とする。
【０１０３】
[参考例ｄ]
正極の副活物質として岩塩層状構造のＬｉＮｉ０．８Ｃｏ０．２Ｏ２を加えた系も検討した。このＬｉＮｉ０．８Ｃｏ０．２Ｏ２は電解液中の酸生成を抑制する効果を有する。
【０１０４】
正極電極を、[Ａ−５（Ｌｉ１．１Ｍｎ１．９Ｏ４）をＬｉＭｎ１．５Ｎｉ０．５Ｏ４で部分的に被覆したもの]：ＬｉＮｉ０．８Ｃｏ０．２Ｏ２：カーボンブラック：ＰＶｄＦ＝８０：１０：６：４（重量％）の混合比で作製したこと以外は、比較例２と同様に１８６５０円筒電池を作製した（参考例ａのＣ−５の正極にＬｉＮｉ０．８Ｃｏ０．２Ｏ２を加え、負極活物質をグラファイトに変更した系）。この電池をＦ−２とする。
【０１０５】
[参考例ｅ]
負極活物質をハードカーボンとしたこと以外は、参考例ｄと同様にして１８６５０円筒電池を作製した。この電池をＧ−１とする。
【０１０６】
[評価試験例３]
実施例２のＤ−１、実施例３のＤ−２、参考例ｂのＥ−１、参考例ｃのＥ−２、比較例２のＦ−１、参考例ｄのＦ−２、参考例ｅのＧ−１の各電池について、比較評価例１および２と同様に、５０℃、４週間保存後の回復容量率、５０℃、３００サイクル後の容量維持率を測定した。
【０１０７】
上記各電池に、比較例１のＢ−５の電池の加え、計８種類の電池について、被覆材の格子定数ａ１と中心材の格子定数ａ２の比、５０℃、４週間保存後の回復容量率、５０℃、３００サイクル後の容量維持率を表２に示す。
【０１０８】
【表２】
Ｂ−５とＤ−１ならびにＤ−２の比較から、ＬｉＭｎ１．５Ｎｉ０．５Ｏ４のＭｎサイトを４価の他カチオンで置換しても、被覆の効果は得られ、回復容量率および容量維持率ともに優れた特性を示した。
【０１１０】
また、Ｂ−５とＥ−１、Ｅ−２の比較から、液相経由で中心材の全面に薄く被覆した場合も同様の効果が得られることが判明した。
【０１１１】
さらに、Ｆ−１、Ｆ−２、Ｇ−１の比較から、Ｌｉ１＋ｘＭｎ２−ｘＯ４にはＬｉＭｎ１．５Ｎｉ０．５Ｏ４などの被覆を行うこと、副活物質としてＬｉＮｉ０．８ＣＯ０．２Ｏ２を含むこと、とりわけ、負極活物質としてはハードカーボンを組み合わせることが、より好ましいことが判明した。
【０１１２】
以上、表1ならびに表２の結果より明らかとなったことを整理すると次のようになる。
（１）４Ｖ級の正極活物質であるＬｉ１＋ｘＭｎ２−ｘＯ４の表面を、ＬｉＭｎ１．５Ｎｉ０．５Ｏ４の５Ｖ級活物質（Ｍｎ価数が実質的に４価であり、金属Ｌｉ対極電位で４．５Ｖ以上に充放電容量を有する材料）で被覆することにより、容量保存特性および充放電サイクル特性が大幅に改善する。
（２）被覆は全面の方が好ましいが、部分的でも有効である。
（３）５Ｖ級活物質のＭｎサイトの一部を他の４価カチオンで置換しても有効である。
（４）被覆材の格子定数ａ１と中心材の格子定数ａ２の比はａ１／ａ２＞０．９９３が好ましい。
（５）正極中には副活物質としてＬｉＮｉ０．８Ｃｏ０．２Ｏ２を含むことが望ましい。
（６）負極活物質はハードカーボンの方が好ましい。
【０１１３】
【発明の効果】
本発明によれば、正極活物質としてスピネル型マンガン酸リチウムを用いた二次電池において、正極からのＭｎ溶出を低減させ、電解液との副反応も抑制することにより、高温サイクル特性ならびに保存特性に優れた二次電池を提供することが可能となる。
【図面の簡単な説明】
【図１】本発明の実施例に係る電池の正極活物質の粉末Ｘ線回折パターンである。
【図２】本発明の実施例に係る電池の初回充電曲線である。
【図３】本発明に係る二次電池の断面構造を示す図である。
【符号の説明】
１ 正極活物質層
２ 負極活物質層
３ 正極集電体
４ 負極集電体
５ 多孔質セパレータ
６ 正極外装缶
７ 負極外装缶
８ 絶縁パッキング部[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a positive electrode for a lithium ion secondary battery and a secondary battery using the same.
[0002]
[Prior art]
Secondary batteries using lithium metal or a lithium compound as a negative electrode have been studied vigorously in recent years because an electromotive force exceeding 4 V can be obtained when lithium cobaltate is used as a positive electrode active material. Lithium cobalt oxide is widely used as a positive electrode active material in today's lithium ion secondary batteries because it exhibits good characteristics in terms of total performance such as potential flatness, capacity, discharge potential, and cycle characteristics.
[0003]
However, cobalt is an expensive material with a small recoverable reserve. Lithium cobaltate has a layered rock salt structure (α-NaFeO2Therefore, an oxygen layer having a large electronegativity is adjacent due to lithium desorption at the time of charging. For this reason, when the amount of extracted lithium is too large, such as in an overcharged state, the structure changes due to electrostatic repulsion between oxygen layers and heat is generated, which has a problem in terms of ensuring safety. From such a background, an alternative material for lithium cobalt oxide has been demanded.
[0004]
As a positive electrode active material of a 4V class secondary battery other than lithium cobaltate, lithium nickelate, spinel type lithium manganate, and the like are considered. However, although lithium nickelate has a capacity greater than that of lithium cobaltate, the crystal structure is the same layered rock salt structure as lithium cobaltate, and Ni during charging4+Due to the instability of the material, the oxygen desorption temperature is lower than that of lithium cobaltate and it is more difficult to ensure safety. Furthermore, considering that the discharge potential is lower than that of lithium cobaltate and the high environmental load of Ni, it is less attractive as an alternative material for lithium cobaltate.
[0005]
On the other hand, spinel-type lithium manganate is made of inexpensive manganese, is a stable spinel-type crystal, and contains almost no extra lithium used only during overcharge. Shows high safety. For this reason, it is a very promising material and has already been put into practical use in part. Spinel-type lithium manganate is more advantageous than cobalt and nickel in terms of resource supply and environmental load. Therefore, in addition to the above-mentioned advantage of low cost, it can store power for road leveling and future electric vehicles. It has a high potential as a power source.
[0006]
[Problems to be solved by the invention]
As described above, spinel type lithium manganate is a material that has advantages in various aspects of cost, safety, stable supply of resources, and environmental load, and is highly expected as a power storage or power source for electric vehicles. . However, in these applications, unlike small portable devices, factors related to long-term reliability such as charge / discharge cycle life and capacity storage characteristics and power characteristics are more important than high energy density. It has been.
[0007]
In addition to the characteristics of the active material itself, the power characteristics are greatly influenced by the structure and structure such as electrode design and current collection design. Fortunately, the power characteristics of batteries using spinel type lithium manganate as the positive electrode active material are the same. Compared to the case where other positive electrode active materials are used in the configuration, the same or better performance is obtained, which seems promising in this aspect.
[0008]
On the other hand, spinel-type lithium manganate has not reached a level where charge / discharge cycle life and capacity storage characteristics at high temperatures can be satisfied, as seen in various reports so far. In other words, improving the high-temperature cycle characteristics and storage characteristics of spinel-type lithium manganate opens the way to the practical use of power storage for electric power storage or electric vehicles. It has a very important meaning.
[0009]
In view of the above circumstances, an object of the present invention is to provide a secondary battery using spinel-type lithium manganate as a positive electrode active material and excellent in high-temperature cycle characteristics and storage characteristics.
[0010]
[Means for Solving the Problems]
According to the present invention for solving the above problems, a positive electrode for a secondary battery comprising a current collector and a positive electrode active material layer provided on the current collector, wherein the positive electrode active material layer comprises lithium ions. Can be occluded / released,Li as the positive electrode active material 1 + x Mn 2-x O 4 And the surface of the fine particles is, LiMn 1.4 Ti 0.1 Ni 0.5 O 4 Or LiMn 1.4 Si 0.1 Ni 0.5 O 4 Spinel type lithium manganese oxide represented byCovered byingA positive electrode for a secondary battery is provided.
[0012]
In the case of a secondary battery using a positive electrode active material containing Mn, one of the causes of charge / discharge cycle deterioration is elution of Mn from the positive electrode active material. This Mn elution phenomenon is closely related to the acid concentration in the electrolytic solution, but when attention is paid to the valence of Mn, trivalent is relatively more easily dissolved than tetravalent. This is considered to be because trivalent Mn causes disproportionation to generate divalent Mn. In the present invention, since the spinel type lithium manganese composite oxide has a configuration in which the surface of the positive electrode active material is partially or entirely covered, the amount of Mn ions eluted into the electrolytic solution can be reduced.
[0014]
By adopting such a configuration, the surface of the positive electrode active material can be effectively covered with the spinel type lithium manganese composite oxide. As a result, the above Mn elution phenomenon can be effectively suppressed.
[0016]
By covering the surface of the positive electrode active material with the spinel type lithium manganese composite oxide, it becomes possible to more stably suppress elution of manganese ions from the positive electrode active material into the electrolytic solution.
[0017]
In the above positive electrode for secondary battery, the spinel type lithium manganese oxide is LiMn.2-aNiaMbO4(0.45 ≦ a ≦ 0.55, 0 ≦ b ≦ 0.3, M is at least one of Si or Ti). Since this compound contains almost no trivalent Mn, it does not show charge / discharge capacity in a voltage range such as 3.0 to 4.2 V (or 4.3 V) which is a normal use range of the battery. In other words, in this voltage region, Li does not enter and exit with a change in the valence of the transition metal in the crystal, and therefore exists very stably without increasing or decreasing the lattice length.
[0018]
Moreover, the said compound has a spinel structure and contains the empty site in the crystal | crystallization. Therefore, it has Li ion conductivity at the same time while stably existing. Furthermore, since it has a lattice constant almost similar to that of the positive electrode active material, the affinity for the positive electrode active material is also high.
[0019]
By including a material that is stable and has Li ion conductivity in the positive electrode, side reaction between the positive electrode active material and the electrolytic solution is suppressed, and capacity deterioration of the positive electrode active material and an increase in resistance on the negative electrode surface are caused. It is suppressed. As a result, charge / discharge cycle characteristics and capacity storage characteristics in a high temperature environment are improved.
[0020]
Furthermore, since the compound has a charge / discharge capacity of 4.5 V or more at the metal Li counter electrode potential, it functions as a buffer material when the battery is overcharged. Therefore, when an overcharge inhibitor or the like is added to the electrolyte, it helps to ensure that its function functions, and is extremely useful in terms of ensuring the safety of the battery.
[0021]
In the positive electrode for a secondary battery, the positive electrode active material is Li1 + xMn2-xO4It can be set as the structure containing. At this time, 0 ≦ x ≦ 0.20 is preferable, and 0.06 ≦ x ≦ 0.20 is more preferable.
[0022]
In the positive electrode for a secondary battery, the valence of Mn in the spinel type lithium manganese composite oxide is preferably in the range of 3.9 to 4.0. By doing in this way, the absolute amount of trivalent Mn in the spinel type lithium manganese composite oxide can be suppressed. Here, in this specification, the valence of Mn is MnO analyzed according to JIS K1467.2% And a value calculated using a numerical value of Mn% analyzed by a titration method.
[0023]
Moreover, in the positive electrode for a secondary battery described above, it is preferable that the average charge voltage with respect to the lithium reference potential of the spinel type lithium manganese oxide is 4.5 V or more. In this way, the spinel type lithium manganese composite oxide does not show charge / discharge capacity in the voltage range (3.0 to 4.2 V (or 4.3 V)) which is the normal use range of the battery. In other words, in this voltage region, Li does not enter and exit with a change in the valence of the transition metal in the crystal, and therefore exists very stably without increasing or decreasing the lattice length.
[0024]
In the positive electrode for a secondary battery, the lattice constant a1 of the spinel type lithium manganese composite oxide and the lattice constant a2 of the positive electrode active material are preferably close to each other from the viewpoint of the affinity between the two materials. Specifically, it is preferable to satisfy 0.9912 ≦ a1 / a2 ≦ 0.9982, and it is more preferable to satisfy 0.9933 ≦ a1 / a2 ≦ 0.9982.
[0025]
The positive electrode for a secondary battery may further include a LiNi composite oxide. Since the LiNi composite oxide has an effect of suppressing acid generation in the electrolytic solution, the above-described Mn elution phenomenon can be further suppressed. As the LiNi composite oxide, for example, LiNi containing Co0.8Co0.2O2Is mentioned.
[0026]
Furthermore, according to the present invention, there is provided a secondary battery comprising the above-described positive electrode for a secondary battery, and a negative electrode disposed to face the positive electrode for the secondary battery via an electrolytic solution. This secondary battery is excellent in long-term reliability because the Mn elution phenomenon from the positive electrode active material is suppressed.
[0027]
In the secondary battery, the negative electrode active material in the negative electrode can be hard carbon. As a result, a battery having excellent long-term reliability such as cycle characteristics can be obtained.
[0028]
As a result of intensive studies aimed at improving cycle characteristics and capacity storage characteristics in a high temperature environment in a secondary battery using a positive electrode active material containing Mn, the present inventors have arrived at the present invention. .
[0029]
As a positive electrode active material, a general formula LiMn having a cubic spinel structure2O4The use of a Li-containing composite oxide represented by the above is a well-known technique as already disclosed in Japanese Patent Application Laid-Open Nos. 55-100284 and 58-220362.
[0030]
However, this material system is currently the mainstream LiCoO2Since the charge / discharge cycle was inferior to that of Li, the Li excess composition as disclosed in JP-A-2-270268 and JP-A-6-187993, that is, Li1 + xMn2-xO4Or other element substitution to the Mn site, especially Cr substitution, as disclosed in JP-A-3-219571, JP-A-4-160769, and further JP-A-5-36412. Has been derived. Although the improvement of the charge / discharge cycle characteristics by these technologies is recognized experimentally, the improvement range required for power storage and power sources for electric vehicles cannot be obtained. The reduction in the amount of Mn ions is accompanied by a demerit that the charge / discharge capacity is reduced.
[0031]
On the other hand, apart from the approach of substitution with Li or other transition metal elements, LiMn2O4Various methods for coating the surface with other materials have been studied. For example, the techniques described in JP-A-9-35715, JP-A-10-172571, JP-A-11-71114, JP-A-2002-68745 and the like can be mentioned.
[0032]
In JP-A-9-35715, LiMn2O4The surface of LiCoO2A technique for coating with is disclosed.
[0033]
In JP-A-10-172571, LiMn2O4As a result of substitution of the Mn site on the surface of the substrate by other elements, the Mn valence is different from the central materialxMnyOzA technique for coating with is disclosed.
[0034]
In JP-A-11-71114, LiMn2O4Co-substituted LixMn2-yCoyO4A technique for coating with is disclosed.
[0035]
Furthermore, in Japanese Patent Application Laid-Open No. 2002-68745, Al substitution type LiMn2-yAlyO4The surface of Li-rich Li1 + xMn2-xO4A technique for coating with is disclosed.
[0036]
From the viewpoint of having a configuration in which the surface of the spinel structure Li-containing composite oxide, which is a positive electrode active material, is coated with another type of spinel structure Li-containing composite oxide, the present invention and JP-A-10-172571, Although similar to Kaihei 11-71114 and JP-A-2002-68745, its technical idea and material composition are completely different for the following reasons.
[0037]
First, in the technique disclosed in the above publication, the coating material also exhibits charge / discharge capacity, whereas in the present invention, the spinel type lithium manganese composite oxide corresponding to the coating material is used in the normal operating voltage range of the battery. Does not contribute to charging / discharging at all.
[0038]
Second, in the technique disclosed in the above-mentioned 3 publications, the Mn valence in the coating material is about 3.5 to 3.65, whereas the Mn valence in the spinel-type lithium manganese composite oxide in the present invention. The number is substantially tetravalent. For example, as spinel type lithium manganese composite oxide, LiMn2-aNiaO4When 0.45 ≦ a ≦ 0.55 is adopted, the Mn valence can be substantially made tetravalent.
[0039]
Thirdly, for the second reason described above, the spinel type lithium manganese oxide in the present invention has a charge / discharge capacity in a voltage range of 4.5 V or higher at a metal Li counter electrode potential, which is not normally used. On the other hand, the coating material in the technique disclosed in the above publication does not show charge / discharge capacity in this voltage region.
[0040]
For the reasons described above, the technology of the present invention is particularly suitable for large-sized batteries for power storage and electric vehicles as compared with the above-described conventional technology, and the improvement range of charge / discharge cycle characteristics and capacity storage characteristics in a high temperature environment is also conventional. Larger than the surface coating.
[0041]
DETAILED DESCRIPTION OF THE INVENTION
The positive electrode active material used in the present invention (hereinafter, the positive electrode active material is appropriately referred to as a central material in the embodiments) is selected from Li-containing oxides having a plateau near 4 V at the metal Li counter electrode. Especially Li1 + xMn2-xO4(X is in the range of 0.06 ≦ x ≦ 0.20). Depending on the priority order of the characteristics of the target battery, the Mn site may be further substituted with other cations as appropriate.
[0042]
The particle shape of the positive electrode active material is not particularly limited as a lump, a sphere, a plate or the like, and the particle size and specific surface area can be appropriately selected in consideration of the positive electrode film thickness, the positive electrode density, the binder type, etc. From the viewpoint of keeping the energy density high, particle shape, particle size distribution, average particle size, specific surface area, and true density are desirable so that the positive electrode density in the portion excluding the collector metal foil is 2.8 g / cc or more. . In addition, among the positive electrode mixture composed of a positive electrode active material, a binder, a conductivity imparting agent, etc., the particle shape, particle size distribution, average particle size, specific surface area such that the weight ratio occupied by the positive electrode active material is 85% or more. -True density is desirable.
[0043]
Li1 + xMn2-xO4The starting material used for the synthesis (where x is in the range of 0.06 ≦ x ≦ 0.20) is Li2CO3, LiOH, Li2O, Li2SO4MnO as a Mn raw material2, Mn2O3, Mn3O4, MnOOH, MnCO3, Mn (NO3)2Etc. can be used. Among the above, Li as a Li raw material2CO3However, as a Mn raw material, MnO2Or Mn2O3Is particularly preferred.
[0044]
Hereinafter, the synthesis method will be described. The above starting materials are appropriately selected, and weighed and mixed so that a predetermined metal composition ratio is obtained. At this time, Mn2O3The particle size of each reagent is preferably 10 μm or less in order to avoid residual heterogeneous phases. The mixing is performed using a ball mill, a jet mill, a pin mill, or the like, and an apparatus can be appropriately selected depending on the particle diameter and hardness of the selected reagent. The obtained mixed powder is fired in air or oxygen at a temperature range of 600 ° C to 950 ° C.
[0045]
The specific surface area of the obtained Li-containing oxide was 3 m.2/ G or less, more preferably 1 m2/ G or less is particularly preferable. 3m2When the specific surface area is at least / g, the required amount of binder increases, which is disadvantageous in terms of the capacity density of the positive electrode.
[0046]
In the present invention, the center material is coated with a Li-containing composite oxide having a Mn valence in the crystal of substantially tetravalent and having a charge / discharge capacity of 4.5 V or more at the metal Li counter electrode potential. . Specifically, LiMn2-aNiaO4Is mentioned. At this time, by setting 0.45 ≦ a ≦ 0.55, trivalent manganese can be sufficiently reduced in the crystal, so that the Mn valence can be kept substantially tetravalent. At the same time, since the compound has a charge / discharge capacity of 4.5 V or more, it is suitable for the present invention.
[0047]
Since these materials charge and discharge at 4.5 V or higher at the metal Li counter electrode potential by oxidation and reduction of a transition metal other than Mn, the Mn site may be substituted with another cation. However, in order to keep the Mn valence to be tetravalent, the substituted species is preferably a tetravalent cation that is lighter than Mn, such as Si or Ti. Thereby, it can contribute to the weight reduction of a battery, and the improvement of an energy density.
[0048]
Li1 + xMn2-xO4The coating can be appropriately selected according to the particle size and film thickness, such as spraying and mixing fine powder of the coating material, heating and baking following mixing, or immersion or spray spraying of each element in a predetermined ratio solution, In the case of whole surface application, the liquid phase is preferable.
[0049]
The composition of the coating material is Li1 + xMn2-xO4It can be performed in the same manner as the synthesis of However, it is desirable to use a composite oxide of Mn and Ni in advance. Further, when another kind of cation is introduced, a composite oxide containing the element kind can also be used. In the case of via the liquid phase, a composite alkoxide may be used.
[0050]
The obtained composite positive electrode active material comprises a binder type and a conductivity-imparting agent appropriately selected according to the characteristics important for the battery, such as rate characteristics, low temperature discharge characteristics, pulse discharge characteristics, energy density, weight reduction, and downsizing. Mix to make an electrode. As the binder, a resin binder that is generally used can be used. For example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), or the like can be used. A metal thin film mainly composed of Al or the like is used as the current collector metal foil.
[0051]
The ratio (weight ratio) between the center material and the coating material in the positive electrode is preferably in the range of 98: 2 to 90:10. Since the covering material in the present invention does not relate to the charge / discharge capacity, if the ratio of the covering material is too large, it is disadvantageous from the viewpoint of the energy density of the battery. On the other hand, when the ratio of the coating material is too small, elution of manganese ions from the central material to the electrolytic solution cannot be effectively suppressed.
[0052]
The negative electrode used in the present invention is preferably selected from Li metal, Li alloy, and carbon material that can occlude and release Li ions, but metal, metal oxide, or composite material of these and carbon material that forms an alloy with Li. Transition metal nitrides can also be selected. Selection of the negative electrode material can be appropriately performed according to the intended use of the battery such as capacity, voltage, weight, size and rate characteristics, low-temperature discharge characteristics, and pulse discharge characteristics.
[0053]
The negative electrode active material is mixed with a binder type appropriately selected according to characteristics important as a battery, such as rate characteristics, low temperature discharge characteristics, pulse discharge characteristics, energy density, weight reduction, and downsizing, to form an electrode. As the binder, commonly used polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE) and the like can be used, and a rubber-based binder can also be used. As the current collector metal foil, a metal thin film mainly composed of Al, Cu or the like is used.
[0054]
Although a separator is not specifically limited, A woven fabric, a glass fiber, a porous synthetic resin film | membrane etc. can be used. For example, a polypropylene or polyethylene based porous film is suitable in terms of a thin film and a large area, film strength and film resistance.
[0055]
As the electrolytic solution in the present invention, for example, carbonates, chlorinated hydrocarbons, ethers, ketones, nitriles and the like can be used. Preferably, at least one kind from ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (GBL), etc. as the high dielectric constant solvent, diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate as the low viscosity solvent At least one selected from (EMC), esters and the like, and a mixed solution thereof is used. EC + DEC, PC + DMC, PC + EMD, PC + EC + DEC, and the like are preferable. However, when the purity of the solvent is low or the content of water is large, the mixing ratio of solvent types having a wide potential window on the high potential side may be increased. Furthermore, a trace amount of additives may be added for the purpose of improving moisture consumption, oxidation resistance, safety, and the like.
[0056]
As a supporting salt, LiBF4, LiPF6LiClO4, LiAsF6, LiSBF6, LiCF3SO3, Li (CF3SO2)2N, LiC4F9SO3, Li (CF3SO2)3C, Li (C2F5SO2)2At least one of N and the like is used, but LiPF6A system containing is preferred. The concentration of the supporting salt is preferably 0.8M to 1.5M, and more preferably 0.9M to 1.2M. This is because if the concentration of the supporting salt is too low, the electrical conductivity may decrease, and if it is too high, it is disadvantageous from the viewpoint of energy density.
[0057]
As the configuration of the battery, various shapes such as a square shape, a paper shape, a stacked shape, a cylindrical shape, and a coin shape can be adopted. The exterior material and other constituent members are not particularly limited, and can be selected according to the battery shape.
[0058]
Here, a battery having a structure as shown in FIG. 3 will be described as an example. A positive electrode active material layer 1 including a center material and a covering material is formed on the positive electrode
[0059]
When a voltage is applied to the positive electrode and the negative electrode, lithium ions are desorbed from the positive electrode active material, and the lithium ions are occluded in the negative electrode active material, resulting in a charged state. In addition, by causing electrical contact between the positive electrode and the negative electrode outside the battery, lithium ions are released from the negative electrode active material, and lithium ions are occluded in the positive electrode active material, which is opposite to that during charging.
[0060]
【Example】
EXAMPLES Hereinafter, although an Example demonstrates this invention further, this invention is not limited to these.
[0061]
[Li1 + xMn2-xO4Synthesis]
Li1 + xMn2-xO4For the synthesis of Li as a starting material2CO3And electrolytic manganese dioxide (EMD).
[0062]
As a pre-stage of mixing the above starting materials, Li having improved reactivity and target particle size1 + xMn2-xO4Li for the purpose of obtaining2CO3Were crushed and classified by EMD. Li1 + xMn2-xO4When used as a positive electrode active material for a battery, a weight average particle diameter of 5 to 30 μm is preferable because of ensuring uniformity of charge / discharge reaction, ease of slurry preparation, safety, etc.1 + xMn2-xO4The same particle size as 5 to 30 μm.
[0063]
On the other hand, Li2CO3Since a particle size of 5 μm or less is desirable for ensuring a uniform reaction, pulverization was performed so that the D50 particle size, which is 50% of the particle size accumulation curve, was 1.4 μm.
[0064]
Thus, EMD and Li aligned to a predetermined particle size2CO3Were mixed so that it might become a predetermined ratio of the range of 0 <= x <= 0.20.
[0065]
The mixed powder was primarily fired at 600 to 800 ° C. in an atmosphere of dry air flow, and then secondarily fired at 600 to 800 ° C. in an atmosphere of dry air flow.
[0066]
The resulting Li1 + xMn2-xO4Fine particles having a particle size of 1 μm or less were removed by an air classifier. At this time, the obtained Li1 + xMn2-xO4Specific surface area of 0.6-0.9m2/ G.
[0067]
The tap density was 2.14 to 2.22 g / cc, the true density was 3.98 to 4.13 g / cc, and the D50 particle size was 11 to 16 μm.
[0068]
The obtained Li1 + xMn2-xO4Corresponds to the value of excess Li amount x (calculated from the charged composition ratio), x = 0.0 is A-1, x = 0.03 is A-2, x = 0.06 is A-3, x = 0.08 is A-4, x = 0.10 is A-5, x = 0.15 is A-6, x = 0.18 is A-7, x = 0.20 is A-8 write.
[0069]
[LiMn1.5Ni0.5O4Covered by]
LiMn1.5Ni0.5O4For the synthesis of Li as a starting material2CO3And (Mn0.75Ni0.25)3O4Was used.
[0070]
In order to ensure the uniformity of mixing, the above starting materials were weighed at a predetermined ratio and mixed for 24 hours in a wet ball mill. Thereafter, firing for 8 hours at 750 ° C. in an oxygen flow atmosphere was repeated twice.
[0071]
The obtained powder was confirmed by powder X-ray diffraction to be a single phase having a cubic spinel structure, and the lattice constant was 8.177%. It was also confirmed by chemical analysis that the ratio of Mn and Ni was approximately 3: 1. Furthermore, when the Mn valence of this powder was analyzed, it was calculated to be 3.99.
[0072]
Next, the obtained powder was pulverized for 48 hours by a wet planetary ball mill, and LiMn having a D50 particle size of 0.7 μm.1.5Ni0.5O4Got.
[0073]
Subsequently, Li as the central material1 + xMn2-xO4And LiMn obtained through the above steps1.5Ni0.5O4Is mixed in ethanol at a weight ratio of 95: 5 to give LiMn around the core material.1.5Ni0.5O4Was attached to form composite particles, and dried at 100 ° C. for 12 hours. Thereafter, heat treatment was performed at 500 ° C. in an environment of a dry air flow atmosphere. In addition, by X-ray powder diffraction, the center material and LiMn1.5Ni0.5O4It was confirmed that the lattice constant of each of the fine particles was not changed. As a result, the central material and LiMn1.5Ni0.5O4It was found that they did not react or dissolve with the fine particles. Further, since Ni was detected only on the surface of the composite particles by energy dispersive X-ray spectroscopy (EDX) analysis, LiMn1.5Ni0.5O4It was thought that the fine particles adhered to the surface of the central material.
[0074]
As an example, x = 0.10 lithium manganate, ie Li1.10Mn1.90O4FIG. 1 shows a powder X-ray diffraction pattern of a powder in the case of using a battery, and FIG. 2 shows a charge curve of a coin-type battery made using metal Li as a negative electrode.
[0075]
In FIG. 1, the peak marked with a triangle is LiMn.1.5Ni0.5O4The other peaks are Li and the rest are Li1.10Mn1.90O4Is the peak. Li1.10Mn1.90O4LiMn on the high angle side of the strong peak due to1.5Ni0.5O4It can be seen that a weak peak is observed. Further, from FIG. 2, the presence of a plateau was confirmed in the region of 4.5 V or more at the metal Li counter electrode potential.
[0076]
[Comparative Example 1]
Eight types of 18650 cylindrical batteries were produced using A-1 to A-8 as the positive electrode active materials.
[0077]
The positive electrode is produced with a mixing ratio of active material: carbon black: PVdF = 90: 6: 4 (wt%), and the negative electrode is mixed with a mixture ratio of hard carbon: carbon black: PVdF = 90: 2: 8 (wt%). Produced.
[0078]
The electrolyte is 1M LiPF6Was a supporting salt, and a mixed solution (30: 70 / volume%) of propylene carbonate (PC) and ethyl methyl carbonate (EMC) was used as a solvent.
[0079]
The eight types of batteries are referred to as B-1 to B-8, corresponding to the positive electrodes A-1 to A-8 used.
[0080]
[Example 1]
As positive electrode active materials, each of A-1 to A-8 is used as a central material, and LiMn is obtained by the above method.1.5Ni0.5O4Eight types of 18650 cylindrical batteries were produced in the same manner as in Comparative Example 1 except that a composite material in which was attached to the surface of the powder of the central material was used.
[0081]
The obtained 18650 cylindrical battery is referred to as C-1 to C-8, respectively, corresponding to the central materials A-1 to A-8 of the active material used.
[0082]
[Evaluation Test Example 1]
Comparative Example 1 andReference example aThe capacity storage characteristics were evaluated using the 18650 cylindrical battery produced in the above.
[0083]
First, each cylindrical battery was charged and discharged once at room temperature. The charge current and discharge current at this time were both 200 mA, and the discharge capacity at this time was defined as the initial capacity. In all the batteries, the cut-off potential on the discharge side was 2.5V, and the cut-off potential on the charge side was 4.3V.
[0084]
Thereafter, each battery was charged at 200 mA to 4.3 V, and further charged for 3 hours at a constant potential, and then left in a constant temperature bath at 50 ° C. for 4 weeks. After leaving, the discharge operation was performed again at room temperature at 200 mA, and the charge / discharge operation was repeated again at 200 mA, and the discharge capacity at that time was defined as the recovery capacity.
[0085]
[Evaluation Test Example 2]
Similarly Comparative Example 1 andReference example aA charge / discharge cycle test was performed using the 18650 cylindrical battery manufactured in the above.
[0086]
The cycle evaluation was performed by repeating the operation of charging to 4.3 V at 500 mA, then performing constant potential charging for 2 hours, and discharging to 2.5 V at 500 mA. The test was conducted at a temperature of 50 ° C.
[0087]
Table 1 shows the center material of the positive electrode active material of each battery, the excess Li composition x of the center material, the ratio of the coating material and the lattice constant a1 of the coating material to the lattice constant a2 of the center material, the recovery capacity after standing at 50 ° C. for 4 weeks. Rate (= 100 × [recovery capacity] / [initial capacity]), capacity retention rate after 300 cycles at 50 ° C. (= 100 × [discharge capacity at 300th cycle] / [discharge capacity at 10th cycle]) . In comparison with B-1 to B-8 batteries of Comparative Example 1,Reference example aIn each of the batteries C-1 to C-8, it was found that both the recovery capacity ratio after standing at 50 ° C. for 4 weeks and the capacity maintenance ratio after 50 cycles at 300 ° C. were improved. C-1 to C-8 batteries exhibited good characteristics in which both the recovery capacity ratio and the capacity retention ratio were 80% or more. The a1 / a2 ratio at this time was 0.9933 or more.
[0088]
[Table 1]
That is, LiMn having a Mn valence of 3.99 and a charge / discharge capacity of 4.5 V or more at the metal Li counter electrode potential1.5Ni0.5O4Li partially coated with Li1 + xMn2-xO4It has been clarified that its capacity storage characteristics and cycle characteristics are greatly improved. At this time, LiMn1.5Ni0.5O4Lattice constant a1 and Li1 + xMn2-xO4It has been found that the relationship of the lattice constant a2 is particularly preferably a1 / a2 ≧ 0.9933.
[0090]
[Example 2]
Subsequently, LiMn1.5Ni0.5O4LiMn in which a part of the Mn site was substituted with Ti1.4Ti0.1Ni0.5O4Was synthesized. As a starting material, Li2CO3And (Mn0.7Ti0.05Ni0.25)3O4LiMn, except that1.5Ni0.5O4The same procedure was followed by pulverization.
[0091]
LiMn obtained1.4Ti0.1Ni0.5O4Was confirmed to be a single phase by powder X-ray diffraction, and its Mn valence was calculated to be 4.00. The D50 particle size was 0.8 μm.
[0092]
Subsequently, A-5 (Li1.1Mn1.9O4) And LiMn obtained through the above process1.4Ti0.1Ni0.5O4Was mixed in ethanol at a weight ratio of 95: 5, dried at 100 ° C. for 12 hours, and then heat-treated at 400 ° C. in a dry air flow atmosphere.
[0093]
A 18650 cylindrical battery was produced in the same manner as in Comparative Example 1 using the composite material thus obtained as the positive electrode. This battery is designated as D-1.
[0094]
[Example 3]
Similarly, LiMn1.5Ni0.5O4LiMn in which a part of the Mn site was replaced with Si1.4Si0.1Ni0.5O4Was synthesized. As a starting material, Li2CO3And (Mn0.75Ni0.25)3O4LiMn, except that SiO is used1.5Ni0.5O4The same procedure was followed by pulverization.
[0095]
LiMn obtained1.4Si0.1Ni0.5O4Was confirmed to be a single phase by powder X-ray diffraction, and its Mn valence was calculated to be 3.97. The D50 particle size was 0.6 μm.
[0096]
Subsequently, A-5 (Li1.1Mn1.9O4) And LiMn obtained through the above process1.4Si0.1Ni0.5O4Was mixed in ethanol at a weight ratio of 95: 5, dried at 100 ° C. for 12 hours, and then heat-treated at 550 ° C. in an environment of a dry air flow atmosphere.
[0097]
A 18650 cylindrical battery was produced in the same manner as in Comparative Example 1 using the composite material thus obtained as the positive electrode. This battery is referred to as D-2.
[0098]
[Reference example b]
The above is a partial coating with a fine powdery coating material, but an overall coating via a liquid phase was also attempted. Li ethoxide is used as the Li raw material, manganese acetate hydrate is used as the Mn raw material, nickel acetylacetonate is used as the Ni raw material, and the metal ratio thereof is 1: 1.5: 0.5 (molar ratio). Then, the solution was reacted in 2-ethoxyethanol (under a nitrogen atmosphere) and stabilized with diethylamine.
[0099]
When the material obtained by drying and solidifying this solution was calcined at 600 ° C. in an oxygen flow atmosphere, each peak was slightly broad in powder X-ray diffraction, but LiMn1.5Ni0.5O4A single phase was obtained. The Mn valence of this material was calculated to be 3.98.
[0100]
Subsequently, a solution similar to the above was prepared, and the solution and the central material A-5 (Li1.1Mn1.9O4) Was soaked and mixed at a volume ratio of 1: 1. The slurry was dried at 150 ° C. and then fired at 600 ° C. in an oxygen flow atmosphere. The sample after repeating this “slurry preparation and subsequent firing” three times was analyzed by powder X-ray diffraction.1.1Mn1.9O4And LiMn1.5Ni0.5O4The two spinel phase peaks were observed.
[0101]
A 18650 cylindrical battery was produced in the same manner as in Comparative Example 1 using the composite material obtained by the above method as the positive electrode active material. This battery is designated E-1.
[Reference example c]
Reference example bA composite material was prepared in the same manner as described above. However, the “slurry preparation and subsequent firing” step was repeated 6 times. That is, the coating thickness isReference example bThis was estimated to be about twice that of the composite material. Using this composite material, a 18650 cylindrical battery was produced in the same manner as in Comparative Example 1. This battery is designated E-2.
[0102]
[Comparative Example 2]
A 18650 cylindrical battery was produced in the same manner as B-5 of Comparative Example 1 except that the negative electrode active material was graphite. This battery is designated as F-1.
[0103]
[Reference example d]
LiNi with a rock salt layer structure as a secondary active material of the positive electrode0.8Co0.2O2We also examined the system to which. This LiNi0.8Co0.2O2Has the effect of suppressing acid formation in the electrolyte.
[0104]
The positive electrode is connected to [A-5 (Li1.1Mn1.9O4) LiMn1.5Ni0.5O4Partially coated with: LiNi0.8Co0.2O2: 18650 cylindrical battery was produced in the same manner as Comparative Example 2 except that it was produced at a mixing ratio of: carbon black: PVdF = 80: 10: 6: 4 (% by weight) (Reference example aLiNi on the positive electrode of C-50.8Co0.2O2And the negative electrode active material was changed to graphite). This battery is designated as F-2.
[0105]
[Reference example e]
Except that the negative electrode active material is hard carbon,Reference example dIn the same manner, a 18650 cylindrical battery was produced. This battery is referred to as G-1.
[0106]
[Evaluation Test Example 3]
D-1 of Example 2, D-2 of Example 3,Reference example bE-1,Reference example cE-2, F-1 of Comparative Example 2,Reference example dF-2,Reference example eFor each battery of G-1, the recovery capacity ratio after storage at 50 ° C. for 4 weeks and the capacity maintenance ratio after 50 cycles at 300 ° C. were measured in the same manner as in Comparative Evaluation Examples 1 and 2.
[0107]
In addition to the batteries of B-5 of Comparative Example 1 in addition to the batteries described above, the ratio of the lattice constant a1 of the covering material to the lattice constant a2 of the center material for a total of 8 types of batteries, recovery capacity after storage at 50 ° C. for 4 weeks Table 2 shows the capacity retention rate after 300 cycles at 50 ° C.
[0108]
[Table 2]
From the comparison of B-5 with D-1 and D-2, LiMn1.5Ni0.5O4Even if the Mn site of the bismuth was replaced with tetravalent other cations, the effect of coating was obtained, and both the recovery capacity ratio and capacity retention ratio were excellent.
[0110]
Moreover, it became clear from the comparison between B-5, E-1, and E-2 that the same effect can be obtained when the entire surface of the central material is thinly coated via the liquid phase.
[0111]
Furthermore, from the comparison of F-1, F-2 and G-1, Li1 + xMn2-xO4LiMn1.5Ni0.5O4LiNi as a secondary active material0.8CO0.2O2In particular, it has been found that it is more preferable to combine hard carbon as the negative electrode active material.
[0112]
The following is a summary of what has become clear from the results in Tables 1 and 2.
(1) Li which is a 4V class positive electrode active material1 + xMn2-xO4The surface of LiMn1.5Ni0.5O45V class active material (a material whose Mn valence is substantially tetravalent and has a charge / discharge capacity of 4.5 V or more at a metal Li counter electrode potential), capacity storage characteristics and charge / discharge cycle characteristics are obtained. Greatly improved.
(2) The entire surface is preferable, but partial coverage is also effective.
(3) It is effective to substitute a part of the Mn site of the 5V class active material with another tetravalent cation.
(4) The ratio of the lattice constant a1 of the covering material and the lattice constant a2 of the central material is preferably a1 / a2> 0.993.
(5) LiNi as a secondary active material in the positive electrode0.8Co0.2O2It is desirable to include.
(6) The negative electrode active material is preferably hard carbon.
[0113]
【The invention's effect】
According to the present invention, in a secondary battery using spinel type lithium manganate as a positive electrode active material, by reducing Mn elution from the positive electrode and suppressing side reactions with the electrolyte, high temperature cycle characteristics and storage characteristics can be obtained. It is possible to provide a secondary battery excellent in.
[Brief description of the drawings]
FIG. 1 is a powder X-ray diffraction pattern of a positive electrode active material of a battery according to an example of the present invention.
FIG. 2 is an initial charge curve of a battery according to an embodiment of the present invention.
FIG. 3 is a diagram showing a cross-sectional structure of a secondary battery according to the present invention.
[Explanation of symbols]
1 Positive electrode active material layer
2 Negative electrode active material layer
3 Positive current collector
4 Negative electrode current collector
5 Porous separator
6 Positive electrode outer can
7 Negative electrode outer can
8 Insulation packing
Claims (7)
該正極活物質層は、リチウムイオンを吸蔵・放出可能であり、正極活性物質としてＬｉ １＋ｘ Ｍｎ ２−ｘ Ｏ ４ からなる微粒子を含み、
該微粒子の表面が、ＬｉＭｎ １．４ Ｔｉ ０．１ Ｎｉ ０．５ Ｏ ４ またはＬｉＭｎ １．４ Ｓｉ ０．１ Ｎｉ ０．５ Ｏ ４ で表されるスピネル型リチウムマンガン複合酸化物によって被覆されていることを特徴とする二次電池用正極。A positive electrode for a secondary battery comprising a current collector and a positive electrode active material layer provided on the current collector,
The positive electrode active material layer is capable of inserting and extracting lithium ions, and includes fine particles made of Li 1 + x Mn 2-x O 4 as a positive electrode active material ,
Surface of the fine particles, are covered by LiMn 1.4 Ti 0.1 Ni 0.5 O 4 or LiMn 1.4 Si 0.1 Ni 0.5 O spinel-type lithium manganese complex oxide represented by 4 A positive electrode for a secondary battery.
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| KR100856638B1 (en) | 2004-03-30 | 2008-09-03 | 마쯔시다덴기산교 가부시키가이샤 | Lithium ion secondary battery and charge/discharge controlling system thereof |
| JP2006012426A (en) * | 2004-06-22 | 2006-01-12 | Nichia Chem Ind Ltd | Positive electrode active material for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery |
| FR2879822B1 (en) * | 2004-12-21 | 2009-05-22 | Commissariat Energie Atomique | OPTIMIZED POSITIVE ELECTRODE MATERIAL FOR LITHIUM BATTERIES, PROCESS FOR PRODUCING THE SAME, ELECTRODE, BATTERY AND BATTERY USING THE MATERIAL |
| JP4954481B2 (en) * | 2005-02-24 | 2012-06-13 | 日本碍子株式会社 | Lithium secondary battery |
| FR2890241B1 (en) * | 2005-08-25 | 2009-05-22 | Commissariat Energie Atomique | HIGH SPEED SPINELLE STRUCTURE POSITIVE ELECTRODE MATERIAL BASED ON NICKEL AND MANGANESE FOR LITHIUM ACCUMULATORS |
| JP5224081B2 (en) * | 2006-04-19 | 2013-07-03 | 株式会社Ｇｓユアサ | Nonaqueous electrolyte secondary battery |
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| JP5199844B2 (en) * | 2008-11-21 | 2013-05-15 | 株式会社日立製作所 | Lithium secondary battery |
| JP6008578B2 (en) * | 2012-05-24 | 2016-10-19 | 日揮触媒化成株式会社 | Method for producing positive electrode active material for secondary battery and secondary battery |
| KR101497190B1 (en) * | 2012-10-18 | 2015-02-27 | 삼성정밀화학 주식회사 | Lithium metal oxide composite for lithium secondary battery, method for preparing thereof, and lithium secondary battery including the same |
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| US12308421B2 (en) | 2016-09-12 | 2025-05-20 | Semiconductor Energy Laboratory Co., Ltd. | Electrode and power storage device comprising graphene compound |
| CN115188931A (en) | 2016-10-12 | 2022-10-14 | 株式会社半导体能源研究所 | Positive electrode active material particle and method for producing positive electrode active material particle |
| CN110546794A (en) | 2017-05-12 | 2019-12-06 | 株式会社半导体能源研究所 | Positive electrode active material particles |
| KR20240023214A (en) | 2017-05-19 | 2024-02-20 | 가부시키가이샤 한도오따이 에네루기 켄큐쇼 | Positive electrode active material, method for manufacturing positive electrode active material, and secondary battery |
| CN112201844A (en) | 2017-06-26 | 2021-01-08 | 株式会社半导体能源研究所 | Manufacturing method of positive electrode active material and secondary battery |
| WO2019044770A1 (en) * | 2017-08-30 | 2019-03-07 | 株式会社村田製作所 | Positive electrode active material, positive electrode, battery, battery pack, electronic device, electric vehicle, power storage device, and electric power system |
| JP7809044B2 (en) * | 2022-05-30 | 2026-01-30 | 住友化学株式会社 | Alkali metal-containing oxide, positive electrode active material, electrode and battery |
| KR20250013156A (en) * | 2022-05-30 | 2025-01-31 | 스미또모 가가꾸 가부시키가이샤 | Alkali metal containing oxide, positive electrode active material, electrode and battery |
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