JP4626105B2 - Lithium ion secondary battery - Google Patents
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- JP4626105B2 JP4626105B2 JP2001234822A JP2001234822A JP4626105B2 JP 4626105 B2 JP4626105 B2 JP 4626105B2 JP 2001234822 A JP2001234822 A JP 2001234822A JP 2001234822 A JP2001234822 A JP 2001234822A JP 4626105 B2 JP4626105 B2 JP 4626105B2
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- 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
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- Y02E60/10—Energy storage using batteries
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に関し、より詳しくは、出力密度が向上したリチウムイオン二次電池に関する。
【０００２】
【従来の技術】
各種電子機器および電動機器の電源として、長時間連続して使用でき、再充電可能な各種二次電池の研究が進められてきた。中でも、ニッケル・カドミウム蓄電池やニッケル・水素蓄電池など民生品として適用されている二次電池と比較して、高エネルギー密度、高出力密度の実現が可能である等の特性を有するリチウムイオン二次電池は、活発な研究開発がなされ、携帯電話、カムコーダー、ノート型パソコン等の携帯用電子機器の電源として実用化されている。
【０００３】
また、地球環境汚染および地球温暖化の問題に適応するものとして電気自動車やハイブリッド自動車への関心が高まっており、これらの動力源としてリチウムイオン二次電池の適用が期待されている。自動車などへの適用にあたって、リチウムイオン二次電池は、高出力密度を得るために電池を複数個直列に結合して組電池を形成したときの制御が容易であり、安定性に優れるといった利点も有している。
【０００４】
リチウムイオン二次電池における重要な特性としてはエネルギー密度、出力密度、サイクル特性等があり、特開平１１−３１４９８号公報、特開平１１−２９７３５４号公報、特開平１１−３２９４０９号公報にはリチウムイオン二次電池のこれらの特性を改善する技術が開示されている。
【０００５】
【発明が解決しようとする課題】
特開平１１−３１４９８号公報には、電極の活物質の比表面積および空隙率を調整することにより容量およびサイクル特性を向上させる技術が開示されている。しかしながら、電極の活物質の比表面積および空隙率の関係について考察されいるだけで、活物質粒径、電極厚み、空隙率の相互作用については十分な考察がされていなかった。このため、電極厚みや粒径の条件によっては十分な出力密度が得られず、また、電極の活物質の比表面積および空隙率のみの調整では二次電池の性能向上に限界があった。
【０００６】
特開平１１−２９７３５４号公報には電解質濃度を規定する技術が開示されているが、活物質粒径や電極厚みと電解質濃度との相関関係が記載されていなかった。このため、活物質粒径や電極厚みの条件によっては電解質濃度を大きくしても電解液のリチウムイオン伝導度が低下するため効果的に出力密度を向上させることができなかった。
【０００７】
また、特開平１１−３２９４０９号公報には、電極の活物質の塗布厚さと活物質の粒径とを規定することによりリチウムイオン二次電池の出力密度を向上させる技術が開示されている。しかしながら、高出力密度を重視する構成であるため、エネルギー密度が低下する問題点があった。
【０００８】
本発明は、このような従来の種々の問題を勘案し、鋭意検討することにより完成されたものであり、出力密度が向上したリチウムイオン二次電池を提供することを目的とするものである。
【０００９】
【課題を解決するための手段】
上記目的を達成するための本発明は、リチウムイオンの吸蔵放出が可能な正極と、リチウムイオンの吸蔵放出が可能な負極と、リチウムイオン伝導性の非水電解液とを含み、活物質の粒径が５μｍ以下であり、活物質層の厚さが２０〜８０μｍであり、前記非水電解液の電解質濃度が１．５〜２．５ｍｏｌ／ｌであることを特徴とするリチウムイオン二次電池である。
【００２１】
【発明の効果】
以上のように構成された本発明によれば、活物質の粒径および活物質層の厚さを所定の範囲に規定し、かつ非水電解液の電解質濃度を所定の範囲に規定することにより、出力密度を向上させることができる。
【００２９】
【発明の実施の形態】
まず、本発明のリチウムイオン二次電池の一般的な形態について説明する。
【００３０】
リチウムイオン二次電池は、リチウムイオンの吸蔵放出が可能な材料からなる正極および負極と、リチウムイオン伝導性のある非水電解質とを含む、充放電可能な電池であり、正極および負極は直接接触してショートしないようにセパレーターで分離される。正極および負極は、通常は正極集電体および負極集電体の両面に正極活物質および負極活物質を塗布形成することによって作製され、正極−セパレーター−負極−セパレーターの順に幾層にも積層した構造や、この順序に積層されたシートを渦巻き状に巻き取ったいわゆるジェリーロールタイプなどの電極素子構造をとることができる。
【００３１】
正極活物質としては、リチウム金属酸化物、リチウム金属酸化物の一部を他の元素で置換した複合酸化物、マンガン酸化物など各種公知の正極活物質を適宜使用することができる。具体的には、リチウム金属酸化物としてはＬｉＣｏＯ2、ＬｉＮｉＯ2、ＬｉＭｎＯ2、ＬｉＭｎ2Ｏ4、ＬｉXＦｅＯY、ＬｉXＶYＯZ等が挙げられ、リチウム金属酸化物の一部を他の元素で置換した複合酸化物としてはＬｉXＣｏYＭZＯ2（ＭはＭｎ、Ｎｉ、Ｖなど）やＬｉXＭｎYＭZＯ2（ＭはＬｉ、Ｎｉ、Ｃｒ、Ｆｅ、Ｃｏなど）等が挙げられ、マンガン酸化物としてはλ−ＭｎＯ2、ＭｎＯ2とＶ2Ｏ5の複合体、三成分複合酸化物であるＭｎＯ2・ｘＶ2Ｏ5（０＜ｘ≦０．３）等が挙げられる。
【００３２】
負極活物質としては、ハードカーボン、ソフトカーボン、グラファイト、活性炭などの炭素材料、ＳｎＢXＰYＯZ、Ｎｂ2Ｏ5、ＬｉＴｉXＯY、ＬｉＦｅXＮY、ＬｉＭｎXＮYなどの金属酸化物などを単独または混合して使用できる。ここで、ハードカーボンとは３０００℃で熱処理しても黒鉛化しない炭素材料をいい、ソフトカーボンとは２８００〜３０００℃で熱処理した際に黒鉛化する炭素材料をいう。なお、ハードカーボンの製造には、フラン樹脂、０．６〜０．８のＨ／Ｃ原子比を有する石油ピッチに酸素架橋した有機材料などを出発原料とする方法など各種公知の技術を用いることができ、ソフトカーボンの製造には、石炭、高分子化合物（ポリ塩化ビニル樹脂、ポリビニルアセテート、ポリビニルブチラートなど）、ピッチ等を出発原料とする方法など各種公知の技術を用いることができる。
【００３３】
上記正極活物質および負極活物質を正極集電体および負極集電体上に形成して正極および負極を作製する際にも各種公知の技術を使用できる。例えば、正極を製造する際には、正極活物質を溶媒中でバインダーと混合してペースト状にし、このペーストを正極集電体にコーティングし、乾燥する方法を用いることができる。負極も同様に、負極活物質を溶媒中でバインダーと混合してペースト状にし、このペーストを負極集電体にコーティングし、乾燥する方法を用いることができ、通常集電体の両面にコーティングが施される。なお、カーボンブラック、グラファイト、アセチレンブラック等の導電剤をペースト中に加えてもよい。活物質とバインダーとの混合割合は、電極の形状に合わせて適宜決定することが好ましく、コーティングには各種公知の方法を用いることができる。
【００３４】
集電体は、リチウムイオン二次電池に使用されている各種公知の材料を用いることができ、具体的には、正極集電体としてはアルミニウム箔などが、負極集電体としては銅箔などが挙げられる。
【００３５】
バインダーとしては、ポリフッ化ビニリデン（ＰＶＤＦ）、ポリテトラフルオロエチレンなどを挙げることができ、溶媒としてはバインダーを溶解させる各種極性溶媒が使用できる。具体的には、ジメチルホルムアミド、ジメチルアセトアミド、メチルホルムアミド、Ｎ−メチルピロリドン（ＮＭＰ）などが挙げられる。なお、バインダーとしてポリフッ化ビニリデンを使用した場合はＮ−メチルピロリドンを用いることが好ましい。
【００３６】
非水電解液としては、リチウムイオン伝導性のある各種溶液が好ましく、エチレンカーボネート（ＥＣ）、プロピレンカーボネート（ＰＣ）、ブチレンカーボネート（ＢＣ）等の環状炭酸エステルを単体または適宜組み合わせて使用することができる。また、電気伝導度を高くし、かつ適切な粘度を有する電解液を得るため、ジメチルカーボネート（ＤＭＣ）、ジエチルカーボネート（ＤＥＣ）、γ−ブチルラクトン、γ−バレロラクトン、酢酸エチル、プロピオン酸メチル等を併用してもよい。
【００３７】
非水電解液中の電解質としては、ＬｉＰＦ6、ＬｉＢＦ4、ＬｉＣｌＯ4、ＬｉＡｓＦ6、ＬｉＣＦ3ＳＯ3などが挙げられる。
【００３８】
セパレーターとしては、ポリエチレン、ポリプロピレンなどのポリオレフィン系樹脂の微多孔膜などを使用できる。
【００３９】
本発明に係る二次電池を製造する際には上記した正極、負極、非水電解液、セパレーターを適宜組み合わせて製造することができる。また、電池缶、電池形状などについても、公知の各種材質、形状を適用することができる。
【００４０】
以下、本願に係る発明について詳細に説明する。
【００４１】
本願の第１の発明は、リチウムイオンの吸蔵放出が可能な正極と、リチウムイオンの吸蔵放出が可能な負極と、リチウムイオン伝導性の非水電解液とを含むリチウムイオン二次電池において、活物質の粒径を５μｍ以下、より好ましくは１μｍ以下、活物質層の厚さを２０〜８０μｍ、より好ましくは２０〜３０μｍに規定するものである。なお、本発明において活物質層とは集電体上に形成されてなる活物質を含む層をいい、例えば上述したように溶媒中で活物質、バインダー等の各種組成物を混合することにより作製されたペーストを集電体表面にコーティングし、乾燥することにより形成された層をいい、通常の方法に従って集電体の両面に活物質層が形成されたときは、両活物質層それぞれについて本発明の規定を適用することが好ましい。また、本発明において粒径とは平均粒径を指すものである。
【００４２】
活物質の粒径が大きいと、大電流放電時は膜厚方向の電極内電解液中のリチウムイオンの輸送よりも活物質粒子内のリチウムイオン拡散が律速段階となってしまい、出力密度低下の原因となる。従って、活物質の粒径は５μｍ以下であることが好ましい。なお、活物質の粒径の下限値は特に規定するものではないが、実際的には０．１μｍ以上であることが適当である。また、膜厚が２０μｍより小さいと活物質量の不足により出力密度が小さくなり好ましくなく、膜厚が８０μｍを超えると、内部抵抗が増し出力密度が小さくなるため好ましくない。
【００４３】
上述した、活物質の粒径が５μｍ以下といった活物質粒径が小さい条件下では、大電流放電時は膜厚方向の電極内電解液中のリチウムイオンの輸送が律速段階になっていると考えられる。よって空隙率を大きくすると電極内の電解液量が増え、膜厚方向の電極内電解液中のリチウムイオンの輸送力が増し、より出力密度を向上させることができる。しかし、空隙率が５０％未満だと活物質の量に対応した電解液量が確保できないため、抵抗が増大し出力密度が低下する。このため空隙率は５０％以上であることが好ましい。また、空隙率が６０％を超えると活物質量の不足、すなわち電極表面積の減少により出力密度が徐々に低下していくため、空隙率は５０〜６０％であることがより好ましい。
【００４４】
逆に、活物質粒径が５μｍよりも大きい条件下では、活物質粒子内のリチウムイオン拡散が律速段階となっているため、空隙率を増加させる、すなわち電極内電解液量を増加させても出力密度の向上は図れず、活物質の量が減少するため却って出力密度が低下する。
【００４５】
このように空隙率を規定することによる出力密度の向上を図るためには、活物質層の膜厚は２０μｍ以上あることが好ましい。これは、膜厚が薄いと膜厚方向の電極内電解液中のリチウムイオン輸送の影響が小さく、空隙率の影響が小さくなるためである。
【００４６】
また、活物質層は空隙率の異なる２層の活物質層が積層された構造とすることもできる。空隙率の異なる２層構造とすることにより、エネルギー密度を犠牲にせずに、出力密度を向上させることができる。具体的には、セパレーター側の活物質層の空隙率を大きくし、集電体側の活物質層の空隙率を小さくすることが好ましい。セパレーター付近の活物質層の空隙率を大きくすることにより、セパレーター付近の電解液量を増加することができ、リチウムイオンの輸送力を高めることができる。また、集電体付近の空隙率を小さくすることにより、集電体付近の活物質の利用率を向上させることができる。このような特性を考慮して、活物質内拡散と電解液中輸送とのバランスをとることにより出力密度を効果的に向上されることが可能となる。また、エネルギー密度は、活物質層の平均空隙率および活物質量に影響されるため、適宜調整することによりエネルギー密度を犠牲にせずに出力密度を向上させることができる。例えば、１層構造（空隙率５０％、厚さ６０μｍ）の電極と、２層構造（集電体側の空隙率４０％、厚さ３０μｍ；セパレーター側の空隙率６０％、厚さ３０μｍ）の電極のエネルギー密度は等しくなる。これらの２電極の、平均空隙率および活物質量が等しいからである。
【００４７】
なお、空隙率の異なる２層の活物質層は、厚さがそれぞれ２０〜３０μｍが好ましく、２０〜２５μｍがより好ましく、２層の活物質層の厚さが異なっていてもよい。活物質層の厚さが３０μｍより大きいとエネルギー密度が小さくなる傾向があり、３０μｍ以下であると集電体側の活物質層の利用率が向上するからである。集電体側の活物質層の空隙率は、３０％以上、５０％未満が好ましく、４０％以上、５０％未満がより好ましい。セパレータ側の活物質層の空隙率は、５０〜６０％が好ましく、５０〜５５％がより好ましい。空隙率をこの範囲に調整することで、より大きな効果を得ることができる。
【００４８】
本願の第２の発明は、活物質粒径の異なる２層の活物質層からなることを特徴とするリチウムイオン二次電池である。このような構成により出力密度の向上が図れる。
【００４９】
大電流放電時は膜厚方向の電極内電解液中のリチウムイオンの輸送が律速段階になり、集電体付近の電極活物質が有効に利用できず、出力密度が低下する原因となる。この問題を解決するために、電極のセパレーター付近の活物質の粒径を大きくすることが好ましい。これによりセパレーター付近の電極表面積が減少し、セパレーター付近の活物質の利用率が低下し、リチウムイオンが集電体付近まで輸送されやすくすることができる。このため、集電体付近の電極活物質の利用率が向上し、トータルとしては、電極活物質の利用率が向上する、すなわち出力密度が向上する。また、１層構造の電池と２層構造の電池との全体での活物質量が等しくなるように適宜調節することによりエネルギー密度を犠牲にせずに出力密度を向上させることができる。
【００５０】
また、２層構造をとることによる効果は、電極のセパレーター付近の活物質の粒径を大きくし、セパレーター付近の活物質の利用率を低下させることによって発現するものである。したがって、セパレーター側の活物質層の厚さが必要以上に厚いとセパレーター付近の活物質の利用率が向上してしまい、本発明の効果が小さくなるので好ましくない。また集電体側の活物質層の厚さについても、必要以上に厚いと出力密度が低下するため好ましくない。このため、それぞれ活物質層の厚さは３０μｍ以下であることが好ましく、２５μｍ以下であることがより好ましい。活物質層の厚さの下限値は、エネルギー密度の低下を防ぐためそれぞれ２０μｍであることが好ましい。活物質層の厚さが２０μｍ未満であると、電池における集電体等の重量比率が大きくなるからである。
【００５１】
また、活物質粒径の異なる２層の活物質層は、集電体側の活物質層の活物質粒径が５μｍ以上またはセパレーター側の活物質層の活物質粒径が５μｍ未満であると、本発明の出力密度を向上させる効果が少なくなるので好ましくない。集電体側の活物質粒径の下限値は特に限られるものではないが、実際的には０．１μｍ以上であることが適当である。また、セパレーター側の活物質層の活物質粒径の上限は、活物質粒径が活物質層の厚さより大きくならない範囲で適宜選択することが好ましい。上記観点から、集電体側の活物質層の活物質粒径は０．１μｍ以上、５μｍ未満が好ましく、１μｍ以上、５μｍ未満がより好ましい。セパレータ側の活物質層の活物質粒径は５〜２０μｍが好ましく、５〜１０μｍがより好ましい。
【００５２】
なお、活物質の粒径は出発原料の粒径、または分級により調整することができ、空隙率は活物質と導電剤を含むペーストを集電体に塗布し、乾燥した後、プレスするときの圧力を変化させることにより調整できる。
【００５３】
上記説明においては、２層構造を例にとって説明したが、３層以上の多層構造においても活物質層の空隙率、活物質の粒径、および活物質層の厚さを調整することにより本発明の効果を得ることが可能である。
【００５４】
多層構造は、まず、１層目を集電体上に塗布形成し、その上に２層目を塗布形成する方法により順次ｎ層まで塗布形成することができる他、異なる大きさを持つ粒子の沈降速度の差異を利用して多層構造にすることも可能である。
【００５５】
なお、本発明の前記第１の発明および第２の発明においては、高い出力密度を得る観点から、正極活物質はリチウムマンガン酸化物であることが好ましい。マンガンはコバルトやニッケルに比べてはるかに安価であり、資源的にも豊富であるため製造コストの点からも好ましい。リチウムマンガン酸化物の具体例としては、ＬｉＭｎＯ2、ＬｉＭｎ2Ｏ4が挙げられる。
【００５６】
また、本発明のリチウムイオン二次電池において、より出力密度を向上させるためには、前記第１の発明および第２の発明において、非水電解液の電解質濃度が１．０〜３．０ｍｏｌ／ｌであることが好ましく、１．５〜２．５ｍｏｌ／ｌであることがより好ましい。このような範囲の電解質濃度を使用することにより活物質内拡散と電解液中輸送とのバランスをとることができ好適な出力密度が得られる。
【００５７】
活物質粒径が小さい、または空隙率が大きな条件下では大電流放電時は膜厚方向の電極内電解液中のリチウムイオンの輸送が律速段階になる。よって電解質濃度を大きくすると、濃度分極が抑えられ、膜厚方向の電極内電解液中のリチウムイオンの輸送力が増し、出力密度が向上する。電解質濃度が３．０ｍｏｌ／ｌを超えると電解液のリチウムイオン伝導度の影響があらわれ出力密度が低下するため好ましくなく、電解質濃度が１．０ｍｏｌ／ｌ未満であると、電池の内部抵抗が増加するため好ましくない。また、電解質濃度を１．５〜２．５ｍｏｌ／ｌの範囲に調製することにより、放電時の電圧を高くかつ安定させることができる。
【００５８】
活物質の粒径と電解質濃度との関係も出力密度に影響を及ぼしうる。すなわち、活物質の粒径が５μｍより大きい条件下では、大電流放電時は膜厚方向の電極内電解液中のリチウムイオンの輸送よりも、活物質粒子内のリチウムイオン拡散が律速段階になる。このため、電解液濃度の影響は少なく、電解質の濃度を上げても出力密度がそれほど向上しない。よって、活物質層が２層以上の多層構造であるときは、集電体側の活物質層の活物質粒径を５μｍ未満にすることが好ましい。
【００５９】
また、電解質は、電気伝導率の高い化合物を使用することにより出力密度を向上させる観点からＬｉＰＦ6、またはＬｉＢＦ4であることが好ましい。
【００６０】
なお、本発明においては活物質の粒径はふるい分け試験や沈降法などの各種粒度分布を測定する方法、空隙率は構成材料の比重、活物質層厚さはマイクロメーターによりそれぞれ測定することができる。
【００６１】
【実施例】
１．活物質粒径、活物質層厚さ、および空隙率の出力密度に及ぼす影響調査
＜参考例１＞
正極活物質として平均粒径３μｍのリチウムマンガン酸化物（ＬｉＭｎＯ２）を用いた。この活物質の比表面積は約３ｍ２／ｇであった。この粒径３μｍの活物質７５質量％と、導電剤としてアセチレンブラック１０質量％と、バインダーとしてＰＶＤＦ１５質量％とを、ＮＭＰ中で混合しアルミ箔（集電体）上に塗布し、活物質層厚さ６０μｍで活物質層の空隙率が異なる複数の正極を作製した。空隙率は溶剤の量、乾燥条件や、電極のプレスにより調整した。負極活物質には金属リチウムを用い、電解液には１ＭのＬｉＰＦ６を溶解させたＰＣとＤＭＣとの混合物（体積比１：１）を用いて正極の活物質層の空隙率が異なる複数のリチウムイオン二次電池を製造した。
【００６２】
＜比較例１＞
平均粒径が３０μｍである以外は参考例１と同様の方法により空隙率の異なる複数のリチウムイオン二次電池を作製した。
【００６３】
＜比較例２＞
活物質層厚さが１０μｍである以外は参考例１と同様の方法により空隙率の異なる複数のリチウムイオン二次電池を作製した。
【００６４】
図１に参考例１、比較例１および比較例２の空隙率と出力密度（空隙率４０％の出力密度を１としたときの相対値）との関係を示す。参考例１の電池の出力密度は空隙率５０〜６０％で最大になったのに対し、比較例１および比較例２の電池は空隙率が大きくなるほど出力密度は低下した。
【００６５】
２．活物質層を空隙率の異なる２層構造にすることの影響調査
＜参考例２＞
正極活物質として平均粒径３μｍのリチウムマンガン酸化物（ＬｉＭｎＯ２）を用いた。この活物質の比表面積は約３ｍ２／ｇであった。この粒径３μｍの活物質７５質量％と、導電剤としてアセチレンブラック１０質量％と、バインダーとしてＰＶＤＦ１５質量％とを、ＮＭＰ中で混合しアルミ箔（集電体）上に塗布し、活物質層の空隙率が４０％で活物質層厚さ３０μｍの正極を作製した。この正極に、同様の正極活物質組成物を塗布し、活物質層の空隙率が６０％で活物質層厚さ３０μｍの活物質層を積層した。空隙率は溶剤の量、乾燥条件や電極のプレスにより調整した。このようにして活物質層の厚さ６０μｍの正極を製造し、負極活物質には金属リチウムを用い、電解液には１ＭのＬｉＰＦ６を溶解させたＰＣとＤＭＣとの混合物（体積比１：１）を用いてリチウムイオン二次電池を製造した。
【００６６】
＜参考例３＞
正極活物質として平均粒径３μｍのリチウムマンガン酸化物（ＬｉＭｎＯ２）を用いた。この活物質の比表面積は約３ｍ２／ｇであった。この粒径３μｍの活物質７５質量％と、導電剤としてアセチレンブラック１０質量％と、バインダーとしてＰＶＤＦ１５質量％とを、ＮＭＰ中で混合しアルミ箔（集電体）上に塗布し、活物質層の空隙率が５０％で活物質層厚さ６０μｍの正極を作製した。空隙率は溶剤の量、乾燥条件や、電極のプレスにより調整した。負極活物質には金属リチウムを用い、電解液には１ＭのＬｉＰＦ６を溶解させたＰＣとＤＭＣとの混合物（体積比１：１）を用いてリチウムイオン二次電池を製造した。
【００６７】
＜参考例４＞
正極活物質として平均粒径３μｍのリチウムマンガン酸化物（ＬｉＭｎＯ２）を用いた。この活物質の比表面積は約３ｍ２／ｇであった。この粒径３μｍの活物質７５質量％と、導電剤としてアセチレンブラック１０質量％と、バインダーとしてＰＶＤＦ１５質量％とを、ＮＭＰ中で混合しアルミ箔（集電体）上に塗布し、活物質層の空隙率が４０％で活物質層厚さ２０μｍの正極を作製した。この正極に、同様の正極活物質組成物を塗布し、活物質層の空隙率が６０％で活物質層厚さ４０μｍの活物質層を積層した。空隙率は溶剤の量、乾燥条件や電極のプレスにより調整した。このようにして活物質層の厚さ６０μｍの正極を製造し、負極活物質には金属リチウムを用い、電解液には１ＭのＬｉＰＦ６を溶解させたＰＣとＤＭＣとの混合物（体積比１：１）を用いてリチウムイオン二次電池を製造した。
【００６８】
図２に参考例２〜４の電池の出力密度（参考例３の出力密度を１としたときの相対値）を示す。出力密度は活物質層を２層構造とすることにより向上した。また、２層構造とした場合は各活物質層の厚さを３０μｍ以下としたとき、より効果的に出力密度を向上させることができた。
【００６９】
また、図３に参考例２〜４の電池のエネルギー密度（参考例３のエネルギー密度を１としたときの相対値）を示す。参考例２と参考例３のエネルギー密度は等しかった。これは、参考例２と参考例３の平均空隙率が等しく、正極活物質量が等しいためである。参考例４は活物質層が１層構造である参考例３に比べて、出力密度は向上したが、エネルギー密度の点では劣った。
【００７０】
３．活物質層を活物質粒径の異なる２層構造にすることの影響調査
＜参考例５＞
正極活物質として平均粒径３μｍおよび９μｍの２種類のリチウムマンガン酸化物（ＬｉＭｎＯ２）を用いた。この平均粒径３μｍの活物質７５質量％と、導電剤としてアセチレンブラック１０質量％と、バインダーとしてＰＶＤＦ１５質量％とを、ＮＭＰ中で混合しアルミ箔（集電体）上に塗布し、活物質層厚さ３０μｍの正極を作製した。この正極に、平均粒径９μｍの活物質を用いて同様にして厚さ３０μｍの活物質層を積層した。このようにして活物質層の厚さ６０μｍの正極を製造し、負極活物質には金属リチウムを用い、電解液には１ＭのＬｉＰＦ６を溶解させたＰＣとＤＭＣとの混合物（体積比１：１）を用いてリチウムイオン二次電池を製造した。
【００７１】
＜参考例６＞
正極活物質として平均粒径３μｍのリチウムマンガン酸化物（ＬｉＭｎＯ２）を用いて、参考例５と同様の方法で活物質層厚さ６０μｍの正極を作製し、負極活物質には金属リチウムを用い、電解液には１ＭのＬｉＰＦ６を溶解させたＰＣとＤＭＣとの混合物（体積比１：１）を用いてリチウムイオン二次電池を製造した。
【００７２】
図４に参考例５および参考例６の出力密度（参考例６の出力密度を１としたときの相対値）を示す。粒径の異なる２層構造とした参考例５の方がより高い出力密度を示した。また、図５に参考例５および参考例６のエネルギー密度（参考例６のエネルギー密度を１としたときの相対値）を示す。参考例５および参考例６ではエネルギー密度が等しかった。これは、参考例５と参考例と６の正極活物質の量が等しいためである。
【００７３】
４．活物質粒径が異なる２層構造における活物質層厚さの影響調査
＜比較例３＞
正極活物質として平均粒径３μｍおよび９μｍの２種類のリチウムマンガン酸化物（ＬｉＭｎＯ2）を用いた。この平均粒径３μｍの活物質７５質量％と、導電剤としてアセチレンブラック１０質量％と、バインダーとしてＰＶＤＦ１５質量％とを、ＮＭＰ中で混合しアルミ箔（集電体）上に塗布し、活物質層厚さ２０μｍの正極を作製した。この正極に、平均粒径９μｍの活物質を用いて同様にして厚さ４０μｍの活物質層を積層した。このようにして活物質層の厚さ６０μｍの正極を製造し、負極活物質には金属リチウムを用い、電解液には１ＭのＬｉＰＦ6を溶解させたＰＣとＤＭＣとの混合物（体積比１：１）を用いてリチウムイオン二次電池を製造した。
【００７４】
図６に比較例３および前記参考例６の出力密度（比較例３の出力密度を１としたときの相対値）を示す。比較例３の出力密度が低いのはセパレーター側の活物質層の活物質粒径が大きいためセパレーター付近の活物質の利用率が低下し、かつセパレーター側の活物質層の厚さを大きくしたため集電体付近の活物質の利用率も低下したためである。また、図７に比較例３および前記参考例６のエネルギー密度（比較例３のエネルギー密度を１としたときの相対値）を示す。比較例３と参考例６とではエネルギー密度が等しかった。これは、比較例３と参考例６との正極活物質量が等しいためである。
【００７５】
５．活物質粒径および電解質濃度の出力密度への影響調査
＜実施例７＞
正極活物質として平均粒径３μｍのリチウムマンガン酸化物（ＬｉＭｎＯ2）を用いた。この粒径３μｍの活物質７５質量％と、導電剤としてアセチレンブラック１０質量％と、バインダーとしてＰＶＤＦ１５質量％とを、ＮＭＰ中で混合しアルミ箔（集電体）上に塗布し、活物質層厚さ６０μｍの正極を作製した。負極活物質には金属リチウムを用い、電解液には濃度の異なるＬｉＰＦ6を溶解させたＰＣとＤＭＣとの混合物（体積比１：１）を用いて複数のリチウムイオン二次電池を製造した。
【００７６】
＜比較例４＞
正極活物質として平均粒径３０μｍのリチウムマンガン酸化物（ＬｉＭｎＯ2）を用いた以外は実施例７と同様の方法でリチウムイオン二次電池を製造した。
【００７７】
図８に実施例７および比較例４の電解質濃度と出力密度（電解質濃度が１ｍｏｌ／ｌのときの出力密度を１としたときの相対値）との関係を示す。実施例７は電解質濃度２ｍｏｌ／ｌのとき出力密度が最大となった。これに対して、比較例４は電解質濃度を大きくしても出力密度の向上は見られなかった。
【００７８】
６．活物質層の厚さおよび電解質濃度の出力密度への影響調査
＜実施例８＞
正極活物質として平均粒径３μｍのリチウムマンガン酸化物（ＬｉＭｎＯ2）を用いた。この粒径３μｍの活物質７５質量％と、導電剤としてアセチレンブラック１０質量％と、バインダーとしてＰＶＤＦ１５質量％とを、ＮＭＰ中で混合しアルミ箔（集電体）上に塗布し、活物質層厚さの異なる正極を複数作製した。負極活物質には金属リチウムを用い、電解液には濃度２ｍｏｌ／ｌのＬｉＢＦ4を溶解させたＰＣとＤＭＣとの混合物（体積比１：１）を用いて正極の活物質層の厚さが異なる複数のリチウムイオン二次電池を製造した。
【００７９】
＜参考例９＞
電解液に１ｍｏｌ／ｌのＬｉＢＦ４を用いた以外は実施例８と同様の方法でリチウムイオン二次電池を製造した。
【００８０】
図９に実施例８および参考例９の活物質層厚さと出力密度（参考例９の活物質層厚さが１００μｍのときの出力密度を１としたときの相対値）との関係を示す。電解質および電解質濃度が好適な実施例８の出力密度が約２倍高かった。
【００８１】
上記実施例７および８、参考例１〜６および９、ならびに比較例１〜４の構成を表１〜６に示す。
【００８２】
【表１】
【表２】
【表３】
【表４】
【表５】
【表６】
【図面の簡単な説明】
【図１】 空隙率と出力密度との関係を示すグラフである。
【図２】 参考例と出力密度との関係を示すグラフである。
【図３】 参考例とエネルギー密度との関係を示すグラフである。
【図４】 参考例の出力密度を示すグラフである。
【図５】 参考例のエネルギー密度を示すグラフである。
【図６】 参考例および比較例の出力密度を示すグラフである。
【図７】 参考例および比較例のエネルギー密度を示すグラフである。
【図８】 電解質濃度と出力密度との関係を示すグラフである。
【図９】 活物質層厚さと出力密度との関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lithium ion secondary battery, and more particularly to a lithium ion secondary battery with improved power density.
[0002]
[Prior art]
Research has been conducted on various rechargeable batteries that can be used continuously for a long time as a power source for various electronic devices and electric devices. Above all, compared to secondary batteries that are applied as consumer products such as nickel-cadmium storage batteries and nickel-hydrogen storage batteries, lithium ion secondary batteries have characteristics such as high energy density and high output density. Has been actively researched and developed, and has been put into practical use as a power source for portable electronic devices such as mobile phones, camcorders, and notebook computers.
[0003]
In addition, interest in electric vehicles and hybrid vehicles is increasing as an adaptation to the problems of global environmental pollution and global warming, and lithium ion secondary batteries are expected to be applied as power sources for these vehicles. When applied to automobiles, etc., lithium ion secondary batteries have the advantage that they are easy to control and have excellent stability when multiple batteries are connected in series to form a battery pack in order to obtain high output density. Have.
[0004]
Important characteristics of the lithium ion secondary battery include energy density, output density, cycle characteristics, and the like. Japanese Patent Application Laid-Open Nos. 11-31498, 11-297354, and 11-329409 disclose lithium ions. Techniques for improving these characteristics of secondary batteries are disclosed.
[0005]
[Problems to be solved by the invention]
Japanese Patent Application Laid-Open No. 11-31498 discloses a technique for improving capacity and cycle characteristics by adjusting the specific surface area and porosity of an active material of an electrode. However, only the relationship between the specific surface area and the porosity of the active material of the electrode has been discussed, and the interaction between the active material particle size, the electrode thickness, and the porosity has not been sufficiently considered. For this reason, a sufficient output density cannot be obtained depending on the conditions of the electrode thickness and the particle diameter, and there is a limit to improving the performance of the secondary battery by adjusting only the specific surface area and porosity of the active material of the electrode.
[0006]
Japanese Patent Application Laid-Open No. 11-297354 discloses a technique for defining the electrolyte concentration, but does not describe the correlation between the active material particle size or electrode thickness and the electrolyte concentration. For this reason, depending on the conditions of the active material particle size and electrode thickness, even if the electrolyte concentration is increased, the lithium ion conductivity of the electrolytic solution is lowered, so that the output density cannot be effectively improved.
[0007]
Japanese Patent Application Laid-Open No. 11-329409 discloses a technique for improving the output density of a lithium ion secondary battery by defining the coating thickness of the active material of the electrode and the particle size of the active material. However, there is a problem that the energy density is lowered because the configuration emphasizes high output density.
[0008]
The present invention has been completed by taking into consideration various problems of the prior art and intensively studying, and an object of the present invention is to provide a lithium ion secondary battery with improved output density.
[0009]
[Means for Solving the Problems]
To achieve the above object, the present invention provides: A positive electrode capable of occluding and releasing lithium ions; a negative electrode capable of occluding and releasing lithium ions; and a lithium ion conductive non-aqueous electrolyte, wherein the active material has a particle size of 5 μm or less, A lithium ion secondary battery having a thickness of 20 to 80 μm and an electrolyte concentration of the non-aqueous electrolyte of 1.5 to 2.5 mol / l .
[0021]
【The invention's effect】
According to the present invention configured as described above, The output density can be improved by defining the particle size of the active material and the thickness of the active material layer within a predetermined range and the electrolyte concentration of the non-aqueous electrolyte within a predetermined range. .
[0029]
DETAILED DESCRIPTION OF THE INVENTION
First, the general form of the lithium ion secondary battery of this invention is demonstrated.
[0030]
A lithium ion secondary battery is a chargeable / dischargeable battery including a positive electrode and a negative electrode made of a material capable of occluding and releasing lithium ions, and a non-aqueous electrolyte having lithium ion conductivity. The positive electrode and the negative electrode are in direct contact with each other. Then, it is separated with a separator so as not to short-circuit. The positive electrode and the negative electrode are usually produced by coating and forming a positive electrode active material and a negative electrode active material on both sides of the positive electrode current collector and the negative electrode current collector, and laminated in layers of positive electrode-separator-negative electrode-separator in this order. A structure or an electrode element structure such as a so-called jelly roll type in which sheets stacked in this order are wound in a spiral shape can be employed.
[0031]
As the positive electrode active material, various known positive electrode active materials such as lithium metal oxides, composite oxides in which a part of the lithium metal oxide is substituted with other elements, and manganese oxides can be appropriately used. Specifically, as the lithium metal oxide, LiCoO 2 , LiNiO 2 LiMnO 2 , LiMn 2 O Four , Li X FeO Y , Li X V Y O Z As the composite oxide in which a part of the lithium metal oxide is substituted with another element, Li X Co Y M Z O 2 (M is Mn, Ni, V, etc.) or Li X Mn Y M Z O 2 (M is Li, Ni, Cr, Fe, Co, etc.) and the like, and the manganese oxide is λ-MnO. 2 , MnO 2 And V 2 O Five Composite, ternary composite oxide MnO 2 XV 2 O Five (0 <x ≦ 0.3).
[0032]
As the negative electrode active material, carbon materials such as hard carbon, soft carbon, graphite and activated carbon, SnB X P Y O Z , Nb 2 O Five , LiTi X O Y LiFe X N Y , LiMn X N Y These metal oxides can be used alone or in combination. Here, hard carbon refers to a carbon material that does not graphitize even when heat-treated at 3000 ° C., and soft carbon refers to a carbon material that graphitizes when heat-treated at 2800 to 3000 ° C. For the production of hard carbon, various known techniques such as a method using a furan resin, an organic material oxygen-crosslinked to a petroleum pitch having an H / C atomic ratio of 0.6 to 0.8, etc. as a starting material are used. Various known techniques such as a method using coal, polymer compounds (polyvinyl chloride resin, polyvinyl acetate, polyvinyl butyrate, etc.), pitch and the like as starting materials can be used for the production of soft carbon.
[0033]
Various known techniques can also be used when the positive electrode active material and the negative electrode active material are formed on the positive electrode current collector and the negative electrode current collector to produce the positive electrode and the negative electrode. For example, when manufacturing a positive electrode, a method of mixing a positive electrode active material with a binder in a solvent to form a paste, coating the paste on a positive electrode current collector, and drying can be used. Similarly, the negative electrode can be formed by mixing a negative electrode active material with a binder in a solvent to form a paste, coating the paste on the negative electrode current collector, and drying. Usually, both sides of the current collector are coated. Applied. A conductive agent such as carbon black, graphite or acetylene black may be added to the paste. The mixing ratio of the active material and the binder is preferably determined appropriately according to the shape of the electrode, and various known methods can be used for coating.
[0034]
As the current collector, various known materials used in lithium ion secondary batteries can be used. Specifically, an aluminum foil or the like is used as a positive electrode current collector, and a copper foil or the like is used as a negative electrode current collector. Is mentioned.
[0035]
Examples of the binder include polyvinylidene fluoride (PVDF) and polytetrafluoroethylene, and examples of the solvent include various polar solvents that dissolve the binder. Specific examples include dimethylformamide, dimethylacetamide, methylformamide, N-methylpyrrolidone (NMP) and the like. In addition, when polyvinylidene fluoride is used as a binder, it is preferable to use N-methylpyrrolidone.
[0036]
As the non-aqueous electrolyte, various solutions having lithium ion conductivity are preferable, and cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) may be used alone or in appropriate combination. it can. Further, in order to obtain an electrolytic solution having high electrical conductivity and appropriate viscosity, dimethyl carbonate (DMC), diethyl carbonate (DEC), γ-butyl lactone, γ-valerolactone, ethyl acetate, methyl propionate, etc. May be used in combination.
[0037]
As the electrolyte in the non-aqueous electrolyte, LiPF 6 , LiBF Four LiClO Four , LiAsF 6 , LiCF Three SO Three Etc.
[0038]
As the separator, a microporous film of polyolefin resin such as polyethylene or polypropylene can be used.
[0039]
When the secondary battery according to the present invention is manufactured, the above-described positive electrode, negative electrode, non-aqueous electrolyte, and separator can be appropriately combined. Also, various known materials and shapes can be applied to the battery can and the battery shape.
[0040]
Hereinafter, the invention according to the present application will be described in detail.
[0041]
A first invention of the present application is a lithium ion secondary battery including a positive electrode capable of occluding and releasing lithium ions, a negative electrode capable of occluding and releasing lithium ions, and a lithium ion conductive non-aqueous electrolyte. The particle size of the substance is 5 μm or less, more preferably 1 μm or less, and the thickness of the active material layer is 20 to 80 μm, more preferably 20 to 30 μm. In the present invention, the active material layer refers to a layer containing an active material formed on a current collector. For example, the active material layer is prepared by mixing various compositions such as an active material and a binder in a solvent as described above. This is a layer formed by coating the collected paste on the surface of the current collector and drying it. When active material layers are formed on both sides of the current collector in accordance with the usual method, It is preferred to apply the provisions of the invention. Moreover, in this invention, a particle size refers to an average particle diameter.
[0042]
When the particle size of the active material is large, the lithium ion diffusion in the active material particles becomes the rate-determining step rather than the transport of lithium ions in the electrolyte solution in the film thickness direction during large current discharge, and the output density decreases. Cause. Therefore, the particle size of the active material is preferably 5 μm or less. The lower limit value of the particle size of the active material is not particularly specified, but in practice it is suitably 0.1 μm or more. Also, if the film thickness is smaller than 20 μm, the output density is undesirably reduced due to the insufficient amount of active material, and if the film thickness exceeds 80 μm, the internal resistance increases and the output density decreases, which is not preferable.
[0043]
Under the above-mentioned conditions where the active material particle size is small such that the particle size of the active material is 5 μm or less, the transport of lithium ions in the electrolyte solution in the direction of the film thickness is considered to be the rate-limiting step during large current discharge. It is done. Therefore, when the porosity is increased, the amount of the electrolytic solution in the electrode increases, the transport capacity of lithium ions in the electrolytic solution in the electrode in the film thickness direction increases, and the output density can be further improved. However, if the porosity is less than 50%, the amount of the electrolyte corresponding to the amount of the active material cannot be secured, so that the resistance increases and the output density decreases. Therefore, the porosity is preferably 50% or more. In addition, when the porosity exceeds 60%, the output density gradually decreases due to a shortage of the amount of active material, that is, a reduction in the electrode surface area. Therefore, the porosity is more preferably 50 to 60%.
[0044]
Conversely, under conditions where the active material particle size is larger than 5 μm, the diffusion of lithium ions in the active material particles is at the rate-determining stage, so even if the porosity is increased, that is, the amount of electrolyte in the electrode is increased. The output density cannot be improved, and the amount of active material decreases, so the output density decreases.
[0045]
In order to improve the output density by defining the porosity as described above, the thickness of the active material layer is preferably 20 μm or more. This is because when the film thickness is thin, the influence of lithium ion transport in the electrolyte in the electrode in the film thickness direction is small, and the influence of the porosity is small.
[0046]
The active material layer may have a structure in which two active material layers having different porosity are stacked. By using a two-layer structure with different porosity, the output density can be improved without sacrificing the energy density. Specifically, it is preferable to increase the porosity of the active material layer on the separator side and decrease the porosity of the active material layer on the current collector side. By increasing the porosity of the active material layer in the vicinity of the separator, the amount of the electrolyte solution in the vicinity of the separator can be increased, and the lithium ion transport capability can be increased. Further, by reducing the porosity in the vicinity of the current collector, the utilization factor of the active material in the vicinity of the current collector can be improved. Considering such characteristics, the power density can be effectively improved by balancing the diffusion in the active material and the transport in the electrolytic solution. Further, since the energy density is affected by the average porosity and the amount of the active material of the active material layer, the output density can be improved without sacrificing the energy density by appropriately adjusting the energy density. For example, an electrode having a one-layer structure (50% porosity, 60 μm thickness) and a two-layer electrode (40% porosity on the collector side, 30 μm thickness; 60% porosity on the separator side, 30 μm thickness) Are equal in energy density. This is because the average porosity and the amount of active material of these two electrodes are equal.
[0047]
In addition, the thickness of the two active material layers having different porosity is preferably 20 to 30 μm, more preferably 20 to 25 μm, and the thickness of the two active material layers may be different. This is because if the thickness of the active material layer is greater than 30 μm, the energy density tends to decrease, and if it is 30 μm or less, the utilization factor of the active material layer on the current collector side is improved. The porosity of the active material layer on the current collector side is preferably 30% or more and less than 50%, and more preferably 40% or more and less than 50%. The porosity of the active material layer on the separator side is preferably 50 to 60%, more preferably 50 to 55%. By adjusting the porosity in this range, a greater effect can be obtained.
[0048]
A second invention of the present application is a lithium ion secondary battery comprising two active material layers having different active material particle sizes. With such a configuration, the output density can be improved.
[0049]
During large current discharge, the transport of lithium ions in the electrolyte in the electrode in the film thickness direction becomes a rate-determining step, and the electrode active material in the vicinity of the current collector cannot be used effectively, causing the output density to decrease. In order to solve this problem, it is preferable to increase the particle size of the active material in the vicinity of the electrode separator. Thereby, the electrode surface area near the separator is reduced, the utilization factor of the active material near the separator is lowered, and lithium ions can be easily transported to the vicinity of the current collector. For this reason, the utilization factor of the electrode active material in the vicinity of the current collector is improved, and as a total, the utilization factor of the electrode active material is improved, that is, the output density is improved. Further, the output density can be improved without sacrificing the energy density by appropriately adjusting the amounts of active materials in the single-layer battery and the two-layer battery as a whole.
[0050]
The effect of taking the two-layer structure is manifested by increasing the particle size of the active material in the vicinity of the electrode separator and reducing the utilization of the active material in the vicinity of the separator. Therefore, if the thickness of the active material layer on the separator side is thicker than necessary, the utilization factor of the active material in the vicinity of the separator is improved, and the effect of the present invention is reduced. Further, if the thickness of the active material layer on the current collector side is too thick, the output density is lowered, which is not preferable. For this reason, the thickness of each active material layer is preferably 30 μm or less, and more preferably 25 μm or less. The lower limit value of the thickness of the active material layer is preferably 20 μm in order to prevent a decrease in energy density. This is because when the thickness of the active material layer is less than 20 μm, the weight ratio of the current collector and the like in the battery increases.
[0051]
Further, in the two active material layers having different active material particle sizes, the active material particle size of the active material layer on the current collector side is 5 μm or more, or the active material particle size of the active material layer on the separator side is less than 5 μm. This is not preferable because the effect of improving the output density of the present invention is reduced. The lower limit value of the active material particle size on the current collector side is not particularly limited, but in practice it is suitably 0.1 μm or more. In addition, the upper limit of the active material particle size of the active material layer on the separator side is preferably selected as appropriate so long as the active material particle size is not larger than the thickness of the active material layer. From the above viewpoint, the active material particle size of the active material layer on the current collector side is preferably 0.1 μm or more and less than 5 μm, and more preferably 1 μm or more and less than 5 μm. The active material particle size of the active material layer on the separator side is preferably 5 to 20 μm, and more preferably 5 to 10 μm.
[0052]
The particle size of the active material can be adjusted by the particle size of the starting material or classification, and the porosity is applied when a paste containing the active material and a conductive agent is applied to the current collector, dried, and then pressed. It can be adjusted by changing the pressure.
[0053]
In the above description, a two-layer structure has been described as an example, but the present invention can be achieved by adjusting the porosity of the active material layer, the particle size of the active material, and the thickness of the active material layer even in a multilayer structure of three or more layers. It is possible to obtain the effect.
[0054]
In the multilayer structure, first, the first layer can be coated and formed on the current collector, and then the second layer can be coated and formed up to n layers in sequence, and particles having different sizes can be formed. It is also possible to make a multilayer structure by utilizing the difference in sedimentation speed.
[0055]
In the first and second inventions of the present invention, the positive electrode active material is preferably lithium manganese oxide from the viewpoint of obtaining a high power density. Manganese is much cheaper than cobalt and nickel, and is also abundant in terms of resources, so it is preferable from the viewpoint of manufacturing cost. Specific examples of lithium manganese oxide include LiMnO. 2 , LiMn 2 O Four Is mentioned.
[0056]
In the lithium ion secondary battery of the present invention, in order to further improve the output density, in the first and second inventions, the electrolyte concentration of the non-aqueous electrolyte is 1.0 to 3.0 mol / It is preferable that it is 1, and it is more preferable that it is 1.5-2.5 mol / l. By using an electrolyte concentration in such a range, it is possible to balance the diffusion in the active material and the transport in the electrolytic solution, and a suitable output density can be obtained.
[0057]
Under conditions where the particle size of the active material is small or the porosity is large, the transport of lithium ions in the electrolyte solution in the film thickness direction becomes the rate-limiting step during large current discharge. Therefore, when the electrolyte concentration is increased, concentration polarization is suppressed, the transport capacity of lithium ions in the electrolyte solution in the electrode in the film thickness direction is increased, and the output density is improved. If the electrolyte concentration exceeds 3.0 mol / l, the influence of the lithium ion conductivity of the electrolytic solution will appear and the output density will decrease, which is not preferred. If the electrolyte concentration is less than 1.0 mol / l, the internal resistance of the battery will increase. Therefore, it is not preferable. Moreover, the voltage at the time of discharge can be made high and stable by adjusting electrolyte concentration in the range of 1.5-2.5 mol / l.
[0058]
The relationship between the particle size of the active material and the electrolyte concentration can also affect the power density. That is, under the condition that the particle size of the active material is larger than 5 μm, the diffusion of lithium ions in the active material particles becomes the rate-determining step during the large current discharge, rather than the transport of lithium ions in the electrolyte solution in the film thickness direction. . For this reason, there is little influence of electrolyte concentration, and even if it raises the density | concentration of electrolyte, output density does not improve so much. Therefore, when the active material layer has a multilayer structure of two or more layers, it is preferable that the active material particle size of the active material layer on the current collector side be less than 5 μm.
[0059]
The electrolyte is LiPF from the viewpoint of improving the power density by using a compound having high electrical conductivity. 6 Or LiBF Four It is preferable that
[0060]
In the present invention, the particle size of the active material can be measured by a method for measuring various particle size distributions such as a screening test and a sedimentation method, the porosity can be measured by the specific gravity of the constituent material, and the active material layer thickness can be measured by a micrometer. .
[0061]
【Example】
1. Investigation of the effect of active material particle size, active material layer thickness, and porosity on power density
< reference Example 1>
Lithium manganese oxide (LiMnO) having an average particle size of 3 μm as a positive electrode active material 2 ) Was used. The specific surface area of this active material is about 3m 2 / G. 75% by mass of an active material having a particle size of 3 μm, 10% by mass of acetylene black as a conductive agent, and 15% by mass of PVDF as a binder are mixed in NMP and applied onto an aluminum foil (current collector). A plurality of positive electrodes having a thickness of 60 μm and different active material layer porosity were manufactured. The porosity was adjusted by the amount of solvent, drying conditions, and electrode pressing. Metal lithium is used for the negative electrode active material, and 1M LiPF is used for the electrolyte. 6 A plurality of lithium ion secondary batteries having different porosity of the active material layer of the positive electrode were manufactured using a mixture of PC and DMC (1: 1 volume ratio) in which s.
[0062]
<Comparative Example 1>
Except that the average particle size is 30 μm reference A plurality of lithium ion secondary batteries having different porosity were produced in the same manner as in Example 1.
[0063]
<Comparative Example 2>
Except for the active material layer thickness being 10 μm reference A plurality of lithium ion secondary batteries having different porosity were produced in the same manner as in Example 1.
[0064]
Figure 1 reference The relationship between the porosity of Example 1, Comparative Example 1 and Comparative Example 2 and the output density (relative value when the output density with a porosity of 40% is 1) is shown. reference The power density of the battery of Example 1 was maximized at a porosity of 50 to 60%, whereas the power density of the batteries of Comparative Examples 1 and 2 decreased as the porosity increased.
[0065]
2. Investigation of the effect of making the active material layer into a two-layer structure with different porosity
< reference Example 2>
Lithium manganese oxide (LiMnO) having an average particle size of 3 μm as a positive electrode active material 2 ) Was used. The specific surface area of this active material is about 3m 2 / G. 75% by mass of an active material having a particle size of 3 μm, 10% by mass of acetylene black as a conductive agent, and 15% by mass of PVDF as a binder are mixed in NMP and applied onto an aluminum foil (current collector). A positive electrode having a porosity of 40% and an active material layer thickness of 30 μm was produced. The same positive electrode active material composition was applied to this positive electrode, and an active material layer having an active material layer porosity of 60% and an active material layer thickness of 30 μm was laminated. The porosity was adjusted by the amount of solvent, drying conditions and electrode pressing. In this way, a positive electrode having an active material layer thickness of 60 μm was manufactured, metallic lithium was used as the negative electrode active material, and 1M LiPF was used as the electrolyte. 6 A lithium ion secondary battery was manufactured using a mixture of PC and DMC (volume ratio of 1: 1) in which the slag was dissolved.
[0066]
< reference Example 3>
Lithium manganese oxide (LiMnO) having an average particle size of 3 μm as a positive electrode active material 2 ) Was used. The specific surface area of this active material is about 3m 2 / G. 75% by mass of an active material having a particle size of 3 μm, 10% by mass of acetylene black as a conductive agent, and 15% by mass of PVDF as a binder are mixed in NMP and applied onto an aluminum foil (current collector). A positive electrode having a porosity of 50% and an active material layer thickness of 60 μm was produced. The porosity was adjusted by the amount of solvent, drying conditions, and electrode pressing. Metal lithium is used for the negative electrode active material, and 1M LiPF is used for the electrolyte. 6 A lithium ion secondary battery was manufactured using a mixture of PC and DMC (volume ratio of 1: 1) in which the slag was dissolved.
[0067]
< reference Example 4>
Lithium manganese oxide (LiMnO) having an average particle size of 3 μm as a positive electrode active material 2 ) Was used. The specific surface area of this active material is about 3m 2 / G. 75% by mass of an active material having a particle size of 3 μm, 10% by mass of acetylene black as a conductive agent, and 15% by mass of PVDF as a binder are mixed in NMP and applied onto an aluminum foil (current collector). A positive electrode having a porosity of 40% and an active material layer thickness of 20 μm was produced. The same positive electrode active material composition was applied to this positive electrode, and an active material layer having an active material layer porosity of 60% and an active material layer thickness of 40 μm was laminated. The porosity was adjusted by the amount of solvent, drying conditions and electrode pressing. In this way, a positive electrode having an active material layer thickness of 60 μm was manufactured, metallic lithium was used as the negative electrode active material, and 1M LiPF was used as the electrolyte. 6 A lithium ion secondary battery was manufactured using a mixture of PC and DMC (volume ratio of 1: 1) in which the slag was dissolved.
[0068]
Figure 2 reference Output density of the batteries of Examples 2-4 ( reference (The relative value when the output density of Example 3 is 1). The power density was improved by making the active material layer into a two-layer structure. Moreover, when it was set as 2 layer structure, when the thickness of each active material layer was 30 micrometers or less, the output density could be improved more effectively.
[0069]
In addition, in FIG. reference The energy density of the batteries of Examples 2-4 ( reference The relative value when the energy density of Example 3 is 1 is shown. reference Example 2 and reference The energy density of Example 3 was equal. this is, reference Example 2 and reference This is because the average porosity of Example 3 is equal and the amount of positive electrode active material is equal. reference In Example 4, the active material layer has a single-layer structure. reference Compared with Example 3, the output density was improved, but the energy density was inferior.
[0070]
3. Investigation of the effect of making the active material layer into a two-layer structure with different active material particle sizes
< reference Example 5>
Two types of lithium manganese oxide (LiMnO) having an average particle diameter of 3 μm and 9 μm as positive electrode active materials 2 ) Was used. 75% by mass of an active material having an average particle diameter of 3 μm, 10% by mass of acetylene black as a conductive agent, and 15% by mass of PVDF as a binder are mixed in NMP and applied onto an aluminum foil (current collector). A positive electrode having a layer thickness of 30 μm was produced. Similarly, an active material layer having a thickness of 30 μm was laminated on the positive electrode using an active material having an average particle diameter of 9 μm. In this way, a positive electrode having an active material layer thickness of 60 μm was manufactured, metallic lithium was used as the negative electrode active material, and 1M LiPF was used as the electrolyte. 6 A lithium ion secondary battery was manufactured using a mixture of PC and DMC (volume ratio of 1: 1) in which the slag was dissolved.
[0071]
< reference Example 6>
Lithium manganese oxide (LiMnO) having an average particle size of 3 μm as a positive electrode active material 2 )Using, reference A positive electrode having an active material layer thickness of 60 μm was prepared in the same manner as in Example 5, lithium metal was used as the negative electrode active material, and 1M LiPF was used as the electrolyte. 6 A lithium ion secondary battery was manufactured using a mixture of PC and DMC (volume ratio of 1: 1) in which the slag was dissolved.
[0072]
In FIG. reference Example 5 and reference Output density of Example 6 ( reference (The relative value when the output density of Example 6 is 1). Two-layer structure with different particle sizes reference Example 5 showed a higher power density. In addition, in FIG. reference Example 5 and reference Example 6 energy density ( reference The relative value when the energy density of Example 6 is set to 1 is shown. reference Example 5 and reference In Example 6, the energy density was equal. this is, reference Example 5 and reference This is because the amount of the positive electrode active material in Example and 6 is equal.
[0073]
4). Investigation of the effect of active material layer thickness in a two-layer structure with different active material particle sizes
<Comparative Example 3>
Two types of lithium manganese oxide (LiMnO) having an average particle diameter of 3 μm and 9 μm as positive electrode active materials 2 ) Was used. 75% by mass of an active material having an average particle diameter of 3 μm, 10% by mass of acetylene black as a conductive agent, and 15% by mass of PVDF as a binder are mixed in NMP and applied onto an aluminum foil (current collector). A positive electrode having a layer thickness of 20 μm was produced. Similarly, an active material layer having a thickness of 40 μm was laminated on the positive electrode using an active material having an average particle diameter of 9 μm. In this way, a positive electrode having an active material layer thickness of 60 μm was manufactured, metallic lithium was used as the negative electrode active material, and 1M LiPF was used as the electrolyte. 6 A lithium ion secondary battery was manufactured using a mixture of PC and DMC (volume ratio of 1: 1) in which the slag was dissolved.
[0074]
FIG. 6 shows Comparative Example 3 and the above reference The output density of Example 6 (relative value when the output density of Comparative Example 3 is 1) is shown. The output density of Comparative Example 3 is low because the active material particle size of the active material layer on the separator side is large, the utilization factor of the active material near the separator is reduced, and the thickness of the active material layer on the separator side is increased. This is because the utilization factor of the active material in the vicinity of the electric body has also decreased. Further, in FIG. reference The energy density of Example 6 (relative value when the energy density of Comparative Example 3 is 1) is shown. Comparative Example 3 reference In Example 6, the energy density was equal. This is similar to Comparative Example 3 reference This is because the positive electrode active material amount is the same as in Example 6.
[0075]
5. Investigation of the effect of active material particle size and electrolyte concentration on power density
<Example 7>
Lithium manganese oxide (LiMnO) having an average particle size of 3 μm as a positive electrode active material 2 ) Was used. 75% by mass of an active material having a particle size of 3 μm, 10% by mass of acetylene black as a conductive agent, and 15% by mass of PVDF as a binder are mixed in NMP and applied onto an aluminum foil (current collector). A positive electrode having a thickness of 60 μm was produced. Lithium metal is used for the negative electrode active material, and LiPF with different concentrations is used for the electrolyte. 6 A plurality of lithium ion secondary batteries were manufactured using a mixture of PC and DMC (volume ratio of 1: 1) in which was dissolved.
[0076]
<Comparative example 4>
Lithium manganese oxide (LiMnO) having an average particle size of 30 μm as a positive electrode active material 2 ) Was used to produce a lithium ion secondary battery in the same manner as in Example 7.
[0077]
FIG. 8 shows the relationship between the electrolyte concentration of Example 7 and Comparative Example 4 and the power density (relative value when the power density is 1 when the electrolyte concentration is 1 mol / l). In Example 7, the output density was maximized when the electrolyte concentration was 2 mol / l. In contrast, in Comparative Example 4, the output density was not improved even when the electrolyte concentration was increased.
[0078]
6). Investigation of the effect of active material layer thickness and electrolyte concentration on power density
<Example 8>
Lithium manganese oxide (LiMnO) having an average particle size of 3 μm as a positive electrode active material 2 ) Was used. 75% by mass of an active material having a particle size of 3 μm, 10% by mass of acetylene black as a conductive agent, and 15% by mass of PVDF as a binder are mixed in NMP and applied onto an aluminum foil (current collector). A plurality of positive electrodes having different thicknesses were produced. Lithium metal is used for the negative electrode active material, and LiBF with a concentration of 2 mol / l is used for the electrolyte. Four A plurality of lithium ion secondary batteries having different thicknesses of the active material layer of the positive electrode were manufactured using a mixture of PC and DMC (1: 1 volume ratio) in which the slag was dissolved.
[0079]
< reference Example 9>
1mol / l LiBF in electrolyte 4 A lithium ion secondary battery was produced in the same manner as in Example 8 except that was used.
[0080]
FIG. 9 shows Example 8 and reference Example 9 active material layer thickness and power density ( reference The relationship with the relative value when the output density is set to 1 when the active material layer thickness of Example 9 is 100 μm is shown. The power density of Example 8 with the preferred electrolyte and electrolyte concentration was about twice as high.
[0081]
Example above 7 and 8 , Reference Examples 1-6 and 9, and The structure of Comparative Examples 1-4 is shown to Tables 1-6.
[0082]
[Table 1]
[Table 2]
[Table 3]
[Table 4]
[Table 5]
[Table 6]
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between porosity and power density.
[Figure 2] reference It is a graph which shows the relationship between an example and output density.
[Fig. 3] reference It is a graph which shows the relationship between an example and energy density.
[Fig. 4] reference It is a graph which shows the output density of an example.
[Figure 5] reference It is a graph which shows the energy density of an example.
[Fig. 6] reference It is a graph which shows the output density of an example and a comparative example.
[Fig. 7] reference It is a graph which shows the energy density of an example and a comparative example.
FIG. 8 is a graph showing the relationship between electrolyte concentration and power density.
FIG. 9 is a graph showing the relationship between active material layer thickness and power density.
Claims (3)
活物質の粒径が５μｍ以下であり、
活物質層の厚さが２０〜８０μｍであり、
前記非水電解液の電解質濃度が１．５〜２．５ｍｏｌ／ｌであることを特徴とするリチウムイオン二次電池。A positive electrode capable of occluding and releasing lithium ions, a negative electrode capable of occluding and releasing lithium ions, and a lithium ion conductive non-aqueous electrolyte,
The particle size of the active material is 5 μm or less,
The thickness of the active material layer is 20 to 80 μm,
The electrolyte concentration of the non-aqueous electrolyte is 1. A lithium ion secondary battery characterized by being 5 to 2.5 mol / l.
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