JP4882135B2 - Positive electrode material for lithium secondary battery, positive electrode for lithium secondary battery, and lithium secondary battery - Google Patents
Positive electrode material for lithium secondary battery, positive electrode for lithium secondary battery, and lithium secondary battery Download PDFInfo
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- JP4882135B2 JP4882135B2 JP2000090069A JP2000090069A JP4882135B2 JP 4882135 B2 JP4882135 B2 JP 4882135B2 JP 2000090069 A JP2000090069 A JP 2000090069A JP 2000090069 A JP2000090069 A JP 2000090069A JP 4882135 B2 JP4882135 B2 JP 4882135B2
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- JP
- Japan
- Prior art keywords
- positive electrode
- lithium
- secondary battery
- lithium secondary
- chalcogenide
- Prior art date
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Description
【０００１】
【発明の属する技術分野】
本発明はリチウム二次電池用正極材料に関し、更にはそれを使用した正極及びリチウム二次電池に関する。
【０００２】
【従来の技術】
負極活物質として金属リチウムに代わって、リチウムイオンの吸蔵・放出が可能な炭素材料等を用いることにより、安全性が大幅に向上し、リチウム二次電池が実用段階に入った。
リチウム二次電池の正極活物質として、ＬｉＣｏＯ2やＬｉＮｉＯ2、ＬｉＭｎ2Ｏ4などのリチウム遷移金属複合酸化物が実用段階に入った。特に、ＬｉＭｎ2Ｏ4などのリチウムマンガン酸化物からなるマンガン系正極活物質は、成分となるマンガンがコバルトやニッケルに比較して埋蔵量が多く、安価であり、加えて過充電での安全性も高いというメリットを有している。ＬｉＣｏＯ2等のリチウムコバルト酸化物やＬｉＮｉＯ2等のリチウムニッケル酸化物の場合は、実用的に十分な高温サイクル特性を有することから、この高温環境下におけるサイクル特性の低下という問題は、マンガン系の抱える特有な問題となっている。
【０００３】
上記問題を克服するため、高温環境下でのサイクル特性改良を目的とした検討が精力的に行われ、報告されている。例えば、J.Electrochem.soc.,Vol.145,No.8(1998)2726-2732ではＭｎの一部をＧａやＣｒのような他元素で置換したもの、Electrochemical Society Proceedings Volume97-18.494 ではＭｎの一部をＣｏで置換したり、酸素の一部をＦで置換して結晶構造の安定性向上を図ったものが高温サイクル特性の改善効果があるという結果を示している。しかしこれらは負極として金属リチウムを使用した時の結果であって、炭素材料のような実用的な負極材料との組み合わせでは、十分な効果が得られていないのが実情である。
【０００４】
また、マンガン系リチウム二次電池においては高温環境下でマンガンが溶出しやすいことが高温保存劣化や高温サイクル劣化の問題点として指摘されており、例えば正極活物質表面を処理したり、正極材中にＭｎ溶出抑制効果のある物質を添加するといった検討も鋭意行われている。しかしながら、これら従来の技術では未だ高温環境下でのサイクル特性は実用レベルまで達していない。
【０００５】
なお、特開平８−２５０１２０号公報には、特定のカルコゲン化物からなる被膜を粒子表面に有するリチウム−遷移金属複合酸化物が正極活物質として使用することが記載されている。しかしながら、高温サイクル特性の改善という点では不十分であった。
【０００６】
【発明が解決しようとする課題】
リチウムマンガン酸化物を正極活物質として使用したリチウム二次電池は、リチウムコバルト酸化物やリチウムニッケル酸化物を使用した場合に比較して、充放電容量が低い。そのため、できるだけ容量を引き出そうとして深充放電を繰り返すことになる。しかしながら、特にリチウムが殆ど放出された充電端のリチウムマンガン酸化物は、高温環境下において特異的に活性な状態となり、活物質自体の変質のみならず、マンガン溶出や電解液の分解、負極表面に形成された被膜の破壊等々、様々な悪影響を及ぼすものと考えられる。
【０００７】
本発明は、上記のようなリチウムマンガン酸化物に特有の問題点を解決するためになされたもので、その目的は、リチウムマンガン酸化物に特有な問題である高温環境下での特異的活性化状態の緩和・低減を図り、以て高温サイクル特性の改善されたリチウム二次電池を提供しようとするものである。
【０００８】
【課題を解決するための手段】
本発明者等は、かかる課題を解決するためには、高温環境下、電池内部で安定に存在できる添加剤を用いて触媒活性の低減を図ることが必須と考え、そのような添加剤を見出すべく鋭意検討を重ねた結果、６族元素のカルコゲン化物（クロムカルコゲニド、モリブデンカルコゲニド、タングステンカルコゲニド等）を存在させることによって、高温サイクル特性が改善されることを見出し、本発明に至った。
【０００９】
前記した特定金属元素のカルコゲン化物が特異的に改善効果を発揮した理由として、高温環境下での正極活物質の活性化状態の緩和・低減が図られたことに加え、電解液や負極表面に対しても、安定化剤として作用する等、何らかの形で良い影響をもたらしたためではないかと考えている。
即ち、本発明の要旨は、活物質としてリチウムマンガン酸化物を含み、６族元素のカルコゲン化物が物理混合されていることを特徴とするリチウム二次電池用正極材料に存する。
【００１０】
本発明の好ましい要旨としては、６族元素が、Ｃｒ、Ｍｏ又はＷである上記の正極材料；カルコゲン化物が硫化物である上記の正極材料；カルコゲン化物のリチウムマンガン酸化物に対する混合割合が０．１〜２０モル％である上記の正極材料；リチウムマンガン酸化物が、マンガンサイトの一部が他元素で置換されたリチウムマンガン酸化物である上記の正極材料；マンガンサイトを置換する他元素が、Ａｌ、Ｔｉ、Ｖ、Ｃｒ、Ｆｅ、Ｃｏ、Ｎｉ、Ｃｕ、Ｚｎ、Ｍｇ、Ｇａ及びＺｒからなる群から選ばれる少なくとも一種の金属元素である上記の正極材料が挙げられる。
【００１１】
また、本発明の別の要旨として、上記の正極材料とバインダーとを有することを特徴とするリチウム二次電池用正極が挙げられ、リチウムマンガン酸化物とカルコゲン化物とが、分散して存在してなるリチウム二次電池用正極が好ましい。
更に本発明の別の要旨として、上記の正極と、負極と、電解質層とを有することを特徴とするリチウム二次電池；リチウムマンガン酸化物を活物質として用いた正極と、負極と、電化質層とを有するリチウム二次電池において、負極及び／又は電解質層に、６族元素のカルコゲン化物が含まれてなるリチウム二次電池が挙げられ、負極の活物質が炭素材料であることが好ましい。
【００１２】
なお、特開平８−２５０１２０号公報には、特定のカルコゲン化物からなる被膜を粒子表面に有するリチウム−遷移金属複合酸化物が正極活物質として使用することが記載されている。しかしながら、上記公知文献はリチウムマンガン酸化物という特定の正極材料に注目したものでもなく、また、マンガン系に特有の問題を解決するものでもなく、更には、本発明で規定する６族元素のカルコゲニドを熱処理することなく物理混合しただけのものが特異的に優れた効果を示すことも記載されていない。
【００１３】
【発明の実施の形態】
以下、好適な一実施の形態を用いて本発明を説明するが、本発明の趣旨を超えない限り、以下に限定されるものではない。
本発明において用いられる６族元素のカルコゲン化物としては、クロム、モリブデン、タングステンの硫化物、セレン化物、テルル化物を挙げることができる。具体的には、ＣｒＳ、Ｃｒ7Ｓ8、Ｃｒ5Ｓ6、Ｃｒ3Ｓ4、Ｃｒ2Ｓ3、ＣｒＳｅ、Ｃｒ2Ｓｅ3、Ｃｒ7Ｓｅ8、Ｃｒ3Ｓｅ4、Ｃｒ5Ｓｅ8、Ｃｒ7Ｓｅ12、ＣｒＴｅ、Ｃｒ7Ｔｅ8、Ｃｒ5Ｔｅ6、Ｃｒ3Ｔｅ4、Ｃｒ2Ｔｅ3、Ｃｒ5Ｔｅ8、ＭｏＳ2、Ｍｏ2Ｓ5、ＭｏＳ3、Ｍｏ3Ｓ4、ＭｏＳ4、ＭｏＳｅ、ＭｏＴｅ、ＷＳ2、ＷＳ3、ＷＳｅ2、ＷＴｅ2が挙げられる。これらの化合物は無論複数種を併用してよい。また、６Ａ族元素のカルコゲン化物の６族元素の一部を他の元素で置換したものであっても良く、また、カルコゲン原子の一部が酸素元素等の他原子で置換されていてもよい。また、不定比なものであってもよい。なお、６族元素やカルコゲン原子を他の元素で置換する場合、置換元素は２種以上であってもよい。
【００１４】
特に好ましい６族元素のカルコゲン化物としては、具体的には、Ｃｒ2Ｓ3、ＭｏＳ2及びＷＳ2 からなる群から選ばれる少なくとも一種を挙げることができる。また、硫黄、セレン、テルルのカルコゲン元素の中でも、資源、毒性等の点も勘案すると硫黄が最も好ましい。
６族元素のカルコゲン化物の使用量は、リチウムマンガン酸化物に対して、通常０．１モル％以上、好ましくは２モル％以上であり、また、通常２０モル％以下、好ましくは１０モル％以下、さらに好ましくは８モル％以下、最も好ましくは６モル％以下である。使用量が多くなると放電容量や高温サイクル特性が低下する可能性があり、逆に少なくなると高温サイクル向上効果を得難くなる可能性がある。
【００１５】
６族元素のカルコゲン化物の平均粒径と比表面積は通常正極に用いる活物質の平均粒径や比表面積から大きく逸脱するものでなければ問題ないが、リチウムマンガン酸化物との接触効率を良くするため、平均粒径はリチウムマンガン酸化物の平均粒径以下であり、比表面積はリチウムマンガン酸化物の比表面積以上であるのが好ましい。
【００１６】
本発明で使用する６族元素のカルコゲン化物の比表面積は、通常０．３ｍ2／ｇ以上、好ましくは１ｍ2／ｇ以上、最も好ましくは１．５ｍ2／ｇ以上であり、通常１００ｍ2／ｇ以下、好ましくは５０ｍ2／ｇ以下、最も好ましくは２０ｍ2／ｇ以下である。比表面積が小さすぎると、十分な添加効果を示さない場合があり、比表面積が大きすぎるとそれ自体化学的に不安定になって、かえって悪影響を及ぼす恐れがある。なお、比表面積の測定はＢＥＴ法に従う。
【００１７】
本願発明で用いる６Ａ族元素のカルコゲン化物の平均粒径は、通常０．０５〜３０μｍ、好ましくは０．５〜１０μｍ、最も好ましくは１〜５μｍである。平均粒径が小さすぎるとそれ自体化学的に不安定になって、かえって悪影響を及ぼす恐れがあり、大きすぎると十分な添加効果が発現しなかったり、電極作製時に問題となる場合がある。
【００１８】
本発明で使用するリチウムマンガン酸化物のＢＥＴ比表面積の、６族元素のカルコゲン化物のＢＥＴ比表面積に対する比率は、通常０．０１〜１、好ましくは０．０５〜０．８、さらに好ましくは０．１〜０．５である。この比率が上記範囲を逸脱すると、所望の性能を得ることが困難になることがある。
本発明で使用するリチウムマンガン酸化物の平均粒径の６族元素のカルコゲン化物の平均粒径に対する比率は、通常１〜２０、好ましくは１．５〜１０、更に好ましくは２〜５である。この比率が上記範囲を逸脱すると、所望の性能を得ることが困難になることがある。
【００１９】
リチウムマンガン酸化物を含む正極材料中に６族元素のカルコゲン化物を存在させるには、物理混合の採用が好ましい。本発明における物理混合とは、複数の物質を単に混ぜ合わせることを意味し、６族元素のカルコゲン化物の添加効果（高温サイクルの改善）が失われる程の化学変化を起こしてしまうような高温での熱処理等を伴わない混合を意味する。複数の物質をかき混ぜて正極材料中に６族元素のカルコゲン化物を分散させたものが好ましく、均一に分散されていることが好ましい。なお、６族元素のカルコゲン化物の添加効果（高温サイクルの改善）が失われているか否かは、実施例の欄で後述する試験例（電池評価）に従って電池評価することができ、６族元素のカルコゲン化物を添加しなかった場合（比較例１）に比べて高温サイクルの改善が認められなければ添加効果が失われていると判断される。６族元素のカルコゲン化物の添加効果が減少もしくは失われる化学変化としては、６族元素のカルコゲン化物の分解酸化反応等が考えられる。熱処理等による被覆は６族元素のカルコゲン化物が変質する可能性が高く、目的とする効果を失ってしまう恐れがある。一方、物理混合は、簡便な添加法であり、かつ変質の影響がなく、本来の効果を十分に発揮しうる点で好ましい。物理混合は、乾式混合でも湿式混合でもよい。物理混合には、乳鉢、ボールミル、ジェットミル、レディゲミキサー等を使用することができる。
【００２０】
本発明において用いられるリチウムマンガン酸化物とは、活物質としてＬｉを可逆的に吸蔵・放出できるものであればよく、例えば一般式ＬｉＭｎ2Ｏ4、ＬｉＭｎＯ2 を挙げることができる。本発明の効果が顕著である点で、好ましくはスピネル構造を有するリチウムマンガン酸化物（一般式ＬｉＭｎ2Ｏ4）である。これらは、少量の酸素欠損、不定比性を持っていてもよい。また、酸素サイトの一部が硫黄やハロゲン元素で置換されていてもよい。
【００２１】
本発明で用いるリチウムマンガン酸化物は、マンガンサイトの一部が他の元素で置換されているのが好ましい。その結果、結晶構造の安定性を向上させることができ、これと６族元素のカルコゲン化物の物理混合を組み合わせることで相乗的に高温サイクル特性の向上を図ることができる。
この際の置換する他元素（以下、置換元素と表記する）としては、Ａｌ、Ｔｉ、Ｖ、Ｃｒ、Ｆｅ、Ｃｏ、Ｎｉ、Ｃｕ、Ｚｎ、Ｍｇ、Ｇａ、Ｚｒ等が挙げられ、好ましくはＡｌ、Ｃｒ、Ｆｅ、Ｃｏ、Ｎｉ、Ｍｇ、Ｇａ、更に好ましくはＡｌである。なお、マンガンサイトは２種以上の他元素で置換されていてもよい。
【００２２】
置換元素による置換割合は、通常マンガンの２．５モル％以上、好ましくはＭｎの５モル％以上であり、通常Ｍｎの３０モル％以下、好ましくはＭｎの２０モル％以下である。置換割合が少なすぎるとその高温サイクルの改善効果が充分ではない場合があり、多すぎると電池にした場合の容量が低下してしまう場合がある。
【００２３】
リチウムマンガン酸化物は、従来公知の各種の方法にて製造することができ、例えば、リチウム、マンガン、及び必要に応じて置換元素を含有する出発原料を混合後、酸素存在下で焼成・冷却することによって製造することができる。層状構造を有するリチウムマンガン酸化物も、従来公知の各種の方法にて製造することができ、例えば、リチウム、マンガン、及び必要に応じて置換元素を含有する出発原料を混合後、還元雰囲気下で焼成・冷却することによって製造することができる。
【００２４】
なお、マンガンサイトの一部が他の元素で置換されているリチウムマンガン酸化物を製造する際は、上記製造方法において置換元素を含有する出発原料を用いずマンガンサイトが置換されていないリチウムマンガン酸化物を製造し、該リチウムマンガン酸化物を、置換金属元素を含有する出発原料の水溶液、溶融塩あるいは蒸気中で反応させ、その後必要に応じて置換元素をリチウムマンガン複合酸化物粒子内に拡散させるために再度加熱処理を行うことによりマンガンサイトの一部を置換元素で置換することもできる。
【００２５】
出発原料として用いられるリチウム化合物としては、Ｌｉ2ＣＯ3、ＬｉＮＯ3、ＬｉＯＨ、ＬｉＯＨ・Ｈ2Ｏ、ＣＨ3ＣＯＯＬｉ、Ｌｉ2Ｏ、ジカルボン酸Ｌｉ、クエン酸Ｌｉ、脂肪酸Ｌｉ、アルキルリチウム、ハロゲン化物等が挙げられる。
好ましくはＬｉＯＨ・Ｈ2Ｏ、ジカルボン酸Ｌｉ、クエン酸Ｌｉ、脂肪酸Ｌｉ、Ｌｉ2ＣＯ3が挙げられる。
【００２６】
出発原料として用いられるマンガン化合物としては、Ｍｎ2Ｏ3、ＭｎＯ2等のマンガン酸化物、ＭｎＣＯ3、Ｍｎ（ＮＯ3）2 、ＭｎＳＯ4、酢酸マンガン、ジカルボン酸マンガン、クエン酸マンガン、脂肪酸マンガン等のマンガン塩、オキシ水酸化物、水酸化物、ハロゲン化物等が挙げられる。Ｍｎ2Ｏ3として、ＭｎＣＯ3やＭｎＯ2などの化合物を熱処理して作製したものを用いてもよい。好ましくはＭｎ2Ｏ3、オキシ水酸化物が挙げられる。
【００２７】
置換元素の化合物としては、酸化物、水酸化物、オキシ水酸化物、硝酸塩、硫酸塩、炭酸塩、ジカルボン酸塩、脂肪酸塩、アンモニウム塩等が挙げられる。これらの出発原料は、通常湿式混合、乾式混合、ボールミル粉砕、共沈等の方法によって混合される。混合の前後、および混合中において粉砕の工程を加えてもよい。
【００２８】
スピネル型リチウムマンガン酸化物の焼成・冷却の方法としては、例えば、仮焼後６００〜９００℃程度の温度で酸素雰囲気下で本焼を行い、次いで５００℃以下程度まで１０℃／ｍｉｎ以下の速度で徐冷する方法や、仮焼後６００〜９００℃程度の温度で空気又は酸素雰囲気下で本焼し、次いで４００℃程度の温度で酸素雰囲気下アニールする方法を挙げることができる。焼成・冷却の条件については、特開平９−３０６４９０号公報、特開平９−３０６４９３号公報、特開平９−２５９８８０号公報等に詳しく記載されている。
【００２９】
層状リチウムマンガン酸化物の焼成の方法としては、例えば、窒素等の還元雰囲気中９００〜１０００℃程度の温度で焼成を行う方法を挙げることができる。
本発明で用いるリチウムマンガン酸化物の比表面積は、好ましくは０．３ｍ2／ｇ以上、より好ましくは０．５ｍ2／ｇ以上であり、また好ましくは１．５ｍ2／ｇ以下、より好ましくは１．０ｍ2／ｇ以下である。比表面積が小さすぎるとレート特性の低下、容量の低下を招き、大きすぎると電解液等と好ましくない反応を引き起こし、サイクル特性を低下させることがある。比表面積の測定はＢＥＴ法に従う。
【００３０】
本願発明で用いるリチウムマンガン酸化物の平均粒径は、通常０．１〜３０μｍ、好ましくは０．２〜１０μｍ、より好ましくは０．３〜５μｍである。平均粒径が小さすぎると電池のサイクル劣化が大きくなったり、安全性に問題が生じたりする場合があり、大きすぎると電池の内部抵抗が大きくなり、出力が出しにくくなる場合がある。
【００３１】
本発明の正極材料は、リチウム二次電池の正極に使用することができる。
本発明の正極は、上記正極材料とバインダーとを有する。好ましくは、正極は、正極集電体と、正極材料とバインダーとを含有する正極層とからなる。正極層中のリチウムマンガン酸化物と６族元素のカルコゲン化物とは、分散して存在させるのが、本発明の効果を十分に発揮しうる点で好ましい。このような正極層は、リチウムマンガン酸化物、６族元素のカルコゲン化物、後述の結着剤( バインダー) 及び必要に応じて導電剤を溶媒でスラリー化したものを正極集電体に塗布し、乾燥することにより製造することができる。スラリー調製前に、事前にリチウムマンガン酸化物と６族元素のカルコゲン化物とを物理混合しておくこともできる。
【００３２】
正極中には、ＬｉＣｏＯ2、ＬｉＮｉＯ2、ＬｉＦｅＯ2、ＬｉＣｏＰＯ4、ＬｉＦｅＰＯ4等のように、リチウムマンガン酸化物以外のリチウムイオンを吸蔵・放出しうる活物質をさらに含有していてもよい。
正極中の活物質の割合は、通常１０重量％以上、好ましくは３０重量％以上、さらに好ましくは５０重量％以上であり、通常９９．９重量％以下、好ましくは９９重量％以下である。
【００３３】
また、正極に使用されるバインダーとしては、例えば、ポリフッ化ビニリデン、ポリテトラフルオロエチレン、フッ素化ポリフッ化ビニリデン、ＥＰＤＭ（エチレン−プロピレン−ジエン三元共重合体）、ＳＢＲ（スチレン−ブタジエンゴム）、ＮＢＲ（アクリロニトリル−ブタジエンゴム）、フッ素ゴム、ポリ酢酸ビニル、ポリメチルメタクリレート、ポリエチレン、ニトロセルロース等が挙げられる。正極層中のバインダーの割合は、通常０．１重量％以上、好ましくは１重量％以上、さらに好ましくは５重量％以上であり、通常８０重量％以下、好ましくは６０重量％以下、さらに好ましくは４０重量％以下、最も好ましくは１０重量％以下である。バインダーの割合が低すぎると、活物質を十分に保持できずに正極の機械的強度が不足し、サイクル特性等の電池性能を悪化させることがあり、一方高すぎると電池容量や導電性を下げることがある。
【００３４】
正極層は、通常導電性を高めるため導電剤を含有する。導電剤としては、天然黒鉛、人造黒鉛等の黒鉛や、アセチレンブラック等のカーボンブラック、ニードルコークス等の無定形炭素等の炭素材料を挙げることができる。正極中の導電剤の割合は、通常０．０１重量％以上、好ましくは０．１重量％以上、さらに好ましくは１重量％以上であり、通常５０重量％以下、好ましくは３０重量％以下、さらに好ましくは１５重量％以下である。導電剤の割合が低すぎると導電性が不十分になることがあり、逆に高すぎると電池容量が低下することがある。
【００３５】
また、スラリー溶媒としては、通常はバインダーを溶解あるいは分散する有機溶剤が使用される。例えば、Ｎ−メチルピロリドン、ジメチルホルムアミド、ジメチルアセトアミド、メチルエチルケトン、シクロヘキサノン、酢酸メチル、アクリル酸メチル、ジエチルトリアミン、Ｎ−Ｎ−ジメチルアミノプロピルアミン、エチレンオキシド、テトラヒドロフラン等を挙げることができる。また、水に分散剤、増粘剤等を加えてＳＢＲ等のラテックスで活物質をスラリー化することもできる。
【００３６】
活物質層の厚さは、通常１０〜２００μｍ程度である。
正極に使用する集電体の材質としては、アルミニウム、ステンレス鋼、ニッケルメッキ鋼等が用いられ、好ましくはアルミニウムである。
なお、塗布・乾燥によって得られた活物質層は、活物質の充填密度を上げるためローラープレス等により圧密されるのが好ましい。
【００３７】
本発明の正極材料を用いてリチウムイオン二次電池とすることができる。本発明のリチウムイオン二次電池は、前記活物質を正極中に含有するが、通常上記正極と負極及び電解質を有する。
本発明の二次電池の負極に使用される負極の活物質としては、リチウムやリチウムアルミニウム合金合金などのリチウム合金であっても良いが、より安全性の高いリチウムを吸蔵、放出できる炭素材料が好ましい。
【００３８】
前記炭素材料は特に限定されないが、黒鉛及び、石炭系コークス、石油系コークス、石炭系ピッチの炭化物、石油系ピッチの炭化物、あるいはこれらピッチを酸化処理したものの炭化物、ニードルコークス、ピッチコークス、フェノール樹脂、結晶セルロース等の炭化物等及びこれらを一部黒鉛化した炭素材、ファーネスブラック、アセチレンブラック、ピッチ系炭素繊維等が挙げられる。
【００３９】
さらに、ＳｎＯ、ＳｎＯ2、Ｓｎ1-xＭxＯ（Ｍ＝Ｈｇ、Ｐ、Ｂ、Ｓｉ、ＧｅまたはＳｂ、ただし０≦ｘ＜１）、Ｓｎ3Ｏ2（ＯＨ）2 、Ｓｎ3-xＭxＯ2（ＯＨ）2（Ｍ＝Ｍｇ、Ｐ、Ｂ、Ｓｉ、Ｇｅ、Ｓｂ又はＭｎ、ただし０≦ｘ＜３）、ＬｉＳｉＯ2、ＳｉＯ2又はＬｉＳｎＯ2等を挙げることができる。
なお、これらの中から選ばれる２種以上の混合物として用いてもよい。
【００４０】
負極は通常、正極の場合と同様、活物質層を集電体上に形成されてなる。この際使用するバインダーや、必要に応じて使用される導電剤やスラリー溶媒としては、正極で使用するものと同様のものを使用することができる。また、負極の集電体としては、銅、ニッケル、ステンレス鋼、ニッケルメッキ鋼等が使用され、好ましくは銅が用いられる。
【００４１】
正極と負極との間にセパレーターを使用する場合は、微多孔性の高分子フィルムが用いられ、ナイロン、セルロースアセテート、ニトロセルロース、ポリスルホン、ポリアクリロニトリル、ポリフッ化ビニリデン、ポリプロピレン、ポリエチレン、ポリブテン等のポリオレフィン高分子よりなるものが用いられる。セパレータの化学的及び電気化学的安定性は重要な因子である。この点からポリオレフィン系高分子が好ましく、電池セパレータの目的の一つである自己閉塞温度の点からポリエチレン製であることが望ましい。
【００４２】
ポリエチレンセパレーターの場合、高温形状維持性の点から超高分子量ポリエチレンであることが好ましく、その分子量の下限は好ましくは５０万、さらに好ましくは１００万、最も好ましくは１５０万である。他方分子量の上限は、好ましくは５００万、更に好ましくは４００万、最も好ましくは３００万である。分子量が大きすぎると、流動性が低すぎて加熱された時セパレーターの孔が閉塞しない場合があるからである。
【００４３】
また、本発明のリチウム二次電池における電解質には、例えば公知の有機電解液、高分子固体電解質、ゲル状電解質、無機固体電解質等を用いることができるが、中でも有機電解液が好ましい。有機電解液は、有機溶媒と溶質から構成される。
有機溶媒としては特に限定されるものではないが、例えばカーボネート類、エーテル類、ケトン類、スルホラン系化合物、ラクトン類、ニトリル類、塩素化炭化水素類、エーテル類、アミン類、エステル類、アミド類、リン酸エステル化合物等を使用することができる。これらの代表的なものを列挙すると、ジメチルカーボネート、ジエチルカーボネート、プロピレンカーボネート、エチレンカーボネート、ビニレンカーボネート、テトラヒドロフラン、２−メチルテトラヒドロフラン、１，４−ジオキサン、４−メチル−２−ペンタノン、１，２−ジメトキシエタン、１，２−ジエトキシエタン、γ−ブチロラクトン、１，３−ジオキソラン、４−メチル−１，３−ジオキソラン、ジエチルエーテル、スルホラン、メチルスルホラン、アセトニトリル、プロピオニトリル、ベンゾニトリル、ブチロニトリル、バレロニトリル、１，２−ジクロロエタン、ジメチルホルムアミド、ジメチルスルホキシド、リン酸トリメチル、リン酸トリエチル等の単独もしくは二種類以上の混合溶媒が使用できる。
【００４４】
上述の有機溶媒には、電解質を解離させるために高誘電率溶媒が含まれることが好ましい。ここで、高誘電率溶媒とは、２５℃における比誘電率が２０以上の化合物を意味する。高誘電率溶媒の中で、エチレンカーボネート、プロピレンカーボネート及びそれらの水素原子をハロゲン等の他の元素又はアルキル基等で置換した化合物が電解液中に含まれることが好ましい。高誘電率化合物の、電解液に占める割合は、好ましくは２０重量％以上、更に好ましくは３０重量％以上、最も好ましくは４０重量％以上である。該化合物の含有量が少ないと、所望の電池特性が得られない場合があるからである。
【００４５】
またこの溶媒に溶解させる溶質として特に限定されるものではないが、従来公知のいずれもが使用でき、ＬｉＣｌＯ4、ＬｉＡｓＦ6、ＬｉＰＦ6、ＬｉＢＦ4、ＬｉＢ（Ｃ6Ｈ5）4 、ＬｉＣｌ、ＬｉＢｒ、ＣＨ3ＳＯ3Ｌｉ、ＣＦ3ＳＯ3Ｌｉ、ＬｉＮ（ＳＯ2ＣＦ3）2、ＬｉＮ（ＳＯ2Ｃ2Ｆ5）2、ＬｉＣ（ＳＯ2ＣＦ3）3、ＬｉＮ（ＳＯ3ＣＦ3）2等が挙げられ、これらのうち少なくとも１種以上のものを用いることができる。また、ＣＯ2 、 Ｎ2Ｏ、ＣＯ、ＳＯ2 等のガスやポリサルファイドＳx 2-など負極表面にリチウムイオンの効率よい充放電を可能にする良好な皮膜を生成する添加剤を任意の割合で上記単独又は混合溶媒に添加してもよい。
【００４６】
高分子固体電解質を使用する場合にも、この高分子に公知のものを用いることができ、特にリチウムイオンに対するイオン導電性の高い高分子を使用することが好ましく、例えば、ポリエチレンオキサイド、ポリプロピレンオキサイド、ポリエチレンイミン等が好ましく使用され、またこの高分子に対して上記の溶質と共に、上記の溶媒を加えてゲル状電解質として使用することも可能である。
【００４７】
無機固体電解質を使用する場合にも、この無機物に公知の結晶質、非晶質固体電解質を用いることができる。結晶質の固体電解質としては例えば、ＬｉＩ、Ｌｉ3Ｎ、Ｌｉ1+xＭxＴｉ2-x（ＰＯ4）3（Ｍ＝Ａｌ，Ｓｃ，Ｙ，Ｌａ）、Ｌｉ0.5-3xＲＥ0.5+xＴｉＯ3（ＲＥ＝Ｌａ，Ｐｒ，Ｎｄ，Ｓｍ）等が挙げられ、非晶質の固体電解質としては例えば、4.9 ＬｉＩ−34.1Ｌｉ2Ｏ−６１Ｂ2Ｏ5，33.3Ｌｉ2Ｏ−66.7ＳｉＯ2 等の酸化物ガラスや0.45ＬｉＩ−0.37Ｌｉ2Ｓ−0.26Ｂ2Ｓ3，0.30ＬｉＩ−0.42Ｌｉ2Ｓ−0.28ＳｉＳ2等の硫化物ガラス等が挙げられる。これらのうち少なくとも１種以上のものを用いることができる。
【００４８】
以下実施例によって本発明の方法をさらに具体的に説明するが、本発明はこれらにより何ら制限されるものではない。
【００４９】
【実施例】
実施例１
Ｌｉ1.04Ｍｎ1.85Ａｌ0.11Ｏ4なる、Ｍｎサイトの一部がＬｉとＡｌで置換された立方晶スピネル構造を有するリチウムマンガン酸化物を使用し、これにＭｏＳ2を、リチウムマンガン酸化物に対して１モル％の割合で添加混合したものを正極材料として用いた。なお、ここで用いたリチウムマンガン酸化物のＢＥＴ比表面積は０．９ｍ2／ｇ、５分間の超音波分散後レーザー回折式粒度分布測定から求めたメジアン径は７．４μｍであった。また、ＭｏＳ2のＢＥＴ比表面積は０．７ｍ2／ｇ、５分間の超音波分散後、レーザー回折式粒度分布測定から求めたメジアン径は２３．１μｍであった。
【００５０】
実施例２
実施例１と同様のリチウムマンガン酸化物を使用し、これにＷＳ2をリチウムマンガン酸化物に対して１モル％の割合で添加混合したものを正極材料として用いた。なお、ここで用いたＷＳ2のＢＥＴ比表面積は１．８ｍ2／ｇ、５分間の超音波分散後、レーザー回折式粒度分布測定から求めたメジアン径は９．４μｍであった。
【００５１】
実施例３
実施例１と同様のリチウムマンガン酸化物を使用し、これにＣｒ2Ｓ3をリチウムマンガン酸化物に対して１モル％の割合で添加混合したものを正極材料として用いた。なお、ここで用いたＣｒ2Ｓ3のＢＥＴ比表面積は２．０ｍ2／ｇ、５分間の超音波分散後、レーザー回折式粒度分布測定から求めたメジアン径は１８．６μｍであった。
【００５２】
比較例１
実施例１と同様のリチウムマンガン酸化物を、そのまま正極材料とした。即ち、６族元素のカルコゲン化物を使用しなかった。
比較例２
６族元素のカルコゲン化物としてＭｏＳ2をリチウムマンガン酸化物に対して１モル％の割合で添加混合し、６５０℃で１０時間熱処理したものを用いたこと以外実施例１と同様にして正極材料を得た。
【００５３】
比較例３
カルコゲン化物としてＴｉＳ2をリチウムマンガン酸化物に対して５モル％用いたこと以外実施例１と同様にして正極材料を得た。
参考例１
組成ＬｉＮｉ1.8Ｃｏ0.15Ａｌ0.05Ｏ2なるリチウムニッケル酸化物を正極材料として用いた。
【００５４】
試験例（電池評価）
以下の方法で本発明の実施例、比較例及び参考例の電池評価を行った。
１. 正極の作成と容量確認
正極材料を７５重量％ 、アセチレンブラックを２０重量％、ポリテトラフロロエチレンパウダーを５重量％の割合で秤量したものを乳鉢で十分混合し、薄くシート状にし、９ｍｍφ、１２ｍｍφのポンチで打ち抜いた。この際全体重量は各々約８ｍｍg、約１８ｍｇになるように調整した。これをＡｌのエキスパンドメタルに圧着して正極とした。
【００５５】
次に、正極の容量を確認した。
即ち、９ｍｍφに打ち抜いた前記正極を試験極、Ｌｉ金属を対極として電池セルを組んだ。この電池セルに０．５ｍＡ／ｃｍ2の定電流充電すなわち、正極からリチウムイオンを放出させる反応を上限４．３５Ｖで行い、ついで０．５ｍＡ／ｃｍ2の定電流放電すなわち正極にリチウムイオンを吸蔵させる試験を下限３．２Ｖで行った。この際の正極活物質単位重量当たりの初期充電容量をＱs(C)（ｍＡｈ／ｇ） 、初期放電容量をＱs(D)（ｍＡｈ／ｇ）とした。なお、正極活物質にリチウムニッケル酸化物を用いた参考例の場合の正極の容量評価は、上限電圧を４．２Ｖとした。
【００５６】
２. 負極の作成と容量確認
負極活物質としての平均粒径約８〜１０μｍ の黒鉛粉末(ｄ００２＝３．３５Å)と、バインダーとしてのポリフッ化ビニリデンとを重量比で９２．５：７．５の割合で秤量し、これをＮ−メチルピロリドン溶液中で混合し、負極合剤スラリーとした。このスラリーを２０μｍ厚さの銅箔の片面に塗布し、乾燥して溶媒を蒸発させた後、１２ｍｍφに打ち抜き、０．５ｔｏｎ／ｃｍ2でプレス処理をしたものを負極とした。
【００５７】
なお、この負極を試験極、Ｌｉ金属を対極として電池セルを組み、０．２ｍＡ／ｃｍ2の定電流で負極にＬｉイオンを吸蔵させる試験を下限０Ｖで行った際の負極活物質単位重量当たりの初期吸蔵容量をＱf（ｍＡｈ／ｇ）とした。
３. 電池セルの組立
コイン型セルを使用して、電池性能を評価した。即ち、正極缶の上に１２ｍｍφに打ち抜いた前記正極を置き、その上にセパレータとして２５μｍの多孔性ポリエチレンフィルムを置き、ポリプロピレン製ガスケットで押さえた後、前記負極を置き、厚み調整用のスペーサーを置いた後、非水電解液溶液として、１モル／リットルの六フッ化リン酸リチウム( ＬｉＰＦ6)を溶解させたエチレンカーボネート( ＥＣ) とジエチルカーボネート( ＤＥＣ) との体積分率３：７の混合溶媒を用い、これを電池内に加えて充分しみ込ませた後、負極缶を載せ電池を封口した。
【００５８】
なお、この時、正極活物質の重量と負極活物質重量のバランスは、ほぼ
【００５９】
【数１】
正極活物質量〔ｇ〕／負極活物質量〔ｇ〕＝（Ｑf／１．２）／Ｑs(C)
となるよう設定した。
４. 試験方法
この様に得られた電池の高温特性を比較するため、電池の１時間率電流値、即ち１Ｃを
【００６０】
【数２】
１Ｃ[mA]＝Ｑs(D)×正極活物質量〔ｇ〕
と設定し、以下の試験を行った。
まず室温で定電流０．２Ｃ充放電２サイクルおよび定電流１Ｃ充放電１サイクルを行い、次に５０℃の高温で定電流０．２Ｃ充放電１サイクル、ついで定電流１Ｃ充放電１００サイクルの試験を行った。なお充電上限は４．２Ｖ（ただし、リチウムニッケル酸化物を用いた参考例の正極の評価の場合には上限電圧４．１Ｖ）、下限電圧は３．０Ｖとした。
【００６１】
この時５０℃での１Ｃ充放電１００サイクル試験における１サイクル目放電容量Ｑh(1)に対する、１００サイクル目の放電容量Ｑh(100)の割合を高温サイクル容量維持率Ｐ、即ち、
【００６２】
【数３】
Ｐ〔％〕＝｛Ｑh(100)／Ｑh(1)｝×100
とし、この値で電池の高温特性を比較した。
実施例、比較例及び参考例における、５０℃での１Ｃ充放電１００サイクル試験での初期放電容量、及び高温サイクル容量維持率Ｐを表−１に示す。
【００６３】
【表１】
実施例１と比較例１乃至２とを比較すると、二硫化モリブデンを添加混合のみすることによって高温でのサイクル特性が向上することが分かる。また、実施例１乃至３と比較例３とを比較すると、同じ硫化物であっても、本発明のカルコゲニドが特異的に良好な高温サイクル特性を示すことが分かる。更に、比較例１と比較例２を比較すると、比較例２は６族元素のカルコゲン化物の添加効果（高温サイクルの改善）が失われていることがわかる。これは熱処理により６族元素のカルコゲン化物が分解リチウム遷移金属酸化物と反応するなどして効果を発揮し難い化合物へと変質してしまったためと考えられる。これらのことは、特開平８−２５０１２０号公報に記載された様々なカルコゲン化物の中から特定のものだけが優れた高温サイクル特性を示していること、特開平８−２５０１２０号公報に記載の方法では高温サイクルの改善は達成できないことを示している。
【００６４】
さらにまた、参考例１を見ると、リチウムニッケル酸化物を活物質として用いた場合には、そもそも高い放電容量と良好な高温サイクル特性とを示しており、従って、リチウムニッケル酸化物を用いた場合にはそもそも高温サイクル特性を向上させる動機付けそのものが少ないことが分かる。このことは、リチウムマンガン酸化物とリチウムニッケル酸化物とを全くの同列に扱っている特開平８−２５０１２０号公報においては、高温サイクル特性を向上させることは注目されておらず、従って、リチウムマンガン酸化物の特有の問題たる高温サイクル特性の問題を解決する手段を全く示唆していないことを示している。
【００６５】
【発明の効果】
本発明により、容量やレート特性、サイクル特性に優れ、安全性や生産性に優れた電池に使用できる正極材料を提供することができる。特に、リチウムマンガン酸化物を活物質に用いた場合に特有の問題である高温でのサイクル特性を向上させることができる。
【図面の簡単な説明】
【図１】 サイクル−放電容量相関図[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a positive electrode material for a lithium secondary battery, and further relates to a positive electrode and a lithium secondary battery using the positive electrode material.
[0002]
[Prior art]
By using a carbon material or the like capable of occluding and releasing lithium ions instead of metallic lithium as the negative electrode active material, the safety has been greatly improved, and the lithium secondary battery has entered the practical stage.
LiCoO as a positive electrode active material for lithium secondary batteries2And LiNiO2, LiMn2OFourLithium transition metal complex oxides such as have entered the practical stage. In particular, LiMn2OFourThe manganese-based positive electrode active material made of lithium manganese oxide has the advantage that the component manganese is more reserve than cobalt and nickel, is inexpensive, and has high safety in overcharging. is doing. LiCoO2Lithium cobalt oxide such as LiNiO2Since lithium nickel oxides such as these have practically sufficient high-temperature cycle characteristics, the problem of deterioration of the cycle characteristics in a high-temperature environment is a particular problem of manganese.
[0003]
In order to overcome the above problems, studies aimed at improving cycle characteristics in a high temperature environment have been vigorously conducted and reported. For example, in J. Electrochem. Soc., Vol. 145, No. 8 (1998) 2726-2732, a part of Mn is substituted with other elements such as Ga and Cr, and in Electrochemical Society Proceedings Volume 97-18.494, Mn The result of improving the stability of the crystal structure by substituting partly with Co or substituting part of oxygen with F has the effect of improving the high-temperature cycle characteristics. However, these are the results when metallic lithium is used as the negative electrode, and the actual situation is that a sufficient effect is not obtained in combination with a practical negative electrode material such as a carbon material.
[0004]
In addition, manganese-based lithium secondary batteries have been pointed out as a problem of high-temperature storage deterioration and high-temperature cycle deterioration because manganese is likely to elute in a high-temperature environment. In addition, studies have been made to add a substance having an effect of suppressing Mn elution to the material. However, in these conventional techniques, the cycle characteristics under a high temperature environment have not yet reached a practical level.
[0005]
JP-A-8-250120 describes that a lithium-transition metal composite oxide having a coating made of a specific chalcogenide on the particle surface is used as a positive electrode active material. However, it was insufficient in terms of improving the high-temperature cycle characteristics.
[0006]
[Problems to be solved by the invention]
A lithium secondary battery using lithium manganese oxide as a positive electrode active material has a lower charge / discharge capacity than when lithium cobalt oxide or lithium nickel oxide is used. Therefore, deep charge / discharge is repeated in order to draw out the capacity as much as possible. However, especially the lithium manganese oxide at the charging end from which almost all lithium has been released becomes specifically active in a high temperature environment, not only the alteration of the active material itself, but also the elution of manganese, the decomposition of the electrolyte, and the negative electrode surface. It is considered to have various adverse effects such as destruction of the formed film.
[0007]
The present invention was made to solve the above-mentioned problems peculiar to lithium manganese oxide, and its purpose is specific activation under a high temperature environment, which is a problem peculiar to lithium manganese oxide. An object of the present invention is to provide a lithium secondary battery with improved high-temperature cycle characteristics by reducing or reducing the state.
[0008]
[Means for Solving the Problems]
In order to solve such a problem, the present inventors consider that it is essential to reduce the catalytic activity using an additive that can stably exist inside the battery in a high-temperature environment, and find such an additive. As a result of intensive studies, it has been found that the presence of chalcogenides of group 6 elements (chromium chalcogenide, molybdenum chalcogenide, tungsten chalcogenide, etc.) improves the high-temperature cycle characteristics, leading to the present invention.
[0009]
The reason why the chalcogenide of the specific metal element described above has specifically improved the effect is that the activation state of the positive electrode active material in a high-temperature environment is relaxed and reduced, and the electrolyte solution and the negative electrode surface On the other hand, I think that it may have caused some kind of positive effect, such as acting as a stabilizer.
That is, the gist of the present invention resides in a positive electrode material for a lithium secondary battery, characterized in that it contains lithium manganese oxide as an active material and is physically mixed with a chalcogenide of a group 6 element.
[0010]
As a preferable gist of the present invention, the positive electrode material in which the Group 6 element is Cr, Mo or W; the positive electrode material in which the chalcogenide is sulfide; the mixing ratio of the chalcogenide to lithium manganese oxide is 0. 1 to 20 mol% of the above positive electrode material; the lithium manganese oxide is a lithium manganese oxide in which a part of the manganese site is replaced with another element; the other element replacing the manganese site is Examples of the positive electrode material include at least one metal element selected from the group consisting of Al, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Mg, Ga, and Zr.
[0011]
Further, another gist of the present invention is a positive electrode for a lithium secondary battery characterized by having the positive electrode material and a binder, wherein the lithium manganese oxide and the chalcogenide are present in a dispersed state. A positive electrode for a lithium secondary battery is preferable.
Furthermore, as another gist of the present invention, a lithium secondary battery comprising the above positive electrode, a negative electrode, and an electrolyte layer; a positive electrode using lithium manganese oxide as an active material, a negative electrode, and an electrolyte In the lithium secondary battery having a layer, a lithium secondary battery in which a chalcogenide of a Group 6 element is included in the negative electrode and / or the electrolyte layer is used, and the active material of the negative electrode is preferably a carbon material.
[0012]
JP-A-8-250120 describes that a lithium-transition metal composite oxide having a coating made of a specific chalcogenide on the particle surface is used as a positive electrode active material. However, the above-mentioned known document does not focus on a specific positive electrode material called lithium manganese oxide, does not solve the problems peculiar to manganese, and further, chalcogenide of group 6 element defined in the present invention. It is also not described that a material obtained by physical mixing without heat treatment exhibits a particularly excellent effect.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, although this invention is demonstrated using suitable one Embodiment, unless it exceeds the meaning of this invention, it is not limited to the following.
Examples of chalcogenides of group 6 elements used in the present invention include chromium, molybdenum, tungsten sulfides, selenides, and tellurides. Specifically, CrS, Cr7S8, CrFiveS6, CrThreeSFour, Cr2SThree, CrSe, Cr2SeThree, Cr7Se8, CrThreeSeFour, CrFiveSe8, Cr7Se12, CrTe, Cr7Te8, CrFiveTe6, CrThreeTeFour, Cr2TeThree, CrFiveTe8, MoS2, Mo2SFive, MoSThree, MoThreeSFour, MoSFour, MoSe, MoTe, WS2, WSThree, WSe2, WTe2Is mentioned. Of course, these compounds may be used in combination. Further, a part of the group 6 element of the chalcogenide of the group 6A element may be substituted with another element, and a part of the chalcogen atom may be substituted with another atom such as an oxygen element. . Further, it may be non-stoichiometric. In addition, when substituting a group 6 element or a chalcogen atom with another element, 2 or more types of substitution elements may be sufficient.
[0014]
As a particularly preferred group 6 element chalcogenide, specifically, Cr2SThree, MoS2And WS2 And at least one selected from the group consisting of: Of the chalcogen elements such as sulfur, selenium and tellurium, sulfur is most preferable in consideration of resources, toxicity and the like.
The amount of group 6 element chalcogenide used is usually 0.1 mol% or more, preferably 2 mol% or more, and usually 20 mol% or less, preferably 10 mol% or less based on lithium manganese oxide. More preferably, it is 8 mol% or less, and most preferably 6 mol% or less. If the amount used is increased, the discharge capacity and the high-temperature cycle characteristics may be reduced. If the amount used is decreased, the effect of improving the high-temperature cycle may be difficult to obtain.
[0015]
The average particle size and specific surface area of the chalcogenide of the group 6 element is not a problem as long as it does not greatly deviate from the average particle size and specific surface area of the active material usually used for the positive electrode, but the contact efficiency with the lithium manganese oxide is improved. Therefore, it is preferable that the average particle size is not more than the average particle size of the lithium manganese oxide and the specific surface area is not less than the specific surface area of the lithium manganese oxide.
[0016]
The specific surface area of the chalcogenide of group 6 element used in the present invention is usually 0.3 m.2/ G or more, preferably 1 m2/ G or more, most preferably 1.5 m2/ G or more, usually 100m2/ G or less, preferably 50 m2/ G or less, most preferably 20 m2/ G or less. If the specific surface area is too small, there may be a case where a sufficient addition effect may not be exhibited. If the specific surface area is too large, the specific surface area may become chemically unstable and adversely affect. The specific surface area is measured according to the BET method.
[0017]
The average particle size of the chalcogenide of the 6A group element used in the present invention is usually 0.05 to 30 μm, preferably 0.5 to 10 μm, and most preferably 1 to 5 μm. If the average particle size is too small, it may become chemically unstable per se, which may adversely affect it. If it is too large, a sufficient additive effect may not be exhibited, or a problem may occur during electrode preparation.
[0018]
The ratio of the BET specific surface area of the lithium manganese oxide used in the present invention to the BET specific surface area of the chalcogenide of the group 6 element is usually 0.01 to 1, preferably 0.05 to 0.8, more preferably 0. .1 to 0.5. If this ratio deviates from the above range, it may be difficult to obtain desired performance.
The ratio of the average particle size of the lithium manganese oxide used in the present invention to the average particle size of the chalcogenide of the group 6 element is usually 1 to 20, preferably 1.5 to 10, and more preferably 2 to 5. If this ratio deviates from the above range, it may be difficult to obtain desired performance.
[0019]
In order to allow a chalcogenide of a group 6 element to exist in a positive electrode material containing lithium manganese oxide, it is preferable to employ physical mixing. The term “physical mixing” in the present invention means simply mixing a plurality of substances at a high temperature that causes a chemical change such that the effect of adding a chalcogenide of a group 6 element (improvement of high temperature cycle) is lost. This means mixing without heat treatment. A material in which a chalcogenide of a group 6 element is dispersed in a positive electrode material by mixing a plurality of substances is preferable, and it is preferable that the material is uniformly dispersed. Whether or not the effect of adding the chalcogenide of the group 6 element (improvement of the high temperature cycle) is lost can be evaluated by the battery according to the test example (battery evaluation) described later in the column of the example. If the improvement of the high temperature cycle is not observed as compared with the case where no chalcogenide is added (Comparative Example 1), it is judged that the addition effect is lost. As a chemical change in which the effect of adding a chalcogenide of a group 6 element is reduced or lost, a decomposition oxidation reaction of the chalcogenide of a group 6 element can be considered. The coating by heat treatment or the like has a high possibility that the chalcogenide of the group 6 element is altered, and there is a possibility that the intended effect may be lost. On the other hand, physical mixing is preferable because it is a simple addition method, has no influence of alteration, and can sufficiently exhibit its original effect. The physical mixing may be dry mixing or wet mixing. A mortar, a ball mill, a jet mill, a Redige mixer, etc. can be used for physical mixing.
[0020]
The lithium manganese oxide used in the present invention only needs to be capable of reversibly occluding and releasing Li as an active material. For example, the general formula LiMn2OFourLiMnO2Can be mentioned. The lithium manganese oxide having a spinel structure (general formula LiMn) is preferable in that the effect of the present invention is remarkable.2OFour). These may have a small amount of oxygen deficiency and non-stoichiometry. In addition, a part of the oxygen site may be substituted with sulfur or a halogen element.
[0021]
In the lithium manganese oxide used in the present invention, a part of the manganese site is preferably substituted with another element. As a result, the stability of the crystal structure can be improved, and by combining this with the physical mixing of chalcogenides of group 6 elements, the high-temperature cycle characteristics can be improved synergistically.
Examples of other elements to be substituted (hereinafter referred to as “substitution elements”) include Al, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Mg, Ga, and Zr, preferably Al. Cr, Fe, Co, Ni, Mg, Ga, more preferably Al. The manganese site may be substituted with two or more other elements.
[0022]
The substitution ratio with the substitution element is usually 2.5 mol% or more of manganese, preferably 5 mol% or more of Mn, and usually 30 mol% or less of Mn, preferably 20 mol% or less of Mn. If the replacement ratio is too small, the effect of improving the high-temperature cycle may not be sufficient, and if it is too large, the capacity of the battery may be reduced.
[0023]
Lithium manganese oxide can be produced by various conventionally known methods. For example, lithium, manganese and, if necessary, a starting material containing a substitution element are mixed, and then fired and cooled in the presence of oxygen. Can be manufactured. Lithium manganese oxide having a layered structure can also be produced by various conventionally known methods. For example, after mixing starting materials containing lithium, manganese, and, if necessary, a substitution element, under a reducing atmosphere It can be manufactured by firing and cooling.
[0024]
In addition, when producing a lithium manganese oxide in which a part of the manganese site is substituted with another element, the lithium manganese oxide in which the manganese site is not substituted without using the starting material containing the substitution element in the above production method. And reacting the lithium manganese oxide in an aqueous solution, molten salt or steam of a starting material containing a substituted metal element, and then diffusing the substituted element into the lithium manganese composite oxide particles as necessary. Therefore, a part of the manganese site can be replaced with a replacement element by performing heat treatment again.
[0025]
Lithium compounds used as starting materials include Li2COThree, LiNOThree, LiOH, LiOH · H2O, CHThreeCOOLi, Li2O, Licarboxylic acid Li, Citric acid Li, Fatty acid Li, Alkyllithium, Halide, etc. are mentioned.
Preferably LiOH.H2O, Licarboxylic acid Li, Citric acid Li, Fatty acid Li, Li2COThreeIs mentioned.
[0026]
As a manganese compound used as a starting material, Mn2OThree, MnO2Manganese oxides such as MnCOThree, Mn (NOThree)2 , MnSOFourAnd manganese salts such as manganese acetate, manganese dicarboxylate, manganese citrate, and manganese fatty acid, oxyhydroxides, hydroxides, halides, and the like. Mn2OThreeAs MnCOThreeAnd MnO2You may use what was produced by heat-processing such compounds. Preferably Mn2OThreeAnd oxyhydroxide.
[0027]
Examples of the substitution element compound include oxides, hydroxides, oxyhydroxides, nitrates, sulfates, carbonates, dicarboxylates, fatty acid salts, and ammonium salts. These starting materials are usually mixed by methods such as wet mixing, dry mixing, ball milling, and coprecipitation. A pulverization step may be added before and after mixing and during mixing.
[0028]
As a method for firing and cooling the spinel type lithium manganese oxide, for example, after calcination, firing is performed in an oxygen atmosphere at a temperature of about 600 to 900 ° C., and then a rate of 10 ° C./min or less to about 500 ° C. And a method of annealing at a temperature of about 600 to 900 ° C. after air calcination in air or an oxygen atmosphere, and then annealing at a temperature of about 400 ° C. in an oxygen atmosphere. The firing and cooling conditions are described in detail in JP-A-9-306490, JP-A-9-306493, JP-A-9-259880, and the like.
[0029]
Examples of the method for firing the layered lithium manganese oxide include a method of firing at a temperature of about 900 to 1000 ° C. in a reducing atmosphere such as nitrogen.
The specific surface area of the lithium manganese oxide used in the present invention is preferably 0.3 m.2/ G or more, more preferably 0.5 m2/ G or more, and preferably 1.5 m2/ G or less, more preferably 1.0 m2/ G or less. If the specific surface area is too small, the rate characteristics and capacity may be reduced. If the specific surface area is too large, an undesirable reaction with the electrolytic solution or the like may be caused, and the cycle characteristics may be deteriorated. The specific surface area is measured according to the BET method.
[0030]
The average particle diameter of the lithium manganese oxide used in the present invention is usually 0.1 to 30 μm, preferably 0.2 to 10 μm, more preferably 0.3 to 5 μm. If the average particle size is too small, the cycle deterioration of the battery may increase or a safety problem may occur. If the average particle size is too large, the internal resistance of the battery may increase and output may be difficult to output.
[0031]
The positive electrode material of the present invention can be used for a positive electrode of a lithium secondary battery.
The positive electrode of the present invention has the positive electrode material and a binder. Preferably, the positive electrode includes a positive electrode current collector and a positive electrode layer containing a positive electrode material and a binder. It is preferable that the lithium manganese oxide and the chalcogenide of the group 6 element in the positive electrode layer be dispersed and present from the viewpoint that the effects of the present invention can be sufficiently exerted. Such a positive electrode layer is obtained by applying a lithium manganese oxide, a chalcogenide of a group 6 element, a binder (binder) described later, and a slurry of a conductive agent as necessary in a solvent to a positive electrode current collector, It can be manufactured by drying. Prior to slurry preparation, lithium manganese oxide and a chalcogenide of a group 6 element can be physically mixed in advance.
[0032]
In the positive electrode, LiCoO2, LiNiO2LiFeO2, LiCoPOFourLiFePOFourAs described above, an active material capable of occluding and releasing lithium ions other than lithium manganese oxide may be further contained.
The proportion of the active material in the positive electrode is usually 10% by weight or more, preferably 30% by weight or more, more preferably 50% by weight or more, and usually 99.9% by weight or less, preferably 99% by weight or less.
[0033]
Examples of the binder used for the positive electrode include polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, EPDM (ethylene-propylene-diene terpolymer), SBR (styrene-butadiene rubber), Examples thereof include NBR (acrylonitrile-butadiene rubber), fluororubber, polyvinyl acetate, polymethyl methacrylate, polyethylene, and nitrocellulose. The ratio of the binder in the positive electrode layer is usually 0.1% by weight or more, preferably 1% by weight or more, more preferably 5% by weight or more, and usually 80% by weight or less, preferably 60% by weight or less, more preferably It is 40% by weight or less, most preferably 10% by weight or less. If the proportion of the binder is too low, the active material cannot be sufficiently retained, and the mechanical strength of the positive electrode may be insufficient, and the battery performance such as cycle characteristics may be deteriorated. Sometimes.
[0034]
The positive electrode layer usually contains a conductive agent in order to increase conductivity. Examples of the conductive agent include graphite such as natural graphite and artificial graphite, carbon black such as acetylene black, and amorphous carbon such as needle coke. The proportion of the conductive agent in the positive electrode is usually 0.01% by weight or more, preferably 0.1% by weight or more, more preferably 1% by weight or more, and usually 50% by weight or less, preferably 30% by weight or less. Preferably it is 15 weight% or less. If the proportion of the conductive agent is too low, the conductivity may be insufficient, and conversely if it is too high, the battery capacity may be reduced.
[0035]
As the slurry solvent, an organic solvent that dissolves or disperses the binder is usually used. For example, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, NN-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran and the like can be mentioned. Moreover, a dispersing agent, a thickener, etc. can be added to water, and an active material can also be slurried with latex, such as SBR.
[0036]
The thickness of the active material layer is usually about 10 to 200 μm.
As the material of the current collector used for the positive electrode, aluminum, stainless steel, nickel-plated steel or the like is used, and preferably aluminum.
Note that the active material layer obtained by coating and drying is preferably consolidated by a roller press or the like in order to increase the packing density of the active material.
[0037]
A lithium ion secondary battery can be obtained using the positive electrode material of the present invention. The lithium ion secondary battery of the present invention contains the active material in a positive electrode, but usually has the positive electrode, a negative electrode, and an electrolyte.
The negative electrode active material used for the negative electrode of the secondary battery of the present invention may be a lithium alloy such as lithium or a lithium aluminum alloy alloy, but a carbon material that can occlude and release lithium with higher safety is used. preferable.
[0038]
The carbon material is not particularly limited, but graphite, coal-based coke, petroleum-based coke, coal-based pitch carbide, petroleum-based pitch carbide, or carbide obtained by oxidizing these pitches, needle coke, pitch coke, phenol resin And carbides such as crystalline cellulose and the like, carbon materials obtained by partially graphitizing these, furnace black, acetylene black, pitch-based carbon fibers, and the like.
[0039]
In addition, SnO, SnO2, Sn1-xMxO (M = Hg, P, B, Si, Ge or Sb, where 0 ≦ x <1), SnThreeO2(OH)2 , Sn3-xMxO2(OH)2(M = Mg, P, B, Si, Ge, Sb or Mn, where 0 ≦ x <3), LiSiO2, SiO2Or LiSnO2Etc.
In addition, you may use as a 2 or more types of mixture chosen from these.
[0040]
The negative electrode is usually formed by forming an active material layer on a current collector as in the case of the positive electrode. As the binder used at this time, the conductive agent and the slurry solvent used as necessary, the same ones used for the positive electrode can be used. In addition, as the negative electrode current collector, copper, nickel, stainless steel, nickel-plated steel, or the like is used, and copper is preferably used.
[0041]
When a separator is used between the positive electrode and the negative electrode, a microporous polymer film is used, and polyolefins such as nylon, cellulose acetate, nitrocellulose, polysulfone, polyacrylonitrile, polyvinylidene fluoride, polypropylene, polyethylene, and polybutene A polymer is used. The chemical and electrochemical stability of the separator is an important factor. In this respect, a polyolefin-based polymer is preferable, and it is preferable that the polymer is made of polyethylene in view of the self-occluding temperature which is one of the purposes of the battery separator.
[0042]
In the case of a polyethylene separator, ultrahigh molecular weight polyethylene is preferable from the viewpoint of high temperature shape maintenance, and the lower limit of the molecular weight is preferably 500,000, more preferably 1,000,000, and most preferably 1.5 million. On the other hand, the upper limit of the molecular weight is preferably 5 million, more preferably 4 million, and most preferably 3 million. This is because if the molecular weight is too large, the pores of the separator may not close when heated because the fluidity is too low.
[0043]
In addition, as the electrolyte in the lithium secondary battery of the present invention, for example, known organic electrolytes, polymer solid electrolytes, gel electrolytes, inorganic solid electrolytes, and the like can be used. Among them, organic electrolytes are preferable. The organic electrolyte is composed of an organic solvent and a solute.
The organic solvent is not particularly limited. For example, carbonates, ethers, ketones, sulfolane compounds, lactones, nitriles, chlorinated hydrocarbons, ethers, amines, esters, amides. A phosphoric acid ester compound or the like can be used. Typical examples of these are dimethyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, vinylene carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 4-methyl-2-pentanone, 1,2- Dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile, benzonitrile, butyronitrile, One or two or more kinds of mixed solvents such as valeronitrile, 1,2-dichloroethane, dimethylformamide, dimethyl sulfoxide, trimethyl phosphate, and triethyl phosphate can be used.
[0044]
The above organic solvent preferably contains a high dielectric constant solvent in order to dissociate the electrolyte. Here, the high dielectric constant solvent means a compound having a relative dielectric constant of 20 or more at 25 ° C. Among the high dielectric constant solvents, it is preferable that the electrolyte solution contains ethylene carbonate, propylene carbonate, and compounds obtained by substituting hydrogen atoms thereof with other elements such as halogen or alkyl groups. The proportion of the high dielectric constant compound in the electrolytic solution is preferably 20% by weight or more, more preferably 30% by weight or more, and most preferably 40% by weight or more. This is because if the content of the compound is small, desired battery characteristics may not be obtained.
[0045]
Further, the solute dissolved in this solvent is not particularly limited, but any conventionally known solute can be used, and LiClO.Four, LiAsF6, LiPF6, LiBFFour, LiB (C6HFive)Four , LiCl, LiBr, CHThreeSOThreeLi, CFThreeSOThreeLi, LiN (SO2CFThree)2, LiN (SO2C2FFive)2, LiC (SO2CFThree)Three, LiN (SOThreeCFThree)2Among them, at least one of them can be used. CO2 , N2O, CO, SO2 Such as gas and polysulfide Sx 2-For example, an additive that produces a good film that enables efficient charge and discharge of lithium ions on the negative electrode surface may be added to the above-mentioned single or mixed solvent in any proportion.
[0046]
Even when a polymer solid electrolyte is used, a known polymer can be used for this polymer, and it is particularly preferable to use a polymer having high ion conductivity with respect to lithium ions. For example, polyethylene oxide, polypropylene oxide, Polyethyleneimine or the like is preferably used, and the polymer can be used as a gel electrolyte by adding the solvent to the polymer together with the solute.
[0047]
Even when an inorganic solid electrolyte is used, a known crystalline or amorphous solid electrolyte can be used for this inorganic substance. Examples of crystalline solid electrolytes include LiI, LiThreeN, Li1 + xMxTi2-x(POFour)Three(M = Al, Sc, Y, La), Li0.5-3xRE0.5 + xTiOThree(RE = La, Pr, Nd, Sm) and the like. As an amorphous solid electrolyte, for example, 4.9 LiI-34.1Li2O-61B2OFive, 33.3Li2O-66.7SiO2 Oxide glass such as 0.45LiI-0.37Li2S-0.26B2SThree, 0.30LiI-0.42Li2S-0.28SiS2And sulfide glass. Of these, at least one of them can be used.
[0048]
The method of the present invention will be described more specifically with reference to the following examples. However, the present invention is not limited to these examples.
[0049]
【Example】
Example 1
Li1.04Mn1.85Al0.11OFourLithium manganese oxide having a cubic spinel structure in which a part of the Mn site is substituted with Li and Al is used.2Was added and mixed at a ratio of 1 mol% with respect to lithium manganese oxide as a positive electrode material. The BET specific surface area of the lithium manganese oxide used here is 0.9 m.2/ G, The median diameter determined from the laser diffraction particle size distribution measurement after ultrasonic dispersion for 5 minutes was 7.4 μm. MoS2BET specific surface area of 0.7m2/ G After 5 minutes of ultrasonic dispersion, the median diameter determined from laser diffraction particle size distribution measurement was 23.1 μm.
[0050]
Example 2
The same lithium manganese oxide as in Example 1 was used, and WS2Was added and mixed in a proportion of 1 mol% with respect to lithium manganese oxide as a positive electrode material. The WS used here2BET specific surface area of 1.8m2/ G After 5 minutes of ultrasonic dispersion, the median diameter determined from laser diffraction particle size distribution measurement was 9.4 μm.
[0051]
Example 3
The same lithium manganese oxide as in Example 1 was used, and Cr2SThreeWas added and mixed in a proportion of 1 mol% with respect to lithium manganese oxide as a positive electrode material. In addition, Cr used here2SThreeBET specific surface area of 2.0m2/ G After 5 minutes of ultrasonic dispersion, the median diameter determined from laser diffraction particle size distribution measurement was 18.6 μm.
[0052]
Comparative Example 1
The same lithium manganese oxide as in Example 1 was used as the positive electrode material as it was. That is, no chalcogenide of group 6 element was used.
Comparative Example 2
MoS as chalcogenide of group 6 element2Was added and mixed at a ratio of 1 mol% with respect to lithium manganese oxide, and a positive electrode material was obtained in the same manner as in Example 1 except that a material heat-treated at 650 ° C. for 10 hours was used.
[0053]
Comparative Example 3
TiS as chalcogenide2Was used in the same manner as in Example 1 except that 5 mol% was used relative to lithium manganese oxide.
Reference example 1
Composition LiNi1.8Co0.15Al0.05O2The resulting lithium nickel oxide was used as the positive electrode material.
[0054]
Test example (battery evaluation)
The battery evaluation of the Example of this invention, the comparative example, and the reference example was performed with the following method.
1. Creation of positive electrode and capacity check
A mixture of 75% by weight of the positive electrode material, 20% by weight of acetylene black and 5% by weight of polytetrafluoroethylene powder was thoroughly mixed in a mortar, made into a thin sheet, and punched out with punches of 9 mmφ and 12 mmφ. At this time, the total weight was adjusted to about 8 mmg and about 18 mg, respectively. This was crimped to Al expanded metal to obtain a positive electrode.
[0055]
Next, the capacity of the positive electrode was confirmed.
That is, a battery cell was assembled using the positive electrode punched out to 9 mmφ as a test electrode and a Li metal as a counter electrode. 0.5mA / cm in this battery cell2Constant current charging, that is, a reaction of releasing lithium ions from the positive electrode at an upper limit of 4.35 V, and then 0.5 mA / cm2The constant current discharge, that is, the test for occluding lithium ions in the positive electrode was performed at the lower limit of 3.2V. The initial charge capacity per unit weight of the positive electrode active material at this time was Qs (C) (mAh / g), and the initial discharge capacity was Qs (D) (mAh / g). In addition, the capacity | capacitance evaluation of the positive electrode in the case of the reference example which used lithium nickel oxide for the positive electrode active material set the upper limit voltage to 4.2V.
[0056]
2. Preparation of negative electrode and capacity check
A graphite powder (d002 = 3.35 Å) having an average particle diameter of about 8 to 10 μm as a negative electrode active material and polyvinylidene fluoride as a binder were weighed at a ratio of 92.5: 7.5, and this was measured. The mixture was mixed in an N-methylpyrrolidone solution to obtain a negative electrode mixture slurry. This slurry was applied to one side of a 20 μm-thick copper foil, dried to evaporate the solvent, then punched out to 12 mmφ, 0.5 ton / cm2The negative electrode was pressed.
[0057]
A battery cell was assembled with this negative electrode as the test electrode and Li metal as the counter electrode, and 0.2 mA / cm.2Qf (mAh / g) was the initial storage capacity per unit weight of the negative electrode active material when a test for occluding Li ions in the negative electrode with a constant current of 0 V was performed at the lower limit of 0 V.
3. Battery cell assembly
Battery performance was evaluated using a coin-type cell. That is, the positive electrode punched out to 12 mmφ is placed on a positive electrode can, a 25 μm porous polyethylene film is placed thereon as a separator, pressed with a polypropylene gasket, the negative electrode is placed, and a spacer for adjusting the thickness is placed. After that, as a non-aqueous electrolyte solution, 1 mol / liter lithium hexafluorophosphate (LiPF6) Dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3: 7, and this was added to the battery and sufficiently impregnated. did.
[0058]
At this time, the balance between the weight of the positive electrode active material and the weight of the negative electrode active material is almost equal.
[0059]
[Expression 1]
Positive electrode active material amount [g] / Negative electrode active material amount [g] = (Qf / 1.2) / Qs (C)
Was set to be.
4. Test method
In order to compare the high temperature characteristics of the batteries obtained in this way, the 1 hour rate current value of the batteries, ie 1C
[0060]
[Expression 2]
1C [mA] = Qs (D) × positive electrode active material amount [g]
The following tests were conducted.
First, 2 cycles of constant current 0.2C charge / discharge at room temperature and 1 cycle of constant current 1C charge / discharge, followed by 1 cycle of constant current 0.2C charge / discharge at a high temperature of 50 ° C., then 100 cycles of constant current 1C charge / discharge Went. The upper limit of charging was 4.2 V (however, in the case of evaluation of the positive electrode of the reference example using lithium nickel oxide, the upper limit voltage was 4.1 V), and the lower limit voltage was 3.0 V.
[0061]
At this time, the ratio of the discharge capacity Qh (100) at the 100th cycle to the discharge capacity Qh (100) at the 100th cycle in the 1C charge /
[0062]
[Equation 3]
P [%] = {Qh (100) / Qh (1)} × 100
The high temperature characteristics of the batteries were compared with this value.
Table 1 shows the initial discharge capacity and the high-temperature cycle capacity maintenance rate P in the 1C charge / discharge 100-cycle test at 50 ° C. in Examples, Comparative Examples, and Reference Examples.
[0063]
[Table 1]
Comparing Example 1 with Comparative Examples 1 and 2, it can be seen that the cycle characteristics at high temperature are improved by only adding and mixing molybdenum disulfide. Further, when Examples 1 to 3 and Comparative Example 3 are compared, it can be seen that the chalcogenide of the present invention exhibits a particularly good high temperature cycle characteristic even with the same sulfide. Further, comparing Comparative Example 1 and Comparative Example 2, it can be seen that Comparative Example 2 loses the effect of adding a chalcogenide of a Group 6 element (improvement of high temperature cycle). This is presumably because the chalcogenide of the group 6 element has been transformed into a compound that hardly exerts its effect due to the reaction with the decomposed lithium transition metal oxide. These facts show that only a specific one of various chalcogenides described in JP-A-8-250120 exhibits excellent high-temperature cycle characteristics, and the method described in JP-A-8-250120. Shows that an improvement in the high temperature cycle cannot be achieved.
[0064]
Furthermore, in Reference Example 1, when lithium nickel oxide is used as the active material, it shows a high discharge capacity and good high-temperature cycle characteristics in the first place. Therefore, when lithium nickel oxide is used. In the first place, it can be seen that there is little motivation to improve the high-temperature cycle characteristics. In Japanese Patent Laid-Open No. 8-250120, which deals with lithium manganese oxide and lithium nickel oxide in exactly the same line, no attention has been paid to improving the high-temperature cycle characteristics. This indicates that there is no suggestion of any means for solving the problem of high-temperature cycle characteristics, which is a characteristic problem of oxides.
[0065]
【The invention's effect】
According to the present invention, it is possible to provide a positive electrode material that is excellent in capacity, rate characteristics, and cycle characteristics, and that can be used for a battery that is excellent in safety and productivity. In particular, it is possible to improve the cycle characteristics at high temperature, which is a particular problem when lithium manganese oxide is used as the active material.
[Brief description of the drawings]
Fig. 1 Cycle-discharge capacity correlation diagram
Claims (8)
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