JP2004265754A

JP2004265754A - Nonaqueous electrolyte secondary battery

Info

Publication number: JP2004265754A
Application number: JP2003055416A
Authority: JP
Inventors: Hideyuki Inomata; 秀行猪俣; Katsuya Imai; 克哉今井
Original assignee: Sanyo Electric Co Ltd
Current assignee: Sanyo Electric Co Ltd
Priority date: 2003-03-03
Filing date: 2003-03-03
Publication date: 2004-09-24

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive nonaqueous electrolyte secondary battery with excellent charge-discharge characteristics, especially cycle characteristics, and little swelling of the battery due to high-temperature standing or a cycle. <P>SOLUTION: In this nonaqueous electrolyte secondary battery, a negative electrode active material is provided with a composite carbon material in which surfaces of carbonaceous particles used as a core are covered with amorphous carbon, vinylene carbonate (VC) is contained in a solvent of a nonaqueous electrolyte, and an additional amount of vinylene carbonate (VC) is ≥ 0.01 and ≤10.0 mass % of the mass of the solvent. Thereby, a new effect that the swelling of the battery after high-temperature standing or the cycle, which is a problem when VC is added, is controlled is obtained, and the nonaqueous electrolyte secondary battery with excellent charge-discharge characteristics and cycle characteristics can be provided. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、リチウムイオンを挿入・脱離可能な炭素質材料からなる負極活物質を含有する負極と、リチウムイオンを挿入・脱離可能な正極活物質を含有する正極と、これらの正極と負極を隔離するセパレータと、非水電解質とを備えた非水電解質二次電池に関する。
【０００２】
【従来の技術】
近年、小型軽量でかつ高容量で充放電可能な電池としてリチウム二次電池で代表される非水電解質二次電池が実用化されるようになり、小型ビデオカメラ、携帯電話、ノートパソコン等の携帯用電子・通信機器等に用いられるようになった。この種のリチウム二次電池は、負極活物質としてリチウムイオンを挿入・脱離可能な炭素質材料を用い、正極活物質としてリチウムイオンを挿入・脱離可能な、ＬｉＣｏＯ_２，ＬｉＮｉＯ_２，ＬｉＭｎ_２Ｏ_４，ＬｉＦｅＯ_２等のリチウム含有遷移金属酸化物を用いることが好適とされている。
【０００３】
また、電解質には六フッ化リン酸リチウム（ＬｉＰＦ_６）を支持塩として、ジメチルカーボネート（ＤＭＣ）、ジエチルカーボネート（ＤＥＣ）、エチルメチルカーボネート（ＥＭＣ）などの鎖状炭酸エステルと、エチレンカーボネート（ＥＣ）やプロピレンカーボネート（ＰＣ）などの環状炭酸エステルとを溶媒として用いることが好適とされている。ここで、黒鉛材料を負極活物質として用いた場合、電解質に上記支持塩と炭酸エステルを用いただけでは十分な電池特性が得られないため、ビニレンカーボネート（ＶＣ：以下ではＶＣという）を添加することで、低温特性やサイクル特性が向上することが確認されている。
【０００４】
例えば、特許文献１（特開平６‐５２８８７号公報）においては、ＶＣまたはその誘導体と、沸点が１５０℃以下の低沸点溶媒との混合溶媒を用いることが提案されている。この公報においては、ＶＣの添加量は２０〜８０容量％が好ましいとされている。特許文献２（特開平７−２２０７５６号公報）においては、ＶＣと非対称の環状炭酸エステルとの混合溶媒を用いることが提案されている。特許文献３（特開平８−９６８５２号公報）においては、ＶＣを高誘電率溶媒として使用し、鎖状エステルと混合して使用することが提案されており、ＶＣの割合が２０〜６０容量％が好ましいとされている。
【０００５】
また、特許文献４（特開平１１−６７２６６号公報）においては、ＰＣと鎖状カーボネートに、ＶＣを０．０１〜１０質量％含有させることが好ましいとされている。特許文献５（特開２００２−１１０２３２号公報）においては、ＥＣ，γ−ブチロラクトン（γ−ＢＬ），ＰＣ，ＶＣを含有する電解液を提案している。これらの各特許文献で提案された方法においては、いずれも黒鉛材料を負極活物質材料として用いることが好ましいとされており、これにより放電電圧が高く、高容量な電池が得られ、かつＶＣ添加により優れた充放電特性、サイクル特性が得られるとされている。
【０００６】
さらに、特許文献６（特開平５−３０７９５９号公報）においては、負極材料として炭素質材料に非晶質な炭素材料を被覆した多相構造を有するものを用いることを提案しており、これによりサイクル特性に優れ自己放電の小さな電池が得られるとされている。特許文献７（特開平９−２１３３２８号公報）においては、残炭量が１２質量部以下０．１質量部以上となるような有機物の炭化物を付着させた複合炭素質物を用いることで良好な電池特性が得られるとしている。
【特許文献１】
特開平６‐５２８８７号公報
【特許文献２】
特開平７−２２０７５６号公報
【特許文献３】
特開平８−９６８５２号公報
【特許文献４】
特開平１１−６７２６６号公報
【特許文献５】
特開２００２−１１０２３２号公報
【特許文献６】
特開平５−３０７９５９号公報
【特許文献７】
特開平９−２１３３２８号公報
【０００７】
【発明が解決しようとする課題】
しかしながら、ビニレンカーボネート（ＶＣ）は高価であるため、ＶＣの使用量が増大すると、この種の電池のコストが上昇するという問題が生じた。このため、より安価で高性能な電池が求められる今日の二次電池市場においては、コスト面から実使用には不向きであるという問題が生じた。また、従来のように黒鉛材料を負極活物質に主として用いた場合、ＶＣを添加した電解質を用いると優れた充放電特性が得られる反面、高温放置時やサイクル時に電池が膨れやすいという問題点が生じた。
【０００８】
一方、非晶質炭素で被覆された炭素材料をそのまま負極活物質として使用すると、放電容量や充放電特性やサイクル特性が低下する。このため、更なる高容量化や高性能化が要求される市場においては、放電容量、充放電特性、サイクル特性などの性能が不十分なものとなる。この結果、この種の市場の要求を充分に満たすことは困難であるという問題を生じた。
そこで、本発明はこれらの問題点を解消するためになされたものであって、安価で、かつ充放電特性、特に、サイクル特性に優れ、さらに高温放置やサイクルによる電池膨れの小さい非水電解質二次電池を提供することを目的とする。
【０００９】
【課題を解決するための手段】
上記課題を解決するために鋭意検討を行った結果、本発明においては、負極活物質は核となる炭素質粒子の表面が非晶質炭素で被覆された複合炭素材を備えているとともに、非水電解質の溶媒にビニレンカーボネート（ＶＣ）を含み、かつ、ビニレンカーボネート（ＶＣ）の添加量が溶媒の質量に対して０．０１質量％以上で、１０．０質量％以下となるようにしている。これにより、ＶＣを添加した際に問題となった、高温放置やサイクル後の電池膨れが抑制されるという新たな効果が得られ、充放電特性、サイクル特性に優れた非水電解質二次電池を得ることが可能となる。
【００１０】
ここで、ＶＣ添加時のデメリットであった電池膨れが、核となる炭素質粒子の表面が非晶質炭素で被覆された複合炭素材を負極活物質として用いることで抑制されるのは、以下のような理由によるものと考えられる。即ち、核となる炭素質粒子の表面に被覆された非晶質炭素とＶＣとの反応により形成される皮膜の性状は、核となる結晶性の高い炭素質粒子がＶＣと反応して形成される皮膜の性状とは異なるためと考えられる。そして、非晶質炭素の方が膨れ抑制効果と充放電特性、サイクル特性向上の効果を合わせ持つ皮膜が形成されるためと考えられる。
【００１１】
この場合、非水電解質に含有されるＶＣの添加量が過度に多くても、特性改善効果も小さくなり、かつコスト的にも高価となって実用的ではない。また、過度に少ないと良好な充放電特性やサイクル特性が得られないことが明らかになった。このため、非水電解質中におけるＶＣの添加量は０．０１質量％以上で、１０．０質量％以下が望ましく、特に０．０１質量％以上で、３．０質量％以下にするのが好ましい。
【００１２】
また、非晶質炭素の被覆量が、核となる炭素質粒子の質量に対して少量では十分な効果が得られず、０．１質量％以上になると副反応の活性点となりやすい炭素材料のエッジ部を十分に覆うようになって十分な効果が得られるようになる。また、非晶質炭素の被覆量が多量になると非晶質成分が増加した分だけ容量低下が生じるとともに添加効果が小さくなるが、核となる炭素質粒子の質量に対して１５．０質量％以下であれば容量低下を生じることなく添加効果を発揮することが可能となる。これらのことから、非晶質炭素の被覆量は核となる炭素質粒子の質量に対して０．１質量％以上で、１５．０質量％以下とするのが望ましい。
【００１３】
また、核となる炭素質粒子の表面が非晶質炭素で被覆された複合炭素材の含有量が少量、即ち、負極活物質の質量に対して３０質量％未満であると、ＶＣを添加した際の高温放置やサイクル後の電池膨れ抑制効果が十分に発揮できないことが明らかになった。このため、複合炭素材の含有量は、負極活物質の質量に対して３０質量％以上にするのが望ましく、特に、５０質量％以上にするのが好ましい。なお、上記複合炭素材の物性値としては、ＢＥＴ法による比表面積が１〜１０ｍ^２／ｇ、平均粒子径が１０〜３０μｍであることが好ましい。
【００１４】
ここで、非晶質炭素は、核となる炭素質粒子の表面に被覆された炭素前駆体が不活性ガス雰囲気下での熱処理により形成されたものである。そして、炭素前駆体としては、液相で炭素化を進行させる有機物として、コールタールピッチ、石炭液化油などの石炭系重質油、原油、ナフサなどの熱分解時に副生するナフサタール等分解系重質油などの石油系重質油、分解系重質油を熱処理することで得られるエチレンタールピッチなど熱処理ピッチがある。さらに、ポリ塩化ビニル、ポリビニルアセテート、ポリビニルブラチラールなどのビニル系高分子やベンゼン、トルエンなどの芳香族単環炭化水素、ナフタレン、アントラセンなどのような縮合多環式炭化水素のカルボン酸、カルボン酸無水物のような誘導体等の物質があげられる。
【００１５】
また、固相で炭素化を進行させる有機物としては、セルロースなどの天然高分子、ポリ塩化ビニリデンやポリアクリロニトリルなどの鎖状ビニル樹脂、ポリフェニレン等の芳香族系ポリマー、フルフリルアルコール樹脂、フェノール−ホルムアルデヒド樹脂、イミド樹脂等熱硬化性樹脂やフルフリルアルコールのような熱硬化性樹脂原料などが用いることができる。
【００１６】
そして、これらの有機物を必要に応じて、適宜溶媒を選択して溶解・希釈してから、加熱などにより炭素質粒子核の表面に付着させる。さらに有機物を付着させた炭素質粒子核を加熱・分解して炭素化を行って、非晶質炭素質層を表面に形成させる。この際、熱処理温度は７００℃以上で、２５００℃以下にするのが望ましい。これは、７００℃未満では炭素以外の不純物が十分に除去することができず、２５００℃を超える高温になると、炭素質が非晶質から結晶質へ変わってしまうためである。
【００１７】
この場合、核となる炭素質材料は天然黒鉛、または黒鉛化工程を経て作製された人造黒鉛であることが好ましい。これは黒鉛性炭素の方が単位質量あたりの放電容量が大きいためである。物性的にはＸ線回折における（００２）面の面間隔（ｄ_００２）が３．３８０Å以下で、Ｘ線回折におけるｃ軸方向の結晶子の大きさ（Ｌｃ）が１００Å以上であることが好ましい。なお、複合炭素材（コアシェル形状炭素質材料）以外の負極活物質としては、カーボンブラック、熱分解炭素、炭素繊維、コークスなどの難黒鉛化炭素、天然黒鉛やその造粒物、あるいはＭＣＭＢやＭＣＦ、コークス、ピッチなどを原材料とした人造黒鉛などの黒鉛性炭素材料を用いることができる。これらのなかでも、特に、黒鉛系炭素材料が、単位質量あたりの放電容量が大きいため好ましい。なお、特に、核となる炭素質材料を単独で用いても構わない。
【００１８】
また、負極用バインダーとしては、例えば水を溶媒とした場合、増粘剤としては、カルボキシメチルセルロース、メチルセルロース、ヒドロキシメチルセルロース、エチルセルロース、ポリビニルアルコール、ポリアクリル酸（塩）、酸化スターチ、リン酸化スターチ、カゼインなどを用いることができる。結着剤としては、スチレン−ブタジエン共重合体、メチル（メタ）アクリレート、エチル（メタ）アクリレート、ブチル（メタ）アクリレート、（メタ）アクリロニトリル、ヒドロキシエチル（メタ）アクリレート等のエチレン性不飽和カルボン酸エステル用いることができる。さらに、アクリル酸、メタクリル酸、イタコン酸、フマル酸、マレイン酸などのエチレン性不飽和カルボン酸などを用いることができる。また、Ｎ−メチル−２−ピロリドン（ＮＭＰ）などの有機溶媒を用いた場合には、ポリフッ化ビニリデンやポリイミド樹脂等を用いることができる。これらは特に好ましいものがあるわけでなく、限定されるものではない。
【００１９】
なお、ＶＣ以外の非水電解質は、エチレンカーボネート、プロピレンカーボネート、ブチレンカーボネート、γ‐ブチロラクトン、クロロエチレンカーボネートなどの環状炭酸エステル、γ−バレロラクトン等の環状エステル、ジメチルカーボネート、ジエチルカーボネート、エチルメチルカーボネートなどの鎖状カーボネート、酢酸メチル、酪酸メチルなどの鎖状エステル、テトロヒドロフランまたはその誘導体、１，３−ジオキサン、１，２−ジメトキシエタン、メチルジグライムなどのエーテル類、ジオキソフラン、またはその誘導体、スルホラン、スルトンまたはその誘導体などの単独またはそれら２種以上の混合物などを挙げることができるが、これらに限定されるものではない。
【００２０】
支持塩にはリチウム塩が使用される。一般的にはＬｉＰＦ_６、ＬｉＢＦ_４、ＬｉＣｌＯ_４、ＬｉＡｓＦ_６、ＬｉＣＦ_３ＳＯ_３，ＬｉＣＦ_３ＣＯ_２、Ｌｉ（ＣＦ_３ＳＯ_２）_２、ＬｉＮ（ＣＦ_３ＳＯ_２）などがある。これらは１種類でも、複数用いても構わない。また特に０．３〜２．０ｍｏｌ／Ｌの濃度で使用することが望ましく、特に、０．５〜１．５ｍｏｌ／Ｌの濃度で使用することが好ましい。
【００２１】
正極材料としてはクロム、バナジウム、マンガン、鉄、コバルト及びニッケルからなる群より選ばれる少なくとも１種類の金属とリチウムとの複合金属酸化物が使用される。このような複合金属化合物としては、例えば、ＬｉＣｏＯ_２，ＬｉＭｎ_２Ｏ_４，ＬｉＮｉＯ_２，ＬｉＮｉ_０．８Ｃｏ_０．２Ｏ_２などがあげられる。さらに一般式ＬｉＣｏ_１−ｘＭ_ｘＯ_２、Ｌｉ_１＋ｘＭｎ_２−ｙＭ_ｚＯ_４、ＬｉＮｉ_ｘＭ_１−ｘＯ_２（Ｍは遷移金属などの元素）など、一部を置換した化合物なども挙げられる。
【００２２】
非水電解質二次電池の構成は特に限定されるものではなく、正極、負極、及び単層または複層のセパレータを有するコイン電池、さらに正極、負極、及びロール状のセパレータを有する円筒型電池や角型電池などが一例として挙げられる。なおセパレータとしては公知のポリエチレンやポリプロピレンなどのポリオレフィンの微多孔膜、織布、不織布などが使用される。上記以外の電池構成上必要な部材の選択についてはなんら制約を設けるものではない。
【００２３】
【発明の実施の形態】
以下に、本発明の一実施の形態を説明するが、本発明はこの実施の形態に何ら限定されるものでなく、本発明の目的を変更しない範囲で適宜変更して実施することが可能である。
【００２４】
１．負極の作製
まず、核となる炭素質材料として、Ｘ線回折における（００２）面の面間隔（ｄ_００２）が３．３５８Åで、ｃ軸方向の結晶子の大きさ（Ｌｃ）が１０００Åで、平均粒径が２０μｍの燐片状天然黒鉛の粉末を用意した。また、この核の表面を被覆して非晶質炭素となる炭素前駆体としての石油ピッチ（軟化点：２５０℃）を用意した。この後、これらを混合して、窒素ガス雰囲気下で加熱しながらよく混練し、１０００℃で３時間保持した後、室温まで冷却した。これにより、黒鉛粒子の核の表面に非晶質炭素からなる被覆層が形成された複合炭素材を得た。
【００２５】
ここで、石油ピッチが無添加で非晶質炭素からなる被覆層が形成されなかったものを炭素質材α１とした。また、非晶質炭素からなる被覆層の被覆量（核となる黒鉛粒子の質量に対する質量割合：以下においても同様である）が０．０１質量％の複合炭素材を炭素質材α２とした。同様に、０．１質量％の複合炭素材を炭素質材α３とし、１．０質量％の複合炭素材を炭素質材α４とし、１０．０質量％の複合炭素材を炭素質材α５とし、１５．０質量％の複合炭素材を炭素質材α６とし、２０．０質量％の複合炭素材を炭素質材α７とした。
【００２６】
ついで、これらの炭素質材α１〜α７に、それぞれメソカーボンマイクロビーズ（ＭＣＭＢ、粒径２５μｍ）を７：３の質量割合で混合して、それぞれを混合負極活物質として調製した。ついで、これらの各混合負極活物質と、結着剤としてのスチレン−ブタジエンゴム（ＳＢＲ）とのディスパージョン（固形分は４８質量％）を水に分散させた後、増粘剤となるカルボキシメチルセルロース（ＣＭＣ）を添加、混合して負極活物質スラリーを調製した。なお、混合負極活物質とＳＢＲとＣＭＣとの乾燥後の質量組成比が混合負極活物質：ＳＢＲ：ＣＭＣ＝９６：２：２となるように調製した。
【００２７】
ついで、銅箔からなる負極集電体を用意し、上述のように調製した各負極活物質スラリーをこの負極集電体の両面に、負極集電体の単位面積当たり１００ｇ／ｍ^２となるようにドクターブレード法により塗布して、負極活物質層を形成した。この後、１００℃で２時間真空乾燥させた後、負極活物質の充填密度が１．６ｇ／ｃｍ^３になるように圧延し、所定の形状に切断して帯状の負極ａ１〜ａ７を作製した。なお、炭素質材α１を用いたものを負極ａ１とした。同様に、炭素質材α２を用いたものを負極ａ２とし、炭素質材α３を用いたものを負極ａ３とし、炭素質材α３を用いたものを負極ａ３とし、炭素質材α４を用いたものを負極ａ４とし、炭素質材α５を用いたものを負極ａ５とし、炭素質材α６を用いたものを負極ａ６とし、炭素質材α７を用いたものを負極ａ７とした。
【００２８】
２．正極の作製
一方、平均粒径５μｍのＬｉＣｏＯ_２粉末（正極活物質）と人造黒鉛粉末（導電剤）とを質量比が９：１となるように混合して正極合剤とした。ついで、この正極合剤と、Ｎ−メチル−２−ピロリドン（ＮＭＰ）にポリフッ化ビニリデンを５質量％溶かした結着剤溶媒とを、固形分の質量比で９５：５となるように混練して正極活物質スラリーを調整した。この正極活物質スラリーをドクターブレード法により正極集電体としてのアルミニウム箔の両面に塗布し、乾燥して、厚さ１００μｍの正極活物質層を形成した。ついで、充填密度が３．４ｇ／ｃｍ^３となるように圧縮した後、所望の大きさに切断し、さらに１２０℃で２時間の真空乾燥を行って、正極を作製した。
【００２９】
３．電解質の調整
非水電解質として、エチレンカーボネート（ＥＣ）とエチルメチルカーボネート（ＥＭＣ）が体積比で４対６となるように添加して混合した後、さらに、ビニレンカーボネート（ＶＣ）を所定量混合して混合溶媒とした。この混合溶媒に支持塩として六フッ化リン酸リチウム（ＬｉＰＦ_６）を１モル／Ｌの濃度となるように溶解して、非水電解質ｃ１〜ｃ７とした。ここで、ＶＣが無添加のものを非水電解質ｃ１とした。また、ＶＣが混合溶媒に対して０．０１質量％添加されたものを非水電解質ｃ２とした。同様に、０．１質量％添加されたものを非水電解質ｃ３とし、１．０質量％添加されたものを非水電解質ｃ４とし、５．０質量％添加されたものを非水電解質ｃ５とし、１０．０質量％添加されたものを非水電解質ｃ６とし、１５．０質量％添加されたものを非水電解質ｃ７とした。
【００３０】
４．電池の組立
ついで、上述のように作製した負極ａ１〜ａ７と、正極とをそれぞれ用い、これらの間に、それぞれポリエチレン製の微多孔膜からなるセパレータを介在させて巻回して渦巻状電極体をそれぞれ作製した。ついで、得られた渦巻状電極体を押圧して扁平状電極体とした後、これらの扁平状電極体を金属製角型外装缶に収納した。この後、金属製角型外装缶の開口部に封口体を溶接し、上述のように調製した各非水電解質ｃ１〜ｃ７を、封口体の注液口からそれぞれ注液して、公称容量が６００ｍＡで厚みが４．４０ｍｍの角型非水電解質電池電池（４６３０４８サイズ）Ａ１〜Ａ７，Ｂ１〜Ｂ７，Ｃ１〜Ｃ７，Ｄ１〜Ｄ７，Ｅ１〜Ｅ７，Ｆ１〜Ｆ７，Ｇ１〜Ｇ７をそれぞれ作製した。
【００３１】
なお、負極ａ１を用いて非水電解質ｃ１〜ｃ７を注液したものを非水電解質電池電池Ａ１〜Ａ７とした。同様に、負極ａ２を用いて非水電解質ｃ１〜ｃ７を注液したものを非水電解質電池電池Ｂ１〜Ｂ７とし、負極ａ３を用いて非水電解質ｃ１〜ｃ７を注液したものを非水電解質電池電池Ｃ１〜Ｃ７とし、負極ａ４を用いて非水電解質ｃ１〜ｃ７を注液したものを非水電解質電池電池Ｄ１〜Ｄ７とし、負極ａ５を用いて非水電解質ｃ１〜ｃ７を注液したものを非水電解質電池電池Ｅ１〜Ｅ７とし、負極ａ６を用いて非水電解質ｃ１〜ｃ７を注液したものを非水電解質電池電池Ｆ１〜Ｆ７とし、負極ａ７を用いて非水電解質ｃ１〜ｃ７を注液したものを非水電解質電池電池Ｇ１〜Ｇ７とした。
【００３２】
５．電池サイクル特性試験
ついで、上述のようにして作製した各電池Ａ１〜Ａ７，Ｂ１〜Ｂ７，Ｃ１〜Ｃ７，Ｄ１〜Ｄ７，Ｅ１〜Ｅ７，Ｆ１〜Ｆ７，Ｇ１〜Ｇ７を用いて、室温（約２５℃）雰囲気において、６００ｍＡ（１ＩｔｍＡ）の充電電流で電池電圧が４．２Ｖになるまで定電流充電した後、４．２Ｖの定電圧で電流値が１０ｍＡになるまで定電圧充電を行った。その後、６００ｍＡ（１ＩｔｍＡ）の放電電流で電池電圧が３．０Ｖになるまで放電させた。そして、このときの放電容量を測定して初期放電容量とした。このような充放電を繰り返して行って、５００サイクル後の放電容量と、電池厚みとを測定した。
【００３３】
そして、初期放電容量に対する５００サイクル後の放電容量との比率を求めて、容量維持率とすると、下記の表１〜表７に示すような結果が得られた。また、表１〜表７の結果から、電解質中のＶＣ添加量を横軸で表し、容量維持率（初期放電容量に対する５００サイクル後の放電容量）を縦軸で表して、電解質中のＶＣ添加量に対する容量維持率の関係を求めると、図１に示すような結果が得られた。また、電解質中のＶＣ添加量を横軸で表し、電池厚み（５００サイクル後の電池厚み）を縦軸で表して、電解質中のＶＣ添加量に対する電池厚みの関係を求めると、図２に示すような結果が得られた。なお、図１，図２において、電池Ａ１〜Ａ７をＡで表し、電池Ｂ１〜Ｂ７をＢで表し、電池Ｃ１〜Ｃ７をＣで表し、電池Ｄ１〜Ｄ７をＤで表し、電池Ｅ１〜Ｅ７をＥで表し、電池Ｆ１〜Ｆ７をＦで表し、電池Ｇ１〜Ｇ７をＧで表している。
【００３４】
【表１】

【００３５】
【表２】

【００３６】
【表３】

【００３７】
【表４】

【００３８】
【表５】

【００３９】
【表６】

【００４０】
【表７】

【００４１】
上記表１〜表７および図１，図２の結果から明らかなように、電解質中（溶媒）にＶＣを添加しなかった電池（Ａ１，Ｂ１，Ｃ１，Ｄ１，Ｅ１，Ｆ１，Ｇ１）においては、非晶質炭素の被覆量が多いほど容量維持率は大きくなるが、非晶質炭素の被覆量が１．０質量％以上の電池（Ｃ１，Ｄ１，Ｅ１，Ｆ１，Ｇ１）においては、大きな差が認められなかった。また、電池厚みは非晶質炭素量の被覆量にかかわらず、ほとんど変化していないことが分かる。
一方、非晶質炭素の被覆量が０の負極（ａ１，ａ２，ａ３，ａ４，ａ５，ａ６，ａ７）を用いた電池（Ａ１，Ａ２，Ａ３，Ａ４，Ａ５，Ａ６，Ａ７）においては、ＶＣの添加量が多いほど容量維持率は大きくなるが、ＶＣの添加量が１．０質量％以上の電池（Ａ４，Ａ５，Ａ６，Ａ７）においては、大きな差がなくなることが分かる。また、ＶＣの添加量にかかわらず、電池厚みはほとんど変化しないことが分かる。
【００４２】
しかしながら、非晶質炭素の被覆量が０．０１質量％以上で、ＶＣの添加量が０．０１質量％以上の電池（Ｂ２〜Ｂ７，Ｃ２〜Ｃ７，Ｄ２〜Ｄ７，Ｅ２〜Ｅ７，Ｆ２〜Ｆ７，Ｇ２〜Ｇ７）においては、そのいずれかが０の電池（Ａ１〜Ａ７，Ｂ１，Ｃ１，Ｄ１，Ｅ１，Ｆ１，Ｇ１）の場合と比較すると、容量維持率が増加し、さらには電池厚みが小さくなり、大きく特性が向上する方向に変化していることが分かる。この傾向としては、非晶質炭素量が０．１％質量以上で、かつ、電解質溶媒にＶＣを０．０１質量％含有する場合に、特に良好な特性が得られる。
【００４３】
ところが、非晶質炭素の被覆量、ＶＣの添加量ともに多ければ多いほど良いものではなく、ある一定量以上になるとその効果はほとんど変化しなくなる。これは、以下のような理由によるものと考えられる。即ち、ＶＣとの反応により非晶質炭素の表面に生成された皮膜の性状が、通常の黒鉛において生成された皮膜の性状とは異なっている。このため、非晶質炭素に生成された皮膜が、サイクル時の電解液の分解などの副反応を抑制する効果を発揮し、特に、ガス発生の抑制効果が大きいと推定される。
そこで、ＶＣの添加量を１．０質量％とした電解質溶媒を用いた電池（Ａ４，Ｂ４，Ｃ４，Ｄ４，Ｅ４，Ｆ４，Ｇ４）の初期放電容量を示すと、下記の表８および図３に示すような結果が得られた。
【００４４】
【表８】

【００４５】
上記表８および図３の結果から明らかなように、非晶質炭素の被覆量が多くなるほど徐々に初期容量は低下する傾向を示すことが分かる。これは核となる黒鉛の表面に形成された非晶質炭素は、核となる黒鉛に比べて単位質量あたりの放電容量が小さいためである。したがって非晶質炭素の被覆量が多いなるほど初期容量は低下することとなるので、非晶質炭素の被覆量は多くとも１５．０質量％とするのが望ましいということができる。
【００４６】
６．高温充電放置試験
ついで、非晶質炭素が未形成の負極ａ１，ａ３，ａ４，ａ６，ａ７を用いた電池Ａ１，Ａ３，Ａ４，Ａ６，Ａ７、および非晶質炭素の被覆量が１．０質量％の負極ｄ１，ｄ３，ｄ４，ｄ６，ｄ７を備えた電池Ｄ１，Ｄ３，Ｄ４，Ｄ６，Ｄ７を用いて、充電後に高温で放置した場合の電池膨れについて検討した。そこで、これらの各電池を室温（約２５℃）で、６００ｍＡ（１ＩｔｍＡ）の充電電流で電池電圧が４．２Ｖになるまで定電流充電し、４．２Ｖの定電圧で電流値が１０ｍＡになるまで定電圧充電して満充電状態にした。この後、８０℃の雰囲気中に１２時間放置した。ついで、これらの各電池の厚みを測定すると、下記の表９に示すような結果が得られた。また、表９の結果に基づいて、電解質（溶媒）中のＶＣ添加量を横軸で表し、電池厚み（高温放置後の電池厚み）を縦軸で表して、ＶＣ添加量に対する電池厚みの関係を求めると、図４に示すような結果が得られた。なお、図４において、電池Ａ１〜Ａ７をＡで表し、電池Ｄ１〜Ｄ７をＤで表している。
【００４７】
【表９】

【００４８】
上記表９および図４の結果から明らかなように、電池Ａ１，Ａ３，Ａ４，Ａ６，Ａ７のように、非晶質炭素がない場合は、ＶＣ量の増加とともに電池厚みが大きく膨れていくことが分かる。一方、非晶質炭素の被覆量を１．０質量％とした電池Ｄ１，Ｄ３，Ｄ４，Ｄ６，Ｄ７においては、驚くべきことに、ＶＣの添加量が増加するに伴って電池膨れが抑制されることが判明した。これは、充電時に非晶炭素層とＶＣとの反応により生成された皮膜が高温時の活物質表面におけるＶＣの分解を抑制しているためと推定される。このことも、ＶＣの添加量が多ければ良いわけではなく、その効果は１．０質量％を超えるとほとんど変わらないことを示している。
【００４９】
７．負極活物質中の複合炭素材の含有量についての検討
ついで、負極活物質中の複合炭素材の含有量について検討した。そこで、非晶質炭素の被覆量が、核となる黒鉛粒子の質量に対して１．０質量％となるように調製された複合炭素材α４を用いて、この複合炭素材α４とＭＣＭＢが２０：８０となるように混合して負極活物質ｂ１を調製した。
【００５０】
同様に、複合炭素材α４とＭＣＭＢが３０：７０となるように混合して負極活物質ｂ２を調製し、複合炭素材α４とＭＣＭＢが４０：６０となるように混合して負極活物質ｂ３を調製し、複合炭素材α４とＭＣＭＢが５０：５０となるように混合して負極活物質ｂ４を調製し、複合炭素材α４とＭＣＭＢが６０：４０となるように混合して負極活物質ｂ５を調製し、複合炭素材α４とＭＣＭＢが７０：３０となるように混合して負極活物質ｂ６（上述したａ４と同じである）を調製し、複合炭素材α４とＭＣＭＢが８０：２０となるように混合して負極活物質ｂ７を調製した。
【００５１】
ついで、これらの負極活物質ｂ１〜ｂ７を用いて、上述同様に負極をそれぞれ作製し、これらの負極と、上述した正極と、上述のように調製した非水電解質ｃ４（ＶＣの添加量が１．０質量％のもの）とを用いて、上述と同様に、公称容量が６００ｍＡで厚みが４．４０ｍｍの角型非水電解質電池電池Ｈ１〜Ｈ７をそれぞれ作製した。なお、負極活物質ｂ１を用いたものを非水電解質電池電池Ｈ１とし、負極活物質ｂ２を用いたものを非水電解質電池電池Ｈ２とし、負極活物質ｂ３を用いたものを非水電解質電池電池Ｈ３とし、負極活物質ｂ４を用いたものを非水電解質電池電池Ｈ４とし、負極活物質ｂ５を用いたものを非水電解質電池電池Ｈ５とし、負極活物質ｂ５を用いたものを非水電解質電池電池Ｈ５とし、負極活物質ｂ６を用いたものを非水電解質電池電池Ｈ６（上述した電池Ｄ４と同じである）とし、負極活物質ｂ７を用いたものを非水電解質電池電池Ｈ７とした。
【００５２】
ついで、これらの各電池Ｈ１〜Ｈ７を用いて、これらの各電池Ｈ１〜Ｈ７をそれぞれ４．２Ｖまで満充電した後、８０℃の雰囲気中に１２時間放置した後、これらの各電池Ｈ１〜Ｈ７の厚みを測定すると、下記の表１０に示すような結果が得られた。また、表１０の結果に基づいて、複合炭素材の添加量を横軸で表し、電池厚み（高温放置後の電池厚み）を縦軸で表して、複合炭素材の含有量に対する電池厚みの関係を求めると、図５に示すような結果が得られた。
【００５３】
【表１０】

【００５４】
上記表１０および図５の結果から明らかなように、複合炭素材の含有量が多くなるほど放置試験後の電池厚みが小さくなる傾向を示している。この結果から、複合炭素材の含有量は負極活物質に対して３０質量％以上であることが好ましいと考えられる。ここで、複合炭素材の含有量が多いほど高温充電放置後の膨れが小さいのは、非晶質炭素とＶＣとの反応により生成される皮膜により、耐高温放置性が向上するためと考えられる。そして、複合炭素材の含有量が多いほどその効果が大きくなるためと推定される。
【００５５】
８．非晶質炭素を形成させるときの焼成温度の検討
ついで、非晶質炭素を形成させるときの焼成温度について検討した。そこで、上述と同様の燐片状天然黒鉛（ｄ_００２が３．３５８ÅでＬｃが１０００Åで平均粒径が２０μｍのもの）と、石油ピッチ（軟化点：２５０℃）とを用いて、上述と同様に、窒素ガス雰囲気下で５００℃の温度で熱処理して、非晶質炭素の被覆量が核となる黒鉛粒子の質量に対して１．０質量％となる複合炭素材を調製して、複合炭素材β１とした。同様に、７００℃の温度で熱処理して複合炭素材β２を調製し、１０００℃の温度で熱処理して複合炭素材β３（上述したα４と同じである）を調製し、１５００℃の温度で熱処理して複合炭素材β４を調製し、２０００℃の温度で熱処理して複合炭素材β５を調製し、２５００℃の温度で熱処理して複合炭素材β６を調製し、３０００℃の温度で熱処理して複合炭素材β７を調製した。
【００５６】
ついで、これらの複合炭素材β１〜β７に、それぞれＭＣＭＢを７：３の質量割合で混合して負極活物質ｊ１〜ｊ７をそれぞれ調製した。なお、複合炭素材β１を用いたものを負極活物質ｊ１とし、複合炭素材β２を用いたものを負極活物質ｊ２とし、複合炭素材β３を用いたものを負極活物質ｊ３（上述したｄ４と同じである）とし、複合炭素材β４を用いたものを負極活物質ｊ４とし、複合炭素材β５を用いたものを負極活物質ｊ５とし、複合炭素材β６を用いたものを負極活物質ｊ６とし、複合炭素材β７を用いたものを負極活物質ｊ７とした。
【００５７】
ついで、これらの負極活物質ｊ１〜ｊ７を用いて、上述同様に負極をそれぞれ作製し、これらの負極と、上述した正極と、上述のように調製した非水電解質ｃ４（ＶＣの添加量が１質量％のもの）とを用いて、上述と同様に、公称容量が６００ｍＡで厚みが４．４０ｍｍの角型非水電解質電池電池Ｊ１〜Ｊ７をそれぞれ作製した。なお、負極活物質ｊ１を用いたものを非水電解質電池電池Ｊ１とし、負極活物質ｊ２を用いたものを非水電解質電池電池Ｊ２とし、負極活物質ｊ３を用いたものを非水電解質電池電池Ｊ３（上述した電池Ｄ４と同じである）とし、負極活物質ｊ４を用いたものを非水電解質電池電池Ｊ４とし、負極活物質ｊ５を用いたものを非水電解質電池電池Ｊ５とし、負極活物質ｊ６を用いたものを非水電解質電池電池Ｊ６とし、負極活物質ｊ７を用いたものを非水電解質電池電池Ｊ７とした。
【００５８】
ついで、これらの各電池Ｊ１〜Ｊ７を用いて、これらの各電池Ｊ１〜Ｊ７をそれぞれ４．２Ｖまで満充電した後、８０℃の雰囲気中に１２時間放置した、これらの各電池Ｊ１〜Ｊ７の厚みを測定すると、下記の表１１に示すような結果が得られた。また、表１１の結果に基づいて、炭素材料の焼成温度（℃）を横軸で表し、初期容量および電池厚み（高温放置後の電池厚み）を縦軸で表して、炭素材料の焼成温度に対する初期容量および電池厚みの関係を求めると、図６に示すような結果が得られた。
【００５９】
【表１１】

【００６０】
上記表１１および図６の結果から明らかなように、焼成温度が低いと初期容量は小さく、また高温放置後の電池厚みも急に大きくなっていることが分かる。これは石油ピッチの炭素以外の成分が十分に抜けきれておらず不純物が多く残り、副反応が生じたためと考えられる。したがって、放電容量面から見ると、焼成温度は７００℃以上とすることが好ましい。また焼成温度が高くなると２５００℃付近までは容量、電池厚みともに大きな差は認められず、良好な特性が得られているが、３０００℃では電池厚みが急に大きくなる。これは表層が黒鉛化されて、高温充電放置時に好ましい非晶質炭素層がほとんど形成されなかったためと考えられる。以上の結果から、焼成温度（熱処理温度）は７００℃以上で、２５００℃以下にするのが望ましいということができる。
【００６１】
【発明の効果】
上述したように、本発明の非水電解液二次電池においては、負極活物質は核となる炭素質粒子の表面が非晶質炭素で被覆された複合炭素材を備えているとともに、非水電解質の溶媒にビニレンカーボネート（ＶＣ）を含み、かつ、ビニレンカーボネート（ＶＣ）の添加量が溶媒の質量に対して０．０１質量％以上で、１０．０質量％以下となるようにしている。これにより、ＶＣを添加した際に問題となった、高温放置やサイクル後の電池膨れが抑制されるという新たな効果が得られ、充放電特性、サイクル特性に優れた非水電解質二次電池を得ることが可能となる。
【図面の簡単な説明】
【図１】電解質中のＶＣ添加量に対する容量維持率の関係を示すグラフである。
【図２】電解質中のＶＣ添加量に対する電池厚みの関係を示すグラフである。
【図３】電解質中のＶＣ添加量を１．０質量％とした場合の非晶質炭素の被覆量に対する初期放電容量の関係を示すグラフである。
【図４】８０℃の雰囲気中に１２時間放置した後の電解質中のＶＣ添加量に対する電池厚みの関係を示すグラフである。
【図５】８０℃の雰囲気中に１２時間放置した後の複合炭素の含有量に対する電池厚みの関係を示すグラフである。
【図６】８０℃の雰囲気中に１２時間放置した後の炭素材料の焼成温度に対する初期容量および電池厚みの関係を示すグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a negative electrode containing a negative electrode active material made of a carbonaceous material capable of inserting and removing lithium ions, a positive electrode containing a positive electrode active material capable of inserting and removing lithium ions, and these positive and negative electrodes. And a non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte.
[0002]
[Prior art]
In recent years, non-aqueous electrolyte secondary batteries represented by lithium secondary batteries have been put into practical use as small, lightweight, high-capacity, chargeable / dischargeable batteries. It has been used for electronic and communication equipment. This type of lithium secondary battery uses a carbonaceous material capable of inserting and removing lithium ions as a negative electrode active material, and is capable of inserting and releasing lithium ions as a positive electrode active material. ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiFeO ₂ It is preferable to use a lithium-containing transition metal oxide such as
[0003]
The electrolyte is lithium hexafluorophosphate (LiPF). ₆ ) As a supporting salt, a chain carbonate such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC), and a cyclic carbonate such as ethylene carbonate (EC) and propylene carbonate (PC). It is preferred to use as a solvent. Here, when a graphite material is used as a negative electrode active material, vinylene carbonate (VC: hereinafter, referred to as VC) should be added because sufficient battery characteristics cannot be obtained only by using the above supporting salt and carbonate as an electrolyte. Thus, it has been confirmed that low-temperature characteristics and cycle characteristics are improved.
[0004]
For example, Patent Document 1 (JP-A-6-52887) proposes to use a mixed solvent of VC or a derivative thereof and a low-boiling solvent having a boiling point of 150 ° C. or lower. According to this publication, the addition amount of VC is preferably 20 to 80% by volume. Patent Document 2 (JP-A-7-220756) proposes to use a mixed solvent of VC and an asymmetric cyclic carbonate. Patent Document 3 (Japanese Patent Application Laid-Open No. Hei 8-96852) proposes using VC as a high-dielectric solvent and mixing it with a chain ester, wherein the proportion of VC is 20 to 60% by volume. Is preferred.
[0005]
Further, Patent Document 4 (Japanese Patent Application Laid-Open No. 11-67266) states that it is preferable that PC and a chain carbonate contain 0.01 to 10% by mass of VC. Patent Document 5 (Japanese Patent Application Laid-Open No. 2002-110232) proposes an electrolyte containing EC, γ-butyrolactone (γ-BL), PC, and VC. In any of the methods proposed in these patent documents, it is preferable to use a graphite material as the negative electrode active material, whereby a battery having a high discharge voltage and a high capacity can be obtained, and VC is added. It is said that more excellent charge / discharge characteristics and cycle characteristics can be obtained.
[0006]
Further, Patent Document 6 (JP-A-5-307959) proposes to use a negative electrode material having a multiphase structure in which a carbonaceous material is coated with an amorphous carbon material. It is said that a battery having excellent cycle characteristics and small self-discharge can be obtained. Patent Literature 7 (Japanese Patent Application Laid-Open No. 9-213328) discloses that a good battery is obtained by using a composite carbonaceous material to which an organic carbide is attached so that the residual carbon amount is 12 parts by mass or less and 0.1 part by mass or more. It is said that characteristics can be obtained.
[Patent Document 1]
JP-A-6-52887
[Patent Document 2]
JP-A-7-220756
[Patent Document 3]
JP-A-8-96852
[Patent Document 4]
JP-A-11-67266
[Patent Document 5]
JP-A-2002-110232
[Patent Document 6]
JP-A-5-307959
[Patent Document 7]
JP-A-9-213328
[0007]
[Problems to be solved by the invention]
However, since vinylene carbonate (VC) is expensive, there has been a problem that as the amount of used VC increases, the cost of this type of battery increases. For this reason, in the secondary battery market where a cheaper and higher-performance battery is required, there has been a problem that it is not suitable for practical use in terms of cost. In addition, when a graphite material is mainly used as a negative electrode active material as in the related art, when a VC-added electrolyte is used, excellent charge / discharge characteristics can be obtained, but the battery tends to swell when left at high temperatures or during cycles. occured.
[0008]
On the other hand, if the carbon material coated with amorphous carbon is used as it is as the negative electrode active material, the discharge capacity, charge / discharge characteristics, and cycle characteristics deteriorate. Therefore, in a market where higher capacity and higher performance are required, performance such as discharge capacity, charge / discharge characteristics, and cycle characteristics becomes insufficient. As a result, there has been a problem that it is difficult to sufficiently satisfy the requirements of this type of market.
Accordingly, the present invention has been made to solve these problems, and is a non-aqueous electrolyte which is inexpensive, has excellent charge / discharge characteristics, particularly excellent cycle characteristics, and has small battery swelling due to high temperature storage or cycling. It is intended to provide a secondary battery.
[0009]
[Means for Solving the Problems]
As a result of intensive studies to solve the above problems, in the present invention, the negative electrode active material includes a composite carbon material in which the surface of carbonaceous particles serving as nuclei is coated with amorphous carbon, Vinylene carbonate (VC) is contained in the solvent of the water electrolyte, and the amount of vinylene carbonate (VC) to be added is 0.01% by mass or more and 10.0% by mass or less based on the mass of the solvent. . As a result, a new effect of suppressing battery swelling after leaving at high temperatures or cycling, which is a problem when VC is added, is obtained, and a nonaqueous electrolyte secondary battery having excellent charge / discharge characteristics and cycle characteristics is obtained. It is possible to obtain.
[0010]
Here, the swelling of the battery, which was a disadvantage at the time of VC addition, is suppressed by using a composite carbon material in which the surface of the carbonaceous particles serving as nuclei is coated with amorphous carbon as the negative electrode active material is as follows. It is thought to be due to the following reasons. That is, the properties of the film formed by the reaction between the amorphous carbon coated on the surface of the carbonaceous particles serving as the nucleus and the VC are such that the carbonaceous particles serving as the nucleus are formed by the reaction with the VC. This is probably due to the different properties of the film. It is considered that amorphous carbon forms a film having both the effect of suppressing swelling and the effect of improving charge / discharge characteristics and cycle characteristics.
[0011]
In this case, even if the amount of VC contained in the non-aqueous electrolyte is excessively large, the effect of improving the characteristics is reduced and the cost is high, which is not practical. In addition, it has been clarified that if the amount is excessively small, good charge / discharge characteristics and cycle characteristics cannot be obtained. For this reason, the addition amount of VC in the nonaqueous electrolyte is preferably 0.01% by mass or more and 10.0% by mass or less, particularly preferably 0.01% by mass or more and 3.0% by mass or less. .
[0012]
Further, if the coating amount of the amorphous carbon is small with respect to the mass of the core carbonaceous particles, a sufficient effect cannot be obtained. A sufficient effect can be obtained by sufficiently covering the edge portion. Also, when the coating amount of the amorphous carbon is large, the capacity is reduced by the increase in the amount of the amorphous component, and the effect of addition is reduced. If it is less than the above, the effect of addition can be exerted without lowering the capacity. From these facts, it is desirable that the coating amount of the amorphous carbon be 0.1% by mass or more and 15.0% by mass or less based on the mass of the carbonaceous particles serving as the core.
[0013]
VC was added when the content of the composite carbon material in which the surface of the carbonaceous particles serving as nuclei was coated with amorphous carbon was small, that is, less than 30% by mass based on the mass of the negative electrode active material. It has been clarified that the effect of suppressing battery swelling after being left at a high temperature or cycling cannot be sufficiently exhibited. For this reason, the content of the composite carbon material is desirably 30% by mass or more, particularly preferably 50% by mass or more, based on the mass of the negative electrode active material. In addition, as a physical property value of the composite carbon material, the specific surface area by the BET method is 1 to 10 m. ² / G and an average particle size of 10 to 30 μm.
[0014]
Here, the amorphous carbon is formed by a heat treatment in an inert gas atmosphere of a carbon precursor coated on the surface of carbonaceous particles serving as nuclei. As the carbon precursor, organic substances that promote carbonization in the liquid phase include coal-based heavy oils such as coal tar pitch and coal liquefied oil, and naphtha tar and other decomposition-based weights that are by-produced during thermal decomposition of crude oil and naphtha. There are heat-treated pitches such as ethylene tar pitch obtained by heat-treating petroleum heavy oil such as heavy oil and cracked heavy oil. Further, carboxylic acids and carboxylic acids of vinyl polymers such as polyvinyl chloride, polyvinyl acetate, and polyvinyl bratylal; aromatic monocyclic hydrocarbons such as benzene and toluene; condensed polycyclic hydrocarbons such as naphthalene and anthracene; Substances such as derivatives such as anhydrides are exemplified.
[0015]
Examples of the organic substance that progresses carbonization in the solid phase include natural polymers such as cellulose, chain vinyl resins such as polyvinylidene chloride and polyacrylonitrile, aromatic polymers such as polyphenylene, furfuryl alcohol resin, and phenol-formaldehyde. A thermosetting resin such as a resin or an imide resin, or a thermosetting resin material such as furfuryl alcohol can be used.
[0016]
Then, these organic substances are dissolved and diluted by selecting a suitable solvent as required, and then attached to the surface of the carbonaceous particle nucleus by heating or the like. Further, the carbonaceous particle nucleus to which the organic substance is adhered is heated and decomposed to carbonize, thereby forming an amorphous carbonaceous layer on the surface. At this time, the heat treatment temperature is desirably 700 ° C. or higher and 2500 ° C. or lower. This is because if the temperature is lower than 700 ° C., impurities other than carbon cannot be sufficiently removed, and if the temperature is higher than 2500 ° C., the carbonaceous material changes from amorphous to crystalline.
[0017]
In this case, the core carbonaceous material is preferably natural graphite or artificial graphite produced through a graphitization step. This is because graphitic carbon has a larger discharge capacity per unit mass. Physically, the plane spacing (d) of the (002) plane in X-ray diffraction ₀₀₂ ) Is 3.380 ° or less, and the crystallite size (Lc) in the c-axis direction in X-ray diffraction is preferably 100 ° or more. As the negative electrode active material other than the composite carbon material (core-shell shaped carbonaceous material), carbon black, pyrolytic carbon, carbon fiber, non-graphitizable carbon such as coke, natural graphite and its granulated material, or MCMB or MCF A graphitic carbon material such as artificial graphite using coke, pitch or the like as a raw material can be used. Among these, graphite-based carbon materials are particularly preferable because of their large discharge capacity per unit mass. In particular, the carbonaceous material serving as a core may be used alone.
[0018]
Further, as a binder for the negative electrode, for example, when water is used as a solvent, as a thickener, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid (salt), oxidized starch, phosphorylated starch, casein Etc. can be used. Examples of the binder include ethylenically unsaturated carboxylic acids such as styrene-butadiene copolymer, methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, (meth) acrylonitrile, and hydroxyethyl (meth) acrylate. Esters can be used. Further, ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid can be used. When an organic solvent such as N-methyl-2-pyrrolidone (NMP) is used, polyvinylidene fluoride, a polyimide resin, or the like can be used. These are not particularly preferred and are not limited.
[0019]
Note that non-aqueous electrolytes other than VC include ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, cyclic carbonates such as chloroethylene carbonate, cyclic esters such as γ-valerolactone, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Linear carbonates, such as linear acetates such as methyl acetate and methyl butyrate, tetrahydrofuran or derivatives thereof, ethers such as 1,3-dioxane, 1,2-dimethoxyethane and methyldiglyme, dioxofuran or derivatives thereof , Sulfolane, sultone or a derivative thereof alone or a mixture of two or more thereof, but is not limited thereto.
[0020]
As the supporting salt, a lithium salt is used. Generally LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ )and so on. One of these may be used, or a plurality of them may be used. Further, it is particularly desirable to use it at a concentration of 0.3 to 2.0 mol / L, and it is particularly preferable to use it at a concentration of 0.5 to 1.5 mol / L.
[0021]
As the positive electrode material, a composite metal oxide of lithium and at least one metal selected from the group consisting of chromium, vanadium, manganese, iron, cobalt, and nickel is used. As such a composite metal compound, for example, LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _0.8 Co _0.2 O ₂ And so on. Furthermore, the general formula LiCo _1-x M _x O ₂ , Li _{1 + x} Mn _2-y M _z O ₄ , LiNi _x M _1-x O ₂ (M is an element such as a transition metal) and a partially substituted compound.
[0022]
The configuration of the non-aqueous electrolyte secondary battery is not particularly limited, a positive electrode, a negative electrode, a coin battery having a single-layer or multiple-layer separator, further a positive electrode, a negative electrode, a cylindrical battery having a roll-shaped separator and A prismatic battery is an example. As the separator, a known microporous film of polyolefin such as polyethylene or polypropylene, a woven fabric, a nonwoven fabric, or the like is used. There are no restrictions on the selection of members necessary for the battery configuration other than those described above.
[0023]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of the present invention will be described. However, the present invention is not limited to this embodiment at all, and can be implemented by appropriately changing the scope of the present invention without changing the object. is there.
[0024]
1. Fabrication of negative electrode
First, as a carbonaceous material serving as a nucleus, the plane spacing (d) of the (002) plane in X-ray diffraction is used. ₀₀₂ ) Of 3.358 °, a crystallite size (Lc) in the c-axis direction of 1000 °, and a scaly natural graphite powder having an average particle size of 20 μm. In addition, a petroleum pitch (softening point: 250 ° C.) was prepared as a carbon precursor to cover the surface of the nucleus to become amorphous carbon. Thereafter, these were mixed, kneaded well while heating in a nitrogen gas atmosphere, kept at 1000 ° C. for 3 hours, and then cooled to room temperature. Thus, a composite carbon material having a coating layer made of amorphous carbon formed on the surface of the core of the graphite particles was obtained.
[0025]
Here, a carbonaceous material α1 in which no coating layer made of amorphous carbon was formed without adding petroleum pitch was used. The carbonaceous material α2 was a composite carbon material in which the coating amount of the coating layer made of amorphous carbon (mass ratio based on the mass of graphite particles serving as nuclei: the same applies to the following description) was 0.01 mass%. Similarly, 0.1 mass% of the composite carbon material is referred to as carbonaceous material α3, 1.0 mass% of the composite carbon material is referred to as carbonaceous material α4, and 10.0 mass% of the composite carbon material is referred to as carbonaceous material α5. , 15.0 mass% of the composite carbon material was designated as carbonaceous material α6, and 20.0 mass% of the composite carbon material was designated as carbonaceous material α7.
[0026]
Next, mesocarbon microbeads (MCMB, particle size 25 μm) were mixed with these carbonaceous materials α1 to α7 at a mass ratio of 7: 3 to prepare each as a mixed negative electrode active material. Next, a dispersion (solid content: 48% by mass) of each of these mixed negative electrode active materials and styrene-butadiene rubber (SBR) as a binder is dispersed in water, and then carboxymethyl cellulose as a thickener is dispersed. (CMC) was added and mixed to prepare a negative electrode active material slurry. The mixed composition of the mixed negative electrode active material, SBR, and CMC after drying was adjusted so that the mixed negative electrode active material: SBR: CMC = 96: 2: 2.
[0027]
Next, a negative electrode current collector made of a copper foil was prepared, and the respective negative electrode active material slurries prepared as described above were applied to both surfaces of the negative electrode current collector at a rate of 100 g / m 2 per unit area of the negative electrode current collector. ² Was applied by a doctor blade method to form a negative electrode active material layer. Then, after vacuum drying at 100 ° C. for 2 hours, the packing density of the negative electrode active material is 1.6 g / cm. ³ And cut into a predetermined shape to produce strip-shaped negative electrodes a1 to a7. A material using the carbonaceous material α1 was referred to as a negative electrode a1. Similarly, the one using carbonaceous material α2 is referred to as negative electrode a2, the one using carbonaceous material α3 as negative electrode a3, the one using carbonaceous material α3 as negative electrode a3, and one using carbonaceous material α4. The negative electrode a4, the one using the carbonaceous material α5 was used as the negative electrode a5, the one using the carbonaceous material α6 was used as the negative electrode a6, and the one using the carbonaceous material α7 was used as the negative electrode a7.
[0028]
2. Preparation of positive electrode
On the other hand, LiCoO having an average particle size of 5 μm ₂ The powder (positive electrode active material) and the artificial graphite powder (conductive agent) were mixed at a mass ratio of 9: 1 to obtain a positive electrode mixture. Next, this positive electrode mixture and a binder solvent in which 5% by mass of polyvinylidene fluoride was dissolved in N-methyl-2-pyrrolidone (NMP) were kneaded so that the solid content mass ratio was 95: 5. Thus, a positive electrode active material slurry was prepared. This positive electrode active material slurry was applied on both sides of an aluminum foil as a positive electrode current collector by a doctor blade method, and dried to form a 100 μm thick positive electrode active material layer. Then, the packing density is 3.4 g / cm ³ After compression to a desired size, the resultant was cut into a desired size, and further vacuum-dried at 120 ° C. for 2 hours to produce a positive electrode.
[0029]
3. Adjusting the electrolyte
As a non-aqueous electrolyte, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were added and mixed at a volume ratio of 4: 6, and then a predetermined amount of vinylene carbonate (VC) was further mixed to form a mixed solvent. And Lithium hexafluorophosphate (LiPF) as a supporting salt in this mixed solvent ₆ ) Was dissolved to a concentration of 1 mol / L to obtain nonaqueous electrolytes c1 to c7. Here, the one to which VC was not added was defined as the non-aqueous electrolyte c1. In addition, a non-aqueous electrolyte c2 in which VC was added in an amount of 0.01% by mass with respect to the mixed solvent was defined as a nonaqueous electrolyte c2. Similarly, a non-aqueous electrolyte c3 was added at 0.1% by mass, a non-aqueous electrolyte c4 was added at 1.0% by mass, and a non-aqueous electrolyte c5 was added at 5.0% by mass. The one added with 10.0% by mass was referred to as a non-aqueous electrolyte c6, and the one added with 15.0% by mass was referred to as a non-aqueous electrolyte c7.
[0030]
4. Battery assembly
Then, the negative electrodes a1 to a7 prepared as described above and the positive electrode were used, and a spirally wound electrode body was produced by winding each of them with a separator made of a microporous polyethylene film interposed therebetween. . Next, the obtained spiral electrode body was pressed into a flat electrode body, and then the flat electrode body was housed in a metal square outer can. Thereafter, a sealing body is welded to the opening of the metal square outer can, and the nonaqueous electrolytes c1 to c7 prepared as described above are respectively injected from the injection holes of the sealing body to have a nominal capacity. Square-shaped nonaqueous electrolyte battery cells (463048 size) A1 to A7, B1 to B7, C1 to C7, D1 to D7, E1 to E7, F1 to F7, and G1 to G7 each having a thickness of 4.40 mm and 600 mA were produced. .
[0031]
In addition, what injected the nonaqueous electrolyte c1 to c7 using the negative electrode a1 was set as the nonaqueous electrolyte battery A1-A7. Similarly, the non-aqueous electrolytes c1 to c7 injected using the negative electrode a2 are referred to as nonaqueous electrolyte batteries B1 to B7, and the nonaqueous electrolytes c1 to c7 injected using the negative electrode a3 are referred to as nonaqueous electrolytes. Battery cells C1 to C7, nonaqueous electrolytes c1 to c7 injected using negative electrode a4 were used as nonaqueous electrolyte battery cells D1 to D7, and nonaqueous electrolytes c1 to c7 were injected using negative electrode a5. Are nonaqueous electrolyte battery cells E1 to E7, nonaqueous electrolyte batteries c1 to c7 are injected using negative electrode a6 as nonaqueous electrolyte battery cells F1 to F7, and nonaqueous electrolytes c1 to c7 are formed using negative electrode a7. What was injected was designated as nonaqueous electrolyte battery cells G1 to G7.
[0032]
5. Battery cycle characteristics test
Then, using each of the batteries A1 to A7, B1 to B7, C1 to C7, D1 to D7, E1 to E7, F1 to F7, and G1 to G7 prepared as described above, at room temperature (about 25 ° C.) After the battery was charged at a constant current of 600 mA (1 ItmA) until the battery voltage reached 4.2 V, the battery was charged at a constant voltage of 4.2 V until the current value reached 10 mA. Thereafter, the battery was discharged at a discharge current of 600 mA (1 ItmA) until the battery voltage reached 3.0 V. Then, the discharge capacity at this time was measured and used as an initial discharge capacity. By repeating such charge and discharge, the discharge capacity after 500 cycles and the battery thickness were measured.
[0033]
Then, when the ratio of the discharge capacity after 500 cycles to the initial discharge capacity was determined and the capacity retention rate was obtained, the results shown in the following Tables 1 to 7 were obtained. Also, from the results in Tables 1 to 7, the abscissa represents the amount of VC added in the electrolyte, and the ordinate represents the capacity retention ratio (discharge capacity after 500 cycles with respect to the initial discharge capacity). When the relationship between the volume and the capacity retention ratio was determined, the results shown in FIG. 1 were obtained. FIG. 2 shows the relationship between the amount of VC added to the electrolyte and the battery thickness (battery thickness after 500 cycles) represented by the abscissa. Such a result was obtained. 1 and 2, batteries A1 to A7 are represented by A, batteries B1 to B7 are represented by B, batteries C1 to C7 are represented by C, batteries D1 to D7 are represented by D, and batteries E1 to E7 are represented by D. The battery is represented by E, the batteries F1 to F7 are represented by F, and the batteries G1 to G7 are represented by G.
[0034]
[Table 1]

[0035]
[Table 2]

[0036]
[Table 3]

[0037]
[Table 4]

[0038]
[Table 5]

[0039]
[Table 6]

[0040]
[Table 7]

[0041]
As is clear from the results of Tables 1 to 7 and FIGS. 1 and 2, in the batteries (A1, B1, C1, D1, E1, F1, G1) in which VC was not added to the electrolyte (solvent). The capacity retention ratio increases as the coating amount of amorphous carbon increases. However, in the batteries (C1, D1, E1, F1, and G1) in which the coating amount of amorphous carbon is 1.0% by mass or more, the capacity retention ratio increases. No difference was observed. Also, it can be seen that the battery thickness hardly changes regardless of the amount of the amorphous carbon coating.
On the other hand, in a battery (A1, A2, A3, A4, A5, A6, A7) using a negative electrode (a1, a2, a3, a4, a5, a6, a7) having an amorphous carbon coverage of 0, It can be seen that the larger the amount of VC added, the larger the capacity retention ratio, but the batteries (A4, A5, A6, A7) where the amount of VC added is 1.0% by mass or more do not have a large difference. Also, it can be seen that the battery thickness hardly changes regardless of the amount of VC added.
[0042]
However, batteries (B2-B7, C2-C7, D2-D7, E2-E7, F2- F7, G2 to G7), the capacity retention ratio is increased and the battery thickness is greater than that of the batteries (A1 to A7, B1, C1, D1, E1, F1, G1) in which one of them is 0. It can be seen that the value has decreased and the characteristics have been greatly improved. As this tendency, particularly good characteristics are obtained when the amount of amorphous carbon is 0.1% by mass or more and the electrolyte solvent contains 0.01% by mass of VC.
[0043]
However, the larger the coating amount of amorphous carbon and the added amount of VC, the better, the better. When the amount exceeds a certain amount, the effect hardly changes. This is considered to be due to the following reasons. That is, the properties of the film formed on the surface of the amorphous carbon by the reaction with VC are different from the properties of the film formed on ordinary graphite. For this reason, it is presumed that the film formed on the amorphous carbon exerts the effect of suppressing side reactions such as decomposition of the electrolyte during cycling, and in particular, the effect of suppressing gas generation is large.
Thus, the initial discharge capacities of the batteries (A4, B4, C4, D4, E4, F4, G4) using the electrolyte solvent in which the addition amount of VC was 1.0% by mass are shown in Table 8 below and FIG. The result shown in FIG.
[0044]
[Table 8]

[0045]
As is clear from the results of Table 8 and FIG. 3, it can be seen that the initial capacity tends to gradually decrease as the coating amount of the amorphous carbon increases. This is because the amorphous carbon formed on the surface of graphite serving as a nucleus has a smaller discharge capacity per unit mass than graphite serving as a nucleus. Therefore, the initial capacity decreases as the coating amount of the amorphous carbon increases, so that it can be said that the coating amount of the amorphous carbon is desirably at most 15.0% by mass.
[0046]
6. High temperature charging test
Next, batteries A1, A3, A4, A6, and A7 using the negative electrodes a1, a3, a4, a6, and a7 in which amorphous carbon was not formed, and a negative electrode having an amorphous carbon coverage of 1.0 mass% Using batteries D1, D3, D4, D6, and D7 provided with d1, d3, d4, d6, and d7, battery swelling when charged and left at high temperature was examined. Therefore, these batteries are charged at a constant current of 600 mA (1 ItmA) at room temperature (about 25 ° C.) until the battery voltage becomes 4.2 V, and the current value becomes 10 mA at a constant voltage of 4.2 V. Until the battery was fully charged. Then, it was left in an atmosphere at 80 ° C. for 12 hours. Then, when the thickness of each of these batteries was measured, the results shown in Table 9 below were obtained. Also, based on the results in Table 9, the abscissa represents the amount of VC added in the electrolyte (solvent), and the ordinate represents the battery thickness (battery thickness after high-temperature storage), showing the relationship between the amount of VC and the amount of VC added. Was obtained, the result as shown in FIG. 4 was obtained. In FIG. 4, batteries A1 to A7 are represented by A, and batteries D1 to D7 are represented by D.
[0047]
[Table 9]

[0048]
As is clear from the results of Table 9 and FIG. 4, when there is no amorphous carbon, as in the batteries A1, A3, A4, A6, and A7, the battery thickness increases significantly as the VC amount increases. I understand. On the other hand, in the batteries D1, D3, D4, D6, and D7 in which the coating amount of the amorphous carbon was 1.0% by mass, the battery swelling was surprisingly suppressed as the amount of VC added increased. Turned out to be. This is presumably because the film generated by the reaction between the amorphous carbon layer and the VC during charging suppresses the decomposition of VC on the surface of the active material at a high temperature. This also indicates that it is not enough if the amount of VC added is large, and that the effect hardly changes if it exceeds 1.0% by mass.
[0049]
7. Study on content of composite carbon material in negative electrode active material
Next, the content of the composite carbon material in the negative electrode active material was examined. Therefore, using the composite carbon material α4 prepared such that the coating amount of the amorphous carbon is 1.0% by mass with respect to the mass of the graphite particles serving as the core, the composite carbon material α4 and MCMB are reduced to 20%. : 80 to prepare a negative electrode active material b1.
[0050]
Similarly, a negative electrode active material b2 is prepared by mixing the composite carbon material α4 and MCMB at a ratio of 30:70, and a negative electrode active material b3 is prepared by mixing the composite carbon material α4 and MCMB at a ratio of 40:60. The composite carbon material α4 and MCMB were mixed so as to have a ratio of 50:50 to prepare a negative electrode active material b4, and the composite carbon material α4 and MCMB were mixed such that the ratio was 60:40 to obtain a negative electrode active material b5. The negative electrode active material b6 (same as a4 described above) is prepared by mixing and mixing the composite carbon material α4 and MCMB to be 70:30, and the composite carbon material α4 and MCMB are set to be 80:20. To prepare a negative electrode active material b7.
[0051]
Next, using these negative electrode active materials b1 to b7, negative electrodes were respectively produced in the same manner as described above, and these negative electrodes, the above positive electrodes, and the nonaqueous electrolyte c4 prepared as described above (when the amount of VC added was 1). In the same manner as described above, rectangular nonaqueous electrolyte batteries H1 to H7 each having a nominal capacity of 600 mA and a thickness of 4.40 mm were produced. A battery using the negative electrode active material b1 is referred to as a nonaqueous electrolyte battery H1, a battery using the negative electrode active material b2 is referred to as a nonaqueous electrolyte battery H2, and a battery using the negative electrode active material b3 is referred to as a nonaqueous electrolyte battery. H3, a non-aqueous electrolyte battery H4 using the negative electrode active material b4, a non-aqueous electrolyte battery H5 using the negative electrode active material b5, and a non-aqueous electrolyte battery using the negative electrode active material b5 As the battery H5, a battery using the negative electrode active material b6 was referred to as a nonaqueous electrolyte battery H6 (same as the battery D4 described above), and a battery using the negative electrode active material b7 was referred to as a nonaqueous electrolyte battery H7.
[0052]
Next, after each of these batteries H1 to H7 was fully charged to 4.2 V using each of these batteries H1 to H7, and left in an atmosphere of 80 ° C. for 12 hours, each of these batteries H1 to H7 was When the thickness was measured, the results shown in Table 10 below were obtained. Also, based on the results in Table 10, the addition amount of the composite carbon material is represented on the horizontal axis, and the battery thickness (battery thickness after standing at high temperature) is represented on the vertical axis, and the relationship between the content of the composite carbon material and the battery thickness. Was obtained, the result as shown in FIG. 5 was obtained.
[0053]
[Table 10]

[0054]
As is clear from the results in Table 10 and FIG. 5, the battery thickness after the standing test tends to decrease as the content of the composite carbon material increases. From this result, it is considered that the content of the composite carbon material is preferably 30% by mass or more based on the negative electrode active material. Here, it is considered that the reason that the larger the content of the composite carbon material is, the smaller the swelling after being left at high temperature charge is because the film formed by the reaction between amorphous carbon and VC improves the high temperature storage resistance. . It is estimated that the effect increases as the content of the composite carbon material increases.
[0055]
8. Examination of firing temperature when forming amorphous carbon
Next, the firing temperature when forming amorphous carbon was examined. Therefore, the same scaly natural graphite (d ₀₀₂ Is heat-treated at a temperature of 500 ° C. in a nitrogen gas atmosphere in the same manner as described above using 3.358 ° C., Lc 1000 °, average particle size of 20 μm) and petroleum pitch (softening point: 250 ° C.). Thus, a composite carbon material was prepared in which the coating amount of amorphous carbon was 1.0% by mass with respect to the mass of the graphite particles serving as the nucleus, to obtain a composite carbon material β1. Similarly, heat treatment is performed at a temperature of 700 ° C. to prepare a composite carbon material β2, heat treatment is performed at a temperature of 1000 ° C. to prepare a composite carbon material β3 (same as α4 described above), and heat treatment is performed at a temperature of 1500 ° C. To prepare a composite carbon material β4, heat-treated at a temperature of 2000 ° C. to prepare a composite carbon material β5, heat-treated at a temperature of 2500 ° C. to prepare a composite carbon material β6, and heat-treated at a temperature of 3000 ° C. A composite carbon material β7 was prepared.
[0056]
Next, the composite carbon materials β1 to β7 were mixed with MCMB at a mass ratio of 7: 3 to prepare negative electrode active materials j1 to j7, respectively. The composite carbon material β1 was used as the negative electrode active material j1, the composite carbon material β2 was used as the negative electrode active material j2, and the composite carbon material β3 was used as the negative electrode active material j3 (d4 and d4). The same is applied), the composite carbon material β4 is used as the negative electrode active material j4, the composite carbon material β5 is used as the negative electrode active material j5, and the composite carbon material β6 is used as the negative electrode active material j6. The composite carbon material β7 was used as a negative electrode active material j7.
[0057]
Next, using these negative electrode active materials j1 to j7, negative electrodes were respectively produced in the same manner as described above, and these negative electrodes, the above positive electrodes, and the nonaqueous electrolyte c4 prepared as described above (when the amount of VC added was 1). In the same manner as described above, rectangular non-aqueous electrolyte batteries J1 to J7 each having a nominal capacity of 600 mA and a thickness of 4.40 mm were manufactured using the same method as described above. A battery using the negative electrode active material j1 is referred to as a nonaqueous electrolyte battery J1, a battery using the negative electrode active material j2 is referred to as a nonaqueous electrolyte battery J2, and a battery using the negative electrode active material j3 is referred to as a nonaqueous electrolyte battery. J3 (same as the above-described battery D4), a battery using the negative electrode active material j4 is referred to as a nonaqueous electrolyte battery J4, and a battery using the negative electrode active material j5 is referred to as a nonaqueous electrolyte battery J5. The battery using j6 was referred to as a nonaqueous electrolyte battery J6, and the battery using the negative electrode active material j7 was referred to as a nonaqueous electrolyte battery J7.
[0058]
Next, after each of these batteries J1 to J7 was fully charged to 4.2 V using each of these batteries J1 to J7, the batteries J1 to J7 were left in an atmosphere of 80 ° C. for 12 hours. When the thickness was measured, the results as shown in Table 11 below were obtained. Also, based on the results in Table 11, the baking temperature (° C.) of the carbon material is represented on the abscissa, and the initial capacity and battery thickness (battery thickness after high-temperature storage) are represented on the ordinate, with respect to the baking temperature of the carbon material. When the relationship between the initial capacity and the battery thickness was obtained, the results shown in FIG. 6 were obtained.
[0059]
[Table 11]

[0060]
As is clear from the results shown in Table 11 and FIG. 6, when the firing temperature is low, the initial capacity is small, and the battery thickness after being left at a high temperature is suddenly increased. This is presumably because components other than carbon in the petroleum pitch were not sufficiently removed and a large amount of impurities remained, causing a side reaction. Therefore, from the viewpoint of the discharge capacity, the firing temperature is preferably set to 700 ° C. or higher. When the firing temperature is increased, no significant difference is observed in the capacity and the battery thickness up to around 2500 ° C., and good characteristics are obtained. However, at 3000 ° C., the battery thickness suddenly increases. This is presumably because the surface layer was graphitized, and a preferable amorphous carbon layer was hardly formed during the high-temperature charging. From the above results, it can be said that the firing temperature (heat treatment temperature) is desirably 700 ° C. or higher and 2500 ° C. or lower.
[0061]
【The invention's effect】
As described above, in the nonaqueous electrolyte secondary battery of the present invention, the negative electrode active material includes the composite carbon material in which the surface of the carbonaceous particles serving as nuclei is coated with amorphous carbon. The solvent of the electrolyte contains vinylene carbonate (VC), and the amount of vinylene carbonate (VC) added is 0.01% by mass or more and 10.0% by mass or less based on the mass of the solvent. As a result, a new effect of suppressing battery swelling after leaving at high temperatures or cycling, which is a problem when VC is added, is obtained, and a nonaqueous electrolyte secondary battery having excellent charge / discharge characteristics and cycle characteristics is obtained. It is possible to obtain.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the amount of VC added in an electrolyte and the capacity retention ratio.
FIG. 2 is a graph showing the relationship between the amount of VC added to the electrolyte and the thickness of the battery.
FIG. 3 is a graph showing the relationship between the amount of amorphous carbon and the initial discharge capacity when the amount of VC in the electrolyte is 1.0% by mass.
FIG. 4 is a graph showing the relationship between the amount of VC added to the electrolyte and the thickness of the battery after being left in an atmosphere at 80 ° C. for 12 hours.
FIG. 5 is a graph showing the relationship between the composite carbon content and the battery thickness after being left in an atmosphere at 80 ° C. for 12 hours.
FIG. 6 is a graph showing a relationship between an initial capacity and a battery thickness with respect to a firing temperature of a carbon material after being left in an atmosphere at 80 ° C. for 12 hours.

Claims

A negative electrode containing a negative electrode active material made of a carbonaceous material capable of inserting and removing lithium ions, a positive electrode containing a positive electrode active material capable of inserting and removing lithium ions, and a separator for separating these positive and negative electrodes And a non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte,
The negative electrode active material includes a composite carbon material in which the surface of carbonaceous particles serving as nuclei is coated with amorphous carbon,
The solvent of the non-aqueous electrolyte contains vinylene carbonate (VC),
A non-aqueous electrolyte secondary battery characterized in that the amount of vinylene carbonate (VC) added is 0.01% by mass or more and 10.0% by mass or less based on the mass of the solvent.

The non-aqueous solution according to claim 1, wherein the coating amount of the amorphous carbon is 0.1% by mass or more and 15.0% by mass or less based on the mass of the carbonaceous particles serving as the core. Electrolyte secondary battery.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the composite carbon material is 30% by mass or more based on the mass of the negative electrode active material. 4.

The said amorphous carbon is what the carbon precursor coat | covered on the surface of the carbonaceous particle used as a nucleus formed by the heat processing in inert gas atmosphere is characterized by the above-mentioned. 3. The non-aqueous electrolyte secondary battery according to any one of 3.

The non-aqueous electrolyte secondary battery according to claim 1, wherein the heat treatment temperature is 700 ° C. or more and 2500 ° C. or less.

The non-aqueous electrolyte secondary battery according to any one of claims 1 to 5, wherein the nucleus carbonaceous particles are natural graphite or artificial graphite produced through a graphitization process.