JP2004185991A

JP2004185991A - Negative electrode material for lithium secondary battery, lithium secondary battery using the same, and negative electrode material manufcturing method

Info

Publication number: JP2004185991A
Application number: JP2002351637A
Authority: JP
Inventors: Yusuke Watarai; 祐介渡会; Kanji Hisayoshi; 完治久芳; Nariyoshi Ri; 成圭李
Original assignee: Mitsubishi Materials Corp
Current assignee: Mitsubishi Materials Corp
Priority date: 2002-12-03
Filing date: 2002-12-03
Publication date: 2004-07-02

Abstract

<P>PROBLEM TO BE SOLVED: To restrict a volume change of the Fe-Si group compound particle when occluding or discharging lithium ions, and to maintain lithium ion occlusion quantity and lithium ion discharge quantity by the Fe-Si group compound particles even if the number of repetition of occlusion and discharge of the lithium ions increases. <P>SOLUTION: This negative electrode material is composed of the Fe-Si group compound particles 11 mainly composed of multi-crystal particles 12 formed from a multi-phase multi-crystal body of α-FeSi<SB>2</SB>12a and β-FeSi<SB>2</SB>12b or single phase multi-crystal body of β-FeSi<SB>2</SB>. The Fe-Si group compound particle 11 has any one or both of a Si group membrane 12c for coating a part or the whole of a surface of the multi-crystal particle 12 with Si or SiO<SB>2</SB>and a Si atom forming solid solution inside a crystal particle of the multi-crystal particles. Furthermore, the Fe-Si group compound particle is a compound expressed with an expression FeSi<SB>2+x</SB>(0.01≤x≤4.0). <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、負極材料と、この負極材料を用いたリチウム二次電池と、この負極材料を製造する方法に関するものである。
【０００２】
【従来の技術】
最近、シリコン、或いはシリコンベースの合金又は化合物がリチウム二次電池の負極材料として注目され、多くの特許出願がなされている（例えば、特許文献１、特許文献２参照）。特許文献１には、シリコンを主成分とする負極材料がリチウムイオンなどの軽金属イオンをドープ及び脱ドープ可能な性質を有するように構成された負極材料が開示されている。この負極材料では、シリコンがシリコン単体（単結晶）であるか、或いはＳｉＯ_２やＳｉＣ等のシリコン化合物である。また負極材料に導電性を付与するために、ｐ型又はｎ型の不純物がドープされる。なお、上記負極材料を用いて負極を製造するには、先ずシリコン単体の単結晶又はシリコン化合物の単結晶を粉砕した粒子をアルゴンガス雰囲気中で加熱して乾燥することによりシリコン粒子を作製する。次にこのシリコン粒子と結着剤と溶媒と導電材とを混合してスラリーを調製する。更にこのスラリーを集電体に塗布し乾燥することにより、負極が製造される。またこの負極と、正極と、非水電解液とを用いて非水電解液二次電池が製造される。
【０００３】
このように製造された負極では、炭素質材料を主成分とする負極に比べて密度が高く、結着剤により互いに結着されたシリコン粒子の層間や微細な空間にリチウムイオンを多量にドープ及び脱ドープできる。従って、上記負極材料を用いた非水電解液二次電池は、充放電容量が上記炭素質材料を主成分とする負極より数倍から１０倍高くなり、単位体積当りのエネルギ密度を増大できるようになっている。
【０００４】
また、特許文献２には、金属質物、黒鉛質物及び有機物である炭素質物前駆体を混合し、不活性ガス雰囲気下で焼成する非水系リチウム二次電池用負極材の製造法が開示されている。この製造法では、金属質物が固相Ａ及びＢからなり、固相Ａからなる核粒子の周囲の一部又は全面が固相Ｂにより被覆される。また固相Ａは構成元素としてシリコンを含む。更に固相Ｂは、周期律表の２族元素、遷移金属元素、１２族元素、１３族元素、並びに炭素とシリコンを除く１４族元素からなる群より選ばれた少なくとも１種の元素と、シリコンとの固溶体又は金属間化合物である。
【０００５】
このように構成された非水系リチウム二次電池用負極材の製造法では、体積容量の大きな金属質物と導電性粒子である黒鉛質物を炭素質物前駆体で一体に接触処理するので、電池の容量を増大し、電池のサイクル寿命を延すことができる。
また金属質物及び黒鉛質物の表面に炭素質物が存在することにより、初回充電時に生じる不可逆容量を抑制できるようになっている。
【０００６】
一方、化学式：ＡＢｘで表される化合物からなり、構成相がＡＢ_２単相であるリチウムイオン二次電池用負極材料が開示されている（例えば、特許文献３参照）。この負極材料では、上記化学式ＡＢｘ中のＡがＭｎ、Ｃｏ、Ｍｏ、Ｃｒ、Ｎｂ、Ｖ、Ｃｕ、Ｆｅ、Ｎｉ、Ｗ、Ｔｉ、Ｚｒ、Ｔａ及びＲｅ（希土類元素）からなる群より選ばれた少なくとも１種の元素であり、ＢがＳｉを必須元素とし、かつＳｉ、Ｃ、Ｇｅ、Ｓｎ、Ｐｂ、Ａｌ及びＰからなる群より選ばれた１種以上の元素であり、更にｘは１．７≦ｘ≦２．３の範囲に設定される。
【０００７】
このように構成されたリチウムイオン二次電池用負極材料として、ＦｅＳｉ_２を例にとると、通常の製造方法では、目的とするα−ＦｅＳｉ_２の単相が得られず、α−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２の混相状態となる。そこでα−ＦｅＳｉ_２の単相を得るには次の方法で製造される。先ずＦｅ及びＳｉの合金を実質的に完全溶融状態にする。次にこの溶融物をその化合物の平衡状態図における液相線温度から液相線温度＋５００℃の間の温度で保持し、１００℃／秒以上の冷却速度で凝固をさせて固相線以下の温度にまで冷却した後に、再び加熱して固相線−１０℃以下の温度で保持する熱処理を施す。これにより均一でしかも構成相がα−ＦｅＳｉ_２の単相からなる負極材料が得られる。この負極材料を用いた二次電池では、充放電の繰返し数が多くなっても充放電容量が低下せず、サイクル寿命を向上できるようになっている。
【０００８】
【特許文献１】
特開平１０−８３８１７号公報
【特許文献２】
特開２００１−２１０３２９号公報
【特許文献３】
特開平１０−３１２８０４号公報
【０００９】
【発明が解決しようとする課題】
しかし、上記特許文献１に示された非水電解液二次電池では、結着剤により互いに結着されたシリコン粒子が単結晶であるため、リチウムイオンの吸蔵及び放出時におけるシリコン粒子の体積変化が大きく、二次電池のサイクル特性が低下し、サイクル寿命が短くなる不具合があった。
また、上記特許文献１に示された非水電解液二次電池及び上記特許文献２に示された非水系リチウム二次電池では、初回の充放電効率が低く、充放電の繰返し数が多くなるに従って充放電容量が低下する問題点があった。
【００１０】
また、上記特許文献１に示された非水電解液二次電池及び上記特許文献２に示された非水系リチウム二次電池では、導電性の低いシリコンを使用しているため、高電流密度での充放電を行うことができない問題点もあった。
更に、上記特許文献３に示されたリチウムイオン二次電池用負極材料では、化学式ＡＢｘのＡをＦｅとしかつＢをＳｉとし、所定の熱処理を行って、構成相をα−ＦｅＳｉ_２の単相とした場合、リチウムイオンの吸蔵及び放出時における構成相の体積変化が小さいため、二次電池のサイクル寿命は向上するけれども、α−ＦｅＳｉ_２の単相によるリチウムイオンの吸蔵量及び放出量が小さいため、二次電池の充放電容量が小さい問題点があった。
【００１１】
本発明の目的は、リチウムイオンの吸蔵及び放出時における体積変化を抑制でき、かつリチウムイオンの吸蔵及び放出の繰返し数が多くなってもリチウムイオンの吸蔵量及び放出量を大きい状態に維持できる、リチウム二次電池用負極材料及びこの製造方法を提供することにある。
本発明の別の目的は、充放電容量を大きく維持した状態で、サイクル特性及びサイクル寿命を向上できるとともに、初回の充放電効率を向上でき、更に高電流密度での充放電を行うことができる、リチウム二次電池を提供することにある。
【００１２】
【課題を解決するための手段】
請求項１に係る発明は、図１に示すように、α−ＦｅＳｉ_２１２ａ及びβ−ＦｅＳｉ_２１２ｂの混相多結晶体からなるか或いはβ−ＦｅＳｉ_２の単相多結晶体からなる多結晶粒子１２を主成分とするＦｅ−Ｓｉ系化合物粒子１１により構成され、このＦｅ−Ｓｉ系化合物粒子１１が、多結晶粒子１２の表面の一部又は全部をＳｉ又はＳｉＯ_２により被覆するＳｉ系膜１２ｃ、或いは多結晶粒子の結晶粒内に固溶されたＳｉ原子のいずれか一方又は双方を有することを特徴とするリチウム二次電池用負極材料である。
【００１３】
この請求項１に記載されたリチウム二次電池用負極材料では、Ｆｅ−Ｓｉ系化合物粒子１１によるリチウムイオンの吸蔵及び放出時における、α−ＦｅＳｉ_２１２ａ及びβ−ＦｅＳｉ_２１２ｂの体積変化が小さいため、Ｆｅ−Ｓｉ系化合物粒子１１の体積変化が抑制されるので、Ｆｅ−Ｓｉ系化合物粒子１１内に亀裂が発生することはなく、Ｆｅ−Ｓｉ系化合物粒子１１が割れて細かくなるのを防止できる。またα−ＦｅＳｉ_２１２ａはリチウムイオンの吸蔵量及び放出量が小さいけれども、β−ＦｅＳｉ_２１２ｂはリチウムイオンの吸蔵量及び放出量が比較的大きく、またＳｉ系膜１２ｃ及びＳｉ原子を固溶した多結晶粒子１２はリチウムイオンの吸蔵量及び放出量が大きい。この結果、Ｆｅ−Ｓｉ系化合物粒子１１によるリチウムイオンの吸蔵及び放出の繰返し数が多くなっても、Ｆｅ−Ｓｉ系化合物粒子１１によるリチウムイオンの吸蔵量及び放出量を大きい状態に維持できる。
【００１４】
請求項２に係る発明は、請求項１に係る発明であって、更にＦｅ−Ｓｉ系化合物粒子が式（１）で表された化合物であることを特徴とする。
ＦｅＳｉ_２＋ｘ ……（１）
ここで、式（１）において、０．０１≦ｘ≦４．０である。
この請求項２に記載されたリチウム二次電池用負極材料では、請求項１に記載されたＦｅ−Ｓｉ系化合物粒子が得られる。即ち、α−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２の混相多結晶体或いはβ−ＦｅＳｉ_２の単相多結晶体からなる多結晶粒子と、Ｓｉ系膜又は固溶Ｓｉ原子のいずれか一方又は双方とを有するＦｅ−Ｓｉ系化合物粒子が得られる。
【００１５】
請求項３に係る発明は、請求項１に係る発明であって、更にＦｅ−Ｓｉ系化合物粒子が式（２）で表された化合物であることを特徴とする。
Ｆｅ_１−ｙＭ_ｙＳｉ_２＋ｘ ……（２）
ここで、式（２）において、０．０１≦ｘ≦４．０であり、０．０１≦ｙ≦０．０５であり、Ｍは、Ｃｏ，Ｃｒ，Ｍｎ及びＡｌからなる群より選ばれた１種又は２種以上の元素である。
この請求項３に記載されたリチウム二次電池用負極材料では、α−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２の混相多結晶体或いはβ−ＦｅＳｉ_２の単相多結晶体からなる多結晶粒子と、Ｓｉ系膜又は固溶Ｓｉ原子のいずれか一方又は双方とを有するとともに、α−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２のいずれか一方又は双方の一部のＦｅがＭに置換されたＦｅ−Ｓｉ系化合物粒子が得られる。このようにＭからなる元素をＦｅ−Ｓｉ系化合物粒子に添加すると、負極材料の導電性が向上するので、二次電池の充放電時の電圧降下が小さくなり、高電流密度での充放電を行うことができる。
【００１６】
請求項４に係る発明は、請求項１ないし３いずれかに係る発明であって、更にＦｅ−Ｓｉ系化合物粒子の平均粒径が１０ｎｍ〜１０μｍであることを特徴とする。
上記Ｆｅ−Ｓｉ系化合物粒子の平均粒径は、マイクロトラック法又は顕微鏡観察により測定される。
【００１７】
請求項５に係る発明は、請求項１ないし４いずれかに係る発明であって、更にＦｅ−Ｓｉ系化合物粒子の表面が厚さ１ｎｍ〜１μｍの炭素質膜により被覆されたことを特徴とする。
この請求項５に記載されたリチウム二次電池用負極材料では、上記炭素質膜によりＦｅ−Ｓｉ系化合物粒子の導電性を高めることができる。
【００１８】
請求項６に係る発明は、請求項１ないし５いずれかに記載のＦｅ−Ｓｉ系化合物粒子１００重量％に結合材を０．５〜４０重量％混合して作製されたリチウム二次電池用負極である。
この請求項６に記載されたリチウム二次電池用負極では、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵及び放出時における、α−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２の体積変化が小さいため、Ｆｅ−Ｓｉ系化合物粒子の体積変化が抑制され、またβ−ＦｅＳｉ_２及びＳｉ系膜等の存在によりリチウムイオンの吸蔵量及び放出量が大きくなる。この結果、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵及び放出の繰返し数が多くなっても、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵量及び放出量を大きい状態に維持できるので、負極のサイクル特性及びサイクル寿命を向上できる。
【００１９】
請求項７に係る発明は、請求項６に記載の負極を用いた非水電解液リチウム二次電池である。
請求項８に係る発明は、請求項６に記載の負極を用いリチウムイオンポリマー二次電池である。
この請求項７に記載された非水電解液リチウム二次電池又は請求項８に記載されたリチウムイオンポリマー二次電池では、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵及び放出時における、α−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２の体積変化が小さいため、Ｆｅ−Ｓｉ系化合物粒子の体積変化が抑制され、またβ−ＦｅＳｉ_２及びＳｉ系膜等の存在によりリチウムイオンの吸蔵量及び放出量が大きくなる。この結果、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵及び放出の繰返し数が多くなっても、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵量及び放出量を大きい状態に維持できるので、二次電池のサイクル特性及びサイクル寿命を向上できる。
【００２０】
請求項９に係る発明は、請求項２に記載の式（１）ＦｅＳｉ_２＋ｘ（０．０１≦ｘ≦４．０）で表されかつα−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２の混相多結晶体からなる多結晶粒子を主成分とするＦｅ−Ｓｉ系化合物粒子を、非酸化雰囲気中で６００〜９００℃に２０〜８０時間保持する熱処理を行う工程を含むリチウム二次電池用負極材料の製造方法である。
この請求項９に記載されたリチウム二次電池用負極材料の製造方法では、Ｆｅ−Ｓｉ系化合物粒子に上記熱処理を行うことにより、β−ＦｅＳｉ_２の単相多結晶体からなる多結晶粒子を主成分とするＦｅ−Ｓｉ系化合物粒子を得ることができる。
【００２１】
請求項１０に係る発明は、請求項１ないし４いずれかに記載のＦｅ−Ｓｉ系化合物粒子１００重量％に炭素質粒子を０．０５〜１９００重量％混合して混合体を作製する工程と、この混合体を成形して成形体を作製する工程と、この成形体を不活性ガス雰囲気中又は真空中で３００〜１０００℃の温度に２〜１０時間保持して焼結体を作製する工程と、この焼結体を粉砕する工程とを含むリチウム二次電池用負極材料の製造方法である。
この請求項１０に記載されたリチウム二次電池用負極材料の製造方法では、Ｆｅ−Ｓｉ系化合物粒子に炭素質粒子を混合して焼結した後に粉砕することにより、Ｆｅ−Ｓｉ系化合物粒子に炭素質粒子が一体化されるので、二次電池の充放電時の電圧降下を低減できるとともに、高電流密度での充放電を行うことができる。
【００２２】
【発明の実施の形態】
次に本発明の実施の形態を図面に基づいて説明する。
［１］負極材料
▲１▼ 負極材料の構成
図１に示すように、非水電解液リチウム二次電池又はリチウムイオンポリマー二次電池の負極材料は、α−ＦｅＳｉ_２１２ａ及びβ−ＦｅＳｉ_２１２ｂの混相多結晶体からなる多結晶粒子１２を主成分とするＦｅ−Ｓｉ系化合物粒子１１により構成される。α−ＦｅＳｉ_２１２ａは正方晶系の結晶であり、β−ＦｅＳｉ_２１２ｂは斜方晶系の結晶である。またこのＦｅ−Ｓｉ系化合物粒子１１の表面の一部又は全部はＳｉ又はＳｉＯ_２からなるＳｉ系膜１２ｃにより被覆される。ＳｉからなるＳｉ系膜１２ｃはダイヤモンド構造であり、ＳｉＯ_２からなるＳｉ系膜１２ｃはアモルファス構造又は少量の結晶部を含むアモルファス構造である。また上記Ｆｅ−Ｓｉ系化合物粒子１１はリチウムイオンを吸蔵又は放出する負極活物質である。
【００２３】
なお、上記多結晶粒子は、α−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２の混相多結晶体からなる多結晶粒子ではなく、β−ＦｅＳｉ_２の単相多結晶体からなる多結晶粒子であってもよい。また多結晶粒子の結晶粒内にはＳｉ原子を固溶してもよい。この場合、多結晶粒子の表面をＳｉ系膜により被覆するとともに、多結晶粒子の結晶粒内にＳｉ原子を固溶したり、或いは多結晶粒子の表面をＳｉ系膜により被覆せずに、多結晶粒子の結晶粒内にＳｉ原子を固溶してもよい。
【００２４】
一方、Ｆｅ−Ｓｉ系化合物粒子１１は式（１）で表された化合物であることが好ましい。
ＦｅＳｉ_２＋ｘ ……（１）
ここで、式（１）において、０．０１≦ｘ≦４．０、好ましくは０．０２≦ｘ≦２．０の範囲にある。上記式（１）中のｘを０．０１≦ｘ≦４．０の範囲に限定したのは、０．０１未満ではＦｅ−Ｓｉ系化合物粒子の充放電容量が低く、４．０を越えると充放電する際にＦｅ−Ｓｉ系化合物粒子の膨張及び収縮が激しく、材料のサイクル寿命が短くなるからである。
【００２５】
なお、Ｆｅ−Ｓｉ系化合物粒子は次の式（２）で表された化合物であってもよい。
Ｆｅ_１−ｙＭ_ｙＳｉ_２＋ｘ ……（２）
ここで、式（２）において、ｘは０．０１≦ｘ≦４．０、好ましくは０．０２≦ｘ≦２．０の範囲にあり、ｙは０．０１≦ｙ≦０．０５、好ましくは０．０１≦ｙ≦０．０４の範囲にあり、Ｍは、Ｃｏ，Ｃｒ，Ｍｎ及びＡｌからなる群より選ばれた１種又は２種以上の元素である。上記式（２）中のｙを０．０１≦ｙ≦０．０５の範囲に限定したのは、０．０１未満では十分な電子伝導度が得られず、０．０５を越えると析出粒子により充放電が阻害されるからである。
【００２６】
また上記Ｆｅ−Ｓｉ系化合物粒子１１の平均粒径は１０ｎｍ〜１０μｍ、好ましくは３０ｎｍ〜８μｍである。上記粒子１１の平均粒径を１０ｎｍ〜１０μｍの範囲に限定したのは、１０ｎｍ未満では二次電池の負極を作製する際に上記Ｆｅ−Ｓｉ系化合物粒子が極めて取扱い難くなり、１０μｍを越えると均一な負極合剤を調製し難く、また高電流密度での放電が困難となるからである。
【００２７】
更に上記Ｆｅ−Ｓｉ系化合物粒子１１の表面は厚さ１ｎｍ〜１μｍ、好ましくは１０ｎｍ〜７００ｎｍの炭素質膜により被覆されることが好ましい。炭素質膜はアセチレンブラックやケッチンブラック等により形成される。上記炭素質膜の厚さを１ｎｍ〜１μｍの範囲に限定したのは、１ｎｍ未満では炭素質膜による被覆が十分でなく電気伝導度が十分に得られず、１μｍを越えると炭素質膜の割合が多すぎて負極材料の充放電容量が制限されてしまうからである。
【００２８】
▲２▼ 負極材料の製造方法
先ずＳｉの粒子又は塊とＦｅの粒子又は塊とを、Ｓｉ／Ｆｅが原子比で（２．０１／１．００）〜（６．００／１．００）、好ましくは（２．０２／１．００）〜（４．００／１．００）の割合になるように混合して混合体を作製する。次にこの混合体をるつぼに投入し、不活性ガス雰囲気中で１４００〜１５５０℃に０．５〜５時間保持した後に、室温まで徐冷してインゴットを作製する。更にこのインゴットを万能粉砕機を用いて粉砕した後に、湿式ボールミルや遊星ボールミル等を用いて粉砕し、平均粒径が１０ｎｍ〜１０μｍであるＦｅ−Ｓｉ系化合物粒子を作製する。このＦｅ−Ｓｉ系化合物粒子は、α−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２の混相多結晶体からなる多結晶粒子を主成分とし、多結晶粒子の表面の一部又は全部をＳｉ又はＳｉＯ_２により被覆するＳｉ系膜と、多結晶粒子の結晶粒内に固溶されたＳｉ原子とを有する。
【００２９】
ここでＳｉ／Ｆｅの原子比での混合割合を（２．０１／１．００）〜（６．００／１．００）の範囲に限定したのは、２．０１／１．００未満や６．００／１．００を越えた場合、上記式（１）ＦｅＳｉ_２＋ｘ（０．０１≦ｘ≦４．０）で表されるＦｅ−Ｓｉ系化合物粒子を得ることができないからである。また上記不活性ガス雰囲気としては、アルゴンガス雰囲気、窒素ガス雰囲気等が挙げられ、インゴットを不活性ガス雰囲気に保持したのはインゴットの酸化を防止するためである。
【００３０】
なお、上記粉砕したＦｅ−Ｓｉ系化合物粒子には、Ｍからなる元素、即ちＣｏ，Ｃｒ，Ｍｎ及びＡｌからなる群より選ばれた１種又は２種以上の元素を添加してもよい。これによりＦｅ−Ｓｉ系化合物粒子が式（２）のＦｅ_１−ｙＭ_ｙＳｉ_２＋ｘで表された化合物となる。Ｆｅ−Ｓｉ系化合物粒子に上記Ｍを添加するには、上記Ｓｉの粒子又は塊とＦｅの粒子又は塊との混合体を作製したときに、この混合体に所定量だけ加えればよい。
【００３１】
また、上記式（１）ＦｅＳｉ_２＋ｘ（０．０１≦ｘ≦４．０）で表されかつα−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２の混相多結晶体からなる多結晶粒子を主成分とするＦｅ−Ｓｉ系化合物粒子を、非酸化雰囲気中で６００〜９００℃、好ましくは８００〜９００℃に、２０〜８０時間、好ましくは２０〜５０時間保持する熱処理を行うと、上記多結晶粒子はβ−ＦｅＳｉ_２の単相多結晶体になる。
【００３２】
ここで、上記非酸化雰囲気としては、アルゴンガス、窒素ガス等の不活性ガス雰囲気や、二酸化炭素雰囲気が挙げられ、Ｆｅ−Ｓｉ系化合物粒子を非酸化雰囲気に保持したのはＦｅ−Ｓｉ系化合物粒子の酸化を防止するためである。また上記熱処理温度を６００〜９００℃の範囲に限定したのは、６００℃未満ではβ−ＦｅＳｉ_２の生成反応が進行し難く、９００℃を越えるとβ−ＦｅＳｉ_２がα−ＦｅＳｉ_２に変化してしまうからである。更に上記熱処理時間を２０〜８０時間の範囲に限定したのは、２０時間未満ではβ−ＦｅＳｉ_２の生成量が少なく、８０時間を越えると生産効率が低下するからである。
【００３３】
また、上記Ｆｅ−Ｓｉ系化合物粒子には次のようにして炭素質粒子を混合して一体化することが好ましい。先ず上記Ｆｅ−Ｓｉ系化合物粒子１００重量％に、平均粒径１ｎｍ〜１μｍの炭素質粒子を０．０５〜１９００重量％、好ましくは０．１〜１０００重量％混合して混合体を作製する。次いでこの混合体を成形してペレット状等の成形体を作製する。次にこの成形体を所定の形状に成形した後に、不活性ガス雰囲気中又は真空中で３００〜１０００℃、好ましくは４００〜８００℃の温度に、２〜１０時間、好ましくは２〜５時間保持して焼結体を作製する。更にこの焼結体を湿式ボールミルや遊星ボールミル等により粉砕する。
【００３４】
ここで、上記炭素質粒子の混合割合を０．０５〜１９００重量％の範囲に限定したのは、０．０５重量％未満では焼成した焼結体に十分な導電性が得られず、１９００重量％を越えると炭素の割合が高すぎて負極材料の単位重量当りの充放電容量が低くなるからである。上記不活性ガス雰囲気としては、アルゴンガス雰囲気、二酸化炭素ガス雰囲気等が挙げられる。また上記熱処理温度を３００〜１０００℃の範囲に限定したのは、３００℃未満では炭素とＦｅ−Ｓｉ系化合物が反応せず、１０００℃を越えるとＦｅ−Ｓｉ系化合物中のβ−ＦｅＳｉ_２がα−ＦｅＳｉ_２に変化してしまうからである。更に上記熱処理時間を２〜１０時間の範囲に限定したのは、２時間未満では炭素とＦｅ−Ｓｉ系化合物が十分に反応せず、１０時間を越えると生産コストの不要な増大を招くからである。
【００３５】
更に、上記Ｆｅ−Ｓｉ系化合物粒子は、次のメカニカル法を用いて炭素質膜により被覆することが好ましい。先ず上記Ｆｅ−Ｓｉ系化合物粒子１００重量％に、平均粒径１ｎｍ〜１μｍの炭素質粒子を０．０５〜１９００重量％、好ましくは０．１〜１０００重量％混合して混合体を作製する。次にこの混合体を不活性ガス雰囲気中でステンレス鋼製の容器及びボールを用いたボールミル等により５〜１００時間混合する。これにより平均厚さが１０〜７００ｎｍである炭素質膜により被覆されたＦｅ−Ｓｉ系化合物粒子が得られる。ここで上記混合体の混合時間を５〜１００時間の範囲に限定したのは、５時間未満では炭素粒子とＦｅ−Ｓｉ系化合物粒子が均一に混合されず、１００時間を越えると生産コストの不要な増大を招くという不具合があるからである。なお、Ｆｅ−Ｓｉ系化合物粒子を炭素質膜により被覆する方法としては、ＣＶＤ法やＰＶＤ法などの気相法があるけれども、これらの方法ではＳｉＣが生成されるおそれがあるので、上記メカニカル法が好ましい。
【００３６】
このように製造された負極材料では、図１に示すように、Ｆｅ−Ｓｉ系化合物粒子１１によるリチウムイオンの吸蔵及び放出時における、Ｆｅ−Ｓｉ系化合物粒子１１の体積変化は抑制される。これは多結晶粒子１２中のα−ＦｅＳｉ_２１２ａ及びβ−ＦｅＳｉ_２１２ｂの体積変化が小さいためである。このためＦｅ−Ｓｉ系化合物粒子１１内に亀裂が発生することがないので、Ｆｅ−Ｓｉ系化合物粒子１１が割れて細かくなるのを防止できる。
【００３７】
またα−ＦｅＳｉ_２１２ａはリチウムイオンの吸蔵量及び放出量が小さいけれども、β−ＦｅＳｉ_２１２ｂはリチウムイオンの吸蔵量及び放出量が比較的大きく、Ｓｉ系膜１２ｃ及びＳｉ原子を固溶した多結晶粒子１２はリチウムイオンの吸蔵量及び放出量が大きい。この結果、Ｆｅ−Ｓｉ系化合物粒子１１によるリチウムイオンの吸蔵及び放出の繰返し数が多くなっても、Ｆｅ−Ｓｉ系化合物粒子１１によるリチウムイオンの吸蔵量及び放出量を大きい状態に維持できる。
【００３８】
なお、上記Ｆｅ−Ｓｉ系化合物粒子にＭからなる元素を添加したり、Ｆｅ−Ｓｉ系化合物粒子と炭素質粒子とを一体化したり、或いはＦｅ−Ｓｉ系化合物粒子を炭素質膜により被覆すれば、Ｆｅ−Ｓｉ系化合物粒子の導電性、即ち負極材料の導電性を高めることができるので、二次電池の充放電時の電圧降下が小さくなり、高電流密度での充放電を行うことができる。
【００３９】
［２］負極
▲１▼ 負極の構成
上記Ｆｅ−Ｓｉ系化合物粒子により構成された負極材料に、ポリフッ化ビニリデン等の結合材を混合することにより、負極が作製される。負極材料１００重量％に、０．５〜４０重量％、好ましくは１〜３０重量％の結合材が混合される。
結合材の混合割合を０．５〜４０重量％の範囲に限定したのは、０．５重量％未満では結着力が不足し、４０重量％を越えると二次電池の単位重量当りの充放電容量が低下するからである。
【００４０】
▲２▼ 負極の製造方法
先ず上記［１］▲２▼により得られた負極材料１００重量％と、ポリフッ化ビニリデン（ＰＶｄＦ）等の結合材０．５〜４０重量％、好ましくは１〜３０重量％とを混合して混合体を作製した後に、この混合体とＮ−メチル−２−ピロリドン（ＮＭＰ）等の溶剤とを混合して、負極スラリーを調製する。次にこの負極スラリーを負極集電体箔の上面に、スクリーン印刷法やドクタブレード法などにより塗布し乾燥することにより負極が作製される。なお、負極スラリーをガラス基板上に塗布し乾燥した後に、ガラス基板から剥離して負極フィルムを作製し、更にこの負極フィルムを負極集電体に重ねて所定の圧力でプレス成形することにより、負極を作製してもよい。
【００４１】
このように製造された負極では、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵及び放出時における、α−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２の体積変化が小さいため、Ｆｅ−Ｓｉ系化合物粒子の体積変化が抑制され、またβ−ＦｅＳｉ_２及びＳｉ系膜等の存在によりリチウムイオンの吸蔵量及び放出量が大きくなる。
この結果、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵及び放出の繰返し数が多くなっても、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵量及び放出量を大きい状態に維持できるので、負極のサイクル特性及びサイクル寿命を向上できる。
【００４２】
［３］二次電池の製造方法
▲１▼ 非水電解液リチウム二次電池
先ず上記［２］▲２▼により作製された負極と、非水電解液［例えば、エチレンカーボネート（ＥＣ）とジエチレンカーボネート（ＤＥＣ）からなる混合溶媒（混合重量比１：１）と過塩素酸リチウムを１モル／リットル溶解させたもの］を含む電解質層と、正極集電体上に結合材、正極材料及び導電助剤からなる正極スラリーをドクタブレード法にて塗布し乾燥することにより形成された正極とを用意する。次に上記負極、電解質層及び正極を積層する。これにより非水電解液リチウム二次電池が得られる。
【００４３】
▲２▼ リチウムイオンポリマー二次電池
先ず上記［２］▲２▼により得られた負極と、ポリエチレンオキシドやポリフッ化ビニリデン等からなるポリマー電解質層と、正極集電体上に結合材、正極材料及び導電助剤からなる正極スラリーをドクタブレード法にて塗布し乾燥することにより形成された正極とを用意する。次に上記負極、電解質層及び正極を積層する。これによりリチウムイオンポリマー二次電池が得られる。
【００４４】
このように製造された非水電解液リチウム二次電池やリチウムイオンポリマー二次電池では、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵及び放出時における、α−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２の体積変化が小さいため、Ｆｅ−Ｓｉ系化合物粒子の体積変化が抑制され、またβ−ＦｅＳｉ_２及びＳｉ系膜等の存在によりリチウムイオンの吸蔵量及び放出量が大きくなる。この結果、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵及び放出の繰返し数が多くなっても、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵量及び放出量を大きい状態に維持できるので、二次電池のサイクル特性及びサイクル寿命を向上できるとともに、初回の充放電効率を向上でき、更に高電流密度での充放電を行うことができる。
【００４５】
【実施例】
次に本発明の実施例を比較例とともに詳しく説明する。
＜実施例１＞
▲１▼ Ｆｅ−Ｓｉ系化合物粒子の製造
先ずＳｉ粉末及びＦｅ粉末を、Ｓｉ／Ｆｅの原子比が７２／２８となるように混合して混合体を作製した。この混合体を石英るつぼに投入し、アルゴンガス雰囲気中で１５００℃に１時間保持した後に、室温まで徐冷してインゴットを作製した。次にこのインゴットを万能粉砕機を用いて粉砕した後に、湿式ボールミル等を用いて粉砕し、平均粒径１μｍのＦｅ−Ｓｉ系化合物粒子からなる負極材料を作製した。なお、上記Ｆｅ−Ｓｉ系化合物粒子は、α−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２の混相多結晶体からなる多結晶粒子と、多結晶粒子の表面の一部をＳｉにより被覆するＳｉ系膜と、多結晶粒子の結晶粒内に固溶されたＳｉ原子とを有した。
【００４６】
▲２▼ 負極の製造
先ず上記負極材料７０重量％と、アセチレンブラック（導電助剤）１５重量％と、ポリフッ化ビニリデン（結合材）１５重量％とを混合した後に、この混合体とＮＭＰ（溶剤）と混合して負極スラリーを調製した。次いで上記負極スラリーをガラス基板上に塗布して乾燥した後に剥離することにより厚さ０．０９ｃｍの負極フィルムを作製した。この負極フィルムを縦×横がそれぞれ１．２ｃｍ×１．２ｃｍの正方形に切断して、２枚の正方形の負極フィルムを得た。次にこれらの負極フィルムを縦×横×厚さがそれぞれ１ｃｍ×１ｃｍ×０．１ｃｍの銅メッシュの負極集電体の両面に配置して積層体を作製した。更にこの積層体に１１０〜１３０℃に加熱されたプレス機で０．５〜３ＭＰａの圧力をかけて圧着した。
これにより負極を得た。この負極を実施例１とした。
【００４７】
＜実施例２＞
Ｓｉ粉末及びＦｅ粉末を、Ｓｉ／Ｆｅの原子比が７５／２５となるように混合して混合体を作製したこと以外は、実施例１と同様にして負極を作製した。この負極を実施例２とした。
＜実施例３＞
Ｓｉ粉末及びＦｅ粉末を、Ｓｉ／Ｆｅの原子比が８０／２０となるように混合して混合体を作製したこと以外は、実施例１と同様にして負極を作製した。この負極を実施例３とした。
【００４８】
＜実施例４＞
Ｓｉ粉末、Ｆｅ粉末及びＣｏ粉末を、Ｓｉ／Ｆｅ／Ｃｏの原子比が７２／２７．７２／０．２８となるように混合して混合体を作製したこと以外は、実施例１と同様にして負極を作製した。この負極を実施例４とした。
＜実施例５＞
Ｓｉ粉末、Ｆｅ粉末及びＭｎ粉末を、Ｓｉ／Ｆｅ／Ｍｎの原子比が７２／２７．７２／０．２８となるように混合して混合体を作製したこと以外は、実施例１と同様にして負極を作製した。この負極を実施例５とした。
【００４９】
＜実施例６＞
実施例１と同様に作製された平均粒径１μｍのＦｅ−Ｓｉ系化合物粒子を、アルゴンガス雰囲気中で８００℃に５０時間保持する熱処理を行って、多結晶粒子をβ−ＦｅＳｉ_２の単相多結晶体としたＦｅ−Ｓｉ系化合物粒子、即ち負極材料を得た。この負極材料を用いて実施例１と同様にして負極を作製した。この負極を実施例６とした。
【００５０】
＜実施例７＞
実施例１と同様に作製された平均粒径１μｍのＦｅ−Ｓｉ系化合物粒子１００重量％に、平均粒径１００ｎｍの炭素質粒子を２３３重量％混合して混合体を作製し、この混合体を直径及び高さがそれぞれ５ｍｍ及び１０ｍｍであるペレット状に成形した。次にこのペレット状の混合体をアルゴンガス雰囲気中で８００℃の温度に４時間保持して焼結体を作製した。更にこの焼結体を乳鉢で粉砕した後に、平均粒径１０μｍのＦｅ−Ｓｉ系化合物粒子を分級した。この分級されたＦｅ−Ｓｉ系化合物粒子、即ち負極材料を用いて実施例１と同様にして負極を作製した。この負極を実施例７とした。なお、上記負極材料を粉末Ｘ線回折装置（ＸＲＤ）により観察したところ、異物であるＳｉＣのピークは観察されなかった。
【００５１】
＜実施例８＞
実施例１と同様に作製された平均粒径１μｍのＦｅ−Ｓｉ系化合物粒子１００重量％に、平均粒径１００ｎｍの炭素質粒子を１００重量％混合して混合体を作製した後に、実施例７と同様にして負極を作製した。この負極を実施例８とした。なお、上記負極材料を粉末Ｘ線回折装置（ＸＲＤ）により観察したところ、異物であるＳｉＣのピークは観察されなかった。
＜実施例９＞
実施例１と同様に作製された平均粒径１μｍのＦｅ−Ｓｉ系化合物粒子１００重量％に、平均粒径１００ｎｍの炭素質粒子を４３重量％混合して混合体を作製した後に、実施例７と同様にして負極を作製した。この負極を実施例９とした。なお、上記負極材料を粉末Ｘ線回折装置（ＸＲＤ）により観察したところ、異物であるＳｉＣのピークは観察されなかった。
【００５２】
＜実施例１０＞
実施例１と同様に作製された平均粒径１μｍのＦｅ−Ｓｉ系化合物粒子１００重量％に、平均粒径１００ｎｍの炭素質粒子を７０重量％混合して混合体を作製し、この混合体をステンレス鋼製のボールとともにステンレス鋼製の容器に投入し、容器内をアルゴンガス雰囲気にする。この状態で容器を２０時間回転させて、Ｆｅ−Ｓｉ系化合物粒子をＳｉ系膜により被覆した。このＦｅ−Ｓｉ系化合物粒子、即ち負極材料を用いて実施例１と同様にして負極を作製した。この負極を実施例１０とした。なお、上記Ｓｉ系膜の平均厚さは０．２μｍであった。このＳｉ系膜の平均厚さは、Ｆｅ−Ｓｉ系化合物粒子を割り、その断面を高分解能の走査型電子顕微鏡（ＳＥＭ）にて観察することにより測定した。
【００５３】
＜比較例１＞
Ｓｉ粉末及びＦｅ粉末を、Ｓｉ／Ｆｅの原子比が６６．３／３３．３となるように混合して混合体を作製したこと以外は、実施例１と同様にして負極を作製した。この負極を比較例１とした。
＜比較例２＞
Ｓｉ粉末及びＦｅ粉末を、Ｓｉ／Ｆｅの原子比が９０／１０となるように混合して混合体を作製したこと以外は、実施例１と同様にして負極を作製した。この負極を比較例２とした。
＜比較例３＞
負極材料として、平均粒径が５μｍである単結晶シリコン粒子を用いたことを除いて、実施例１と同様にして負極を作製した。この負極を比較例３とした。
【００５４】
＜比較試験及び評価＞
図２に示すように、実施例１〜１０及び比較例１〜３の負極２１を充放電サイクル試験装置３１に取付けた。この装置３１は、容器３２に電解液３３（リチウム塩を有機溶媒に溶かしたもの）が貯留され、上記負極２１が正極２２（縦×横×厚さがそれぞれ２ｃｍ×２ｃｍ×０．２ｃｍの金属リチウム板：対極）及び参照極２３（縦×横×厚さがそれぞれ１ｃｍ×１ｃｍ×０．２ｃｍの金属リチウム板）とともに電解液３３に浸され、更に負極２１，正極２２及び参照極２３がポテンシオスタット３４（ポテンショメータ）にそれぞれ電気的に接続された構成となっている。この装置を用いて充放電サイクル試験を行い、各負極の初回放電容量（ｍＡ・時／ｇ）と、初回充放電効率（％）と、放電容量維持率（％）をそれぞれ測定し、その結果を、Ｆｅ−Ｓｉ系化合物粒子の組成と、式（１）ＦｅＳｉ_２＋ｘ中のｘ又は式（２）Ｆｅ_１−ｙＭ_ｙＳｉ_２＋ｘ中のｘ及びｙと、Ｆｅ−Ｓｉ系化合物粒子１００重量％に対する炭素質粒子の混合割合と、Ｆｅ−Ｓｉ系化合物粒子粒子の相（α＋βはα−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２の混相多結晶体であり、βはβ−ＦｅＳｉ_２の単相多結晶体である。）ともに、表１に示す。
【００５５】
なお、充放電試験は、充電及び放電時の電流密度を０．５ｍＡ／ｃｍ^２とし、充電時に初期電圧から０．１ＶまでＣＶＣＣ法で負極にリチウムを吸蔵させ、放電時に２ＶまでＣＣ法で負極からリチウムを放出させることにより行った。初回放電容量は最初の放電時の容量であり、初回充放電効率は［（初回放電容量／初回充電容量）×１００％］より算出した。また放電容量維持率（％）は次の式（３）より算出した。
放電容量維持率＝（２０サイクル目の放電容量／初回放電容量）×１００…（３）
【００５６】
【表１】

【００５７】
表１から明らかなように、初回放電容量は、Ｓｉ含有量の少ない比較例１では４２０ｍＡ・時／ｇと小さく、Ｓｉ含有量の多い比較例２及び３では２１００ｍＡ・時／ｇ及び１３００ｍＡ・時／ｇと大きかったのに対し、Ｓｉ含有量が上記の中間の量である実施例１〜３では６５０〜９４０ｍＡ・時／ｇと上記中間の値を示したけれども、初回充放電効率は、比較例１〜３では６２〜７５％と低かったのに対し、実施例１〜３では８３〜８７％と高くなった。また放電容量維持率は、比較例１〜３では１５〜５８％と低かったのに対し、実施例１〜３では８５〜９２％と高くなった。これによりＳｉ含有量が多すぎると、初回放電容量は大きいけれども、サイクル特性が低下することが判った。これは、Ｓｉ成分の過多により、Ｓｉ本来の問題点である初回充放電効率及びサイクル特性の低下が表面化したものと考えられる。
【００５８】
また、ＣｏやＭｎ等を添加していない実施例１では、初回放電容量が６５０ｍＡ・時／ｇであり、初回充放電効率が８７％であったのに対し、Ｃｏ又はＭｎを添加した実施例４及び５では、初回放電容量が６９０ｍＡ・時／ｇ及び６８０ｍＡ・時／ｇと若干高くなり、初回充放電効率も８９％及び８８％と若干高くなった。なお、Ｃｏ又はＭｎを添加した実施例４及び５では、負極の導電性が向上し、充放電時の電圧降下が小さくなり、高電流密度での充放電も可能になった。
【００５９】
また、α−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２の混相多結晶体からなる多結晶粒子により構成されたＦｅ−Ｓｉ系化合物粒子を含む実施例１では、初回放電容量が６５０ｍＡ・時／ｇであり、初回充放電効率が８７％であり、放電容量維持率が９２％であったのに対し、β−ＦｅＳｉ_２の単相多結晶体からなる多結晶粒子により構成されたＦｅ−Ｓｉ系化合物粒子を含む実施例６では、初回放電容量が６８０ｍＡ・時／ｇと高くなり、初回充放電効率が９１％と高くなり、放電容量維持率が９４％と高くなった。これは、α−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２の混相多結晶体からなる多結晶粒子よりβ−ＦｅＳｉ_２の単相多結晶体からなる多結晶粒子の方が、リチウムイオンの吸蔵量及び放出量が大きいためであると考えられる。
【００６０】
また、炭素質粒子をＦｅ−Ｓｉ系化合物粒子に一体化していない実施例１では、初回放電容量が６５０ｍＡ・時／ｇであり、初回充放電効率が８７％であり、放電容量維持率が９２％であったのに対し、Ｆｅ−Ｓｉ系化合物粒子に炭素質粒子を一体化した実施例７〜８では、初回放電容量が３９０〜５４０ｍＡ・時／ｇと低かったけれども、初回充放電効率が８９〜９５％と高くなり、放電容量維持率が９３〜９８％と高くなった。なお、上記Ｆｅ−Ｓｉ系化合物粒子に炭素質粒子を一体化した実施例７〜８では、負極の導電性が向上し、充放電時の電圧降下が小さくなり、高電流密度での充放電も可能になった。
【００６１】
更に、Ｆｅ−Ｓｉ系化合物粒子を炭素質膜により被覆していない実施例１では、初回放電容量が６５０ｍＡ・時／ｇであり、初回充放電効率が８７％であり、放電容量維持率が９２％であったのに対し、Ｆｅ−Ｓｉ系化合物粒子を炭素質膜により被覆した実施例１０では、初回放電容量が５３ｍＡ・時／ｇと低かったけれども、初回充放電効率が９３％と高くなり、放電容量維持率が９５％と高くなった。
【００６２】
【発明の効果】
以上述べたように、本発明によれば、α−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２の混相多結晶体からなるか或いはβ−ＦｅＳｉ_２の単相多結晶体からなる多結晶粒子を主成分とするＦｅ−Ｓｉ系化合物粒子により負極材料を構成し、Ｆｅ−Ｓｉ系化合物粒子が、多結晶粒子の表面の一部又は全部をＳｉ又はＳｉＯ_２により被覆するＳｉ系膜、或いは多結晶粒子の結晶粒内に固溶されたＳｉ原子のいずれか一方又は双方を有するので、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵及び放出時における、Ｆｅ−Ｓｉ系化合物粒子の体積変化が抑制されるとともに、β−ＦｅＳｉ_２及びＳｉ系膜等によりリチウムイオンの吸蔵量及び放出量が大きくなる。この結果、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵及び放出の繰返し数が多くなっても、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵量及び放出量を大きい状態に維持できる。
【００６３】
またＦｅ−Ｓｉ系化合物粒子が式（１）ＦｅＳｉ_２＋ｘで表された化合物であれば、上記多結晶粒子と、Ｓｉ系膜又は固溶Ｓｉ原子とを有するＦｅ−Ｓｉ系化合物粒子が得られる。
またＦｅ−Ｓｉ系化合物粒子が式（２）Ｆｅ_１−ｙＭ_ｙＳｉ_２＋ｘで表された化合物であれば、α−ＦｅＳｉ_２又はβ−ＦｅＳｉ_２中の一部のＦｅがＭに置換されるので、Ｆｅ−Ｓｉ系化合物粒子の導電性、即ち負極材料の導電性を向上できる。またＦｅ−Ｓｉ系化合物粒子の表面を厚さ１ｎｍ〜１μｍの炭素質膜により被覆すれば、Ｆｅ−Ｓｉ系化合物粒子の導電性、即ち負極材料の導電性を高めることができる。この結果、二次電池の充放電時の電圧降下が小さくなり、高電流密度での充放電を行うことができる。
【００６４】
また上記Ｆｅ−Ｓｉ系化合物粒子に結合材を混合して負極を作製すれば、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵及び放出時における、Ｆｅ−Ｓｉ系化合物粒子の体積変化が抑制されるとともに、β−ＦｅＳｉ_２及びＳｉ系膜等の存在によりリチウムイオンの吸蔵量及び放出量が大きくなる。この結果、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵及び放出の繰返し数が多くなっても、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵量及び放出量を大きい状態に維持できるので、負極のサイクル特性及びサイクル寿命を向上できる。
【００６５】
また上記負極を用いた非水電解液リチウム二次電池又はリチウムイオンポリマー二次電池では、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵及び放出時における、Ｆｅ−Ｓｉ系化合物粒子の体積変化が抑制されるとともに、β−ＦｅＳｉ_２及びＳｉ系膜等の存在によりリチウムイオンの吸蔵量及び放出量が大きくなる。この結果、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵及び放出の繰返し数が多くなっても、Ｆｅ−Ｓｉ系化合物粒子によるリチウムイオンの吸蔵量及び放出量を大きい状態に維持できるので、二次電池のサイクル特性及びサイクル寿命を向上できる。
【００６６】
また式（１）ＦｅＳｉ_２＋ｘで表されかつα−ＦｅＳｉ_２及びβ−ＦｅＳｉ_２の混相多結晶体からなる多結晶粒子を主成分とするＦｅ−Ｓｉ系化合物粒子を、非酸化雰囲気中で所定の熱処理を行えば、β−ＦｅＳｉ_２の単相多結晶体からなる多結晶粒子を主成分とするＦｅ−Ｓｉ系化合物粒子を得ることができる。
更にＦｅ−Ｓｉ系化合物粒子に炭素質粒子を混合して混合体を作製し、この混合体を成形した後に所定の雰囲気中で加熱して焼結体を作製し、この焼結体を粉砕すれば、Ｆｅ−Ｓｉ系化合物粒子に炭素質粒子が一体化されるので、二次電池の充放電時の電圧降下を低減できるとともに、高電流密度での充放電を行うことができる。
【図面の簡単な説明】
【図１】本発明実施形態のリチウム二次電池用負極材料を構成するＦｅ−Ｓｉ系化合物粒子の断面図。
【図２】実施例及び比較例のリチウム二次電池用負極の充放電サイクル試験に用いられる装置。
【符号の説明】
１１Ｆｅ−Ｓｉ系化合物粒子
１２多結晶粒子
１２ａ α−ＦｅＳｉ_２
１２ｂ β−ＦｅＳｉ_２
１２ｃＳｉ系膜[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a negative electrode material, a lithium secondary battery using the negative electrode material, and a method for manufacturing the negative electrode material.
[0002]
[Prior art]
Recently, silicon or a silicon-based alloy or compound has attracted attention as a negative electrode material of a lithium secondary battery, and many patent applications have been filed (for example, see Patent Documents 1 and 2). Patent Literature 1 discloses a negative electrode material configured such that a negative electrode material containing silicon as a main component has a property of being capable of doping and undoping light metal ions such as lithium ions. In this negative electrode material, silicon is silicon alone (single crystal) or SiO₂And silicon compounds such as SiC. In order to impart conductivity to the negative electrode material, a p-type or n-type impurity is doped. In order to manufacture a negative electrode using the above negative electrode material, first, silicon particles are prepared by heating and drying particles obtained by pulverizing a single crystal of silicon alone or a single crystal of a silicon compound in an argon gas atmosphere. Next, the silicon particles, the binder, the solvent, and the conductive material are mixed to prepare a slurry. Further, the slurry is applied to a current collector and dried to produce a negative electrode. Further, a non-aqueous electrolyte secondary battery is manufactured using the negative electrode, the positive electrode, and the non-aqueous electrolyte.
[0003]
In the negative electrode manufactured in this way, the density is higher than that of the negative electrode containing carbonaceous material as a main component, and a large amount of lithium ions are doped into interlayers and fine spaces between silicon particles bonded to each other by a binder. Can be undoped. Therefore, the non-aqueous electrolyte secondary battery using the negative electrode material has a charge / discharge capacity several to ten times higher than that of the negative electrode containing the carbonaceous material as a main component, and can increase the energy density per unit volume. It has become.
[0004]
Patent Document 2 discloses a method for producing a negative electrode material for a non-aqueous lithium secondary battery in which a metal substance, a graphite substance, and a carbonaceous substance precursor that is an organic substance are mixed and fired in an inert gas atmosphere. . In this manufacturing method, the metallic substance is composed of the solid phases A and B, and a part or the entire periphery of the core particles composed of the solid phase A is covered with the solid phase B. The solid phase A contains silicon as a constituent element. Further, the solid phase B comprises at least one element selected from the group consisting of Group 2 elements, transition metal elements, Group 12 elements, and Group 13 elements of the periodic table, and Group 14 elements excluding carbon and silicon; And a solid solution or an intermetallic compound.
[0005]
In the method for manufacturing a negative electrode material for a non-aqueous lithium secondary battery configured as described above, a metal material having a large volume capacity and a graphite material which is a conductive particle are integrally contact-treated with a carbonaceous material precursor. And the cycle life of the battery can be extended.
In addition, the presence of the carbonaceous material on the surfaces of the metal material and the graphite material makes it possible to suppress the irreversible capacity generated during the first charging.
[0006]
On the other hand, it is composed of a compound represented by the chemical formula: ABx, and its constituent phase is AB₂A single-phase negative electrode material for a lithium ion secondary battery is disclosed (for example, see Patent Document 3). In this negative electrode material, A in the chemical formula ABx is selected from the group consisting of Mn, Co, Mo, Cr, Nb, V, Cu, Fe, Ni, W, Ti, Zr, Ta, and Re (rare earth element). B is at least one element, B is Si as an essential element, and B is at least one element selected from the group consisting of Si, C, Ge, Sn, Pb, Al, and P; It is set in the range of 7 ≦ x ≦ 2.3.
[0007]
As a negative electrode material for a lithium ion secondary battery configured as described above, FeSi₂For example, in a normal manufacturing method, the desired α-FeSi₂Of α-FeSi₂And β-FeSi₂In a mixed phase state. Therefore, α-FeSi₂Is obtained by the following method. First, the alloy of Fe and Si is brought into a substantially completely molten state. Next, the melt is held at a temperature between the liquidus temperature and the liquidus temperature + 500 ° C. in the equilibrium diagram of the compound, and is solidified at a cooling rate of 100 ° C./sec or more, and solidified below the solidus temperature. After cooling to a temperature, a heat treatment for heating again to maintain the solidus at a temperature of -10 ° C or lower is performed. As a result, the composition is uniform and the constituent phase is α-FeSi₂Thus, a negative electrode material composed of a single phase is obtained. In a secondary battery using this negative electrode material, the charge / discharge capacity does not decrease even if the number of charge / discharge repetitions increases, and the cycle life can be improved.
[0008]
[Patent Document 1]
JP-A-10-83817
[Patent Document 2]
JP 2001-210329 A
[Patent Document 3]
JP-A-10-318804
[0009]
[Problems to be solved by the invention]
However, in the non-aqueous electrolyte secondary battery disclosed in Patent Document 1, since the silicon particles bound to each other by the binder are single crystals, the volume change of the silicon particles during occlusion and release of lithium ions. , The cycle characteristics of the secondary battery deteriorated, and the cycle life was shortened.
Further, in the non-aqueous electrolyte secondary battery described in Patent Document 1 and the non-aqueous lithium secondary battery described in Patent Document 2, initial charge / discharge efficiency is low, and the number of charge / discharge repetitions is large. , The charge / discharge capacity is reduced.
[0010]
Further, in the non-aqueous electrolyte secondary battery described in Patent Document 1 and the non-aqueous lithium secondary battery described in Patent Document 2, since silicon having low conductivity is used, high current density can be obtained. There was also a problem that it was not possible to perform charging and discharging.
Further, in the negative electrode material for a lithium ion secondary battery described in Patent Document 3, A in the chemical formula ABx is Fe and B is Si, and a predetermined heat treatment is performed to change the constituent phase to α-FeSi.₂When the single-phase is used, the change in the volume of the constituent phases during insertion and extraction of lithium ions is small, so that the cycle life of the secondary battery is improved.₂However, there is a problem that the charge and discharge capacity of the secondary battery is small because the amount of occlusion and release of lithium ions by the single phase is small.
[0011]
An object of the present invention is to suppress a change in volume at the time of occlusion and release of lithium ions, and to maintain a large amount of lithium ion occlusion and release even when the number of repetitions of occlusion and release of lithium ions increases, An object of the present invention is to provide a negative electrode material for a lithium secondary battery and a method for manufacturing the same.
Another object of the present invention is to improve the cycle characteristics and cycle life while maintaining a large charge / discharge capacity, improve the initial charge / discharge efficiency, and perform charge / discharge at a high current density. To provide a lithium secondary battery.
[0012]
[Means for Solving the Problems]
As shown in FIG. 1, the invention according to claim 1 is based on α-FeSi₂12a and β-FeSi₂12b consisting of a mixed phase polycrystal or β-FeSi₂Is composed of Fe-Si-based compound particles 11 mainly composed of polycrystalline particles 12 made of a single-phase polycrystalline material, and the Fe-Si-based compound particles 11 form part or all of the surface of the polycrystalline particles 12. Si or SiO₂A negative electrode material for a lithium secondary battery, characterized in that the negative electrode material has one or both of a Si-based film 12c coated with a Si atom and / or Si atoms dissolved in crystal grains of polycrystalline particles.
[0013]
In the negative electrode material for a lithium secondary battery according to the first aspect, α-FeSi during occlusion and release of lithium ions by the Fe—Si-based compound particles 11.₂12a and β-FeSi₂Since the change in volume of 12b is small, the change in volume of the Fe-Si-based compound particles 11 is suppressed, so that no crack is generated in the Fe-Si-based compound particles 11, and the Fe-Si-based compound particles 11 are broken. Can be prevented from becoming fine. Α-FeSi₂Although 12a has a small lithium ion occlusion and release amount, β-FeSi₂12b has a relatively large amount of occlusion and release of lithium ions, and the Si-based film 12c and the polycrystalline particles 12 having a solid solution of Si atoms have a large amount of occlusion and release of lithium ions. As a result, even if the number of repetitions of occlusion and release of lithium ions by the Fe-Si-based compound particles 11 increases, the amount of occlusion and release of lithium ions by the Fe-Si-based compound particles 11 can be maintained in a large state.
[0014]
The invention according to claim 2 is the invention according to claim 1, wherein the Fe—Si-based compound particles are a compound represented by the formula (1).
FeSi_{2 + x}            ...... (1)
Here, in Expression (1), 0.01 ≦ x ≦ 4.0.
In the negative electrode material for a lithium secondary battery according to the second aspect, the Fe-Si-based compound particles according to the first aspect are obtained. That is, α-FeSi₂And β-FeSi₂Mixed-phase polycrystal or β-FeSi₂Thus, Fe-Si-based compound particles having polycrystalline particles made of a single-phase polycrystalline material and one or both of a Si-based film and solid solution Si atoms are obtained.
[0015]
The invention according to claim 3 is the invention according to claim 1, wherein the Fe—Si-based compound particles are a compound represented by the formula (2).
Fe_1-yM_ySi_{2 + x}      …… (2)
Here, in the formula (2), 0.01 ≦ x ≦ 4.0 and 0.01 ≦ y ≦ 0.05, and M is selected from the group consisting of Co, Cr, Mn and Al. One or more elements.
In the negative electrode material for a lithium secondary battery according to the third aspect, α-FeSi₂And β-FeSi₂Mixed-phase polycrystal or β-FeSi₂And polycrystalline particles comprising a single-phase polycrystalline material, and either or both of a Si-based film and solid solution Si atoms.₂And β-FeSi₂Fe-Si-based compound particles in which one or both of Fe are partially substituted by M are obtained. When the element composed of M is added to the Fe-Si-based compound particles as described above, the conductivity of the negative electrode material is improved, so that the voltage drop during charging and discharging of the secondary battery is reduced, and charging and discharging at a high current density can be performed. It can be carried out.
[0016]
The invention according to a fourth aspect is the invention according to any one of the first to third aspects, further characterized in that the average particle diameter of the Fe—Si-based compound particles is 10 nm to 10 μm.
The average particle diameter of the Fe-Si-based compound particles is measured by a microtrack method or a microscope observation.
[0017]
The invention according to claim 5 is the invention according to any one of claims 1 to 4, wherein the surface of the Fe-Si-based compound particles is further covered with a carbonaceous film having a thickness of 1 nm to 1 µm. .
In the negative electrode material for a lithium secondary battery according to the fifth aspect, the conductivity of the Fe—Si-based compound particles can be increased by the carbonaceous film.
[0018]
According to a sixth aspect of the present invention, there is provided a negative electrode for a lithium secondary battery produced by mixing a binder in an amount of 0.5 to 40% by weight with 100% by weight of the Fe-Si compound particles according to any one of the first to fifth aspects. It is.
In the negative electrode for a lithium secondary battery according to claim 6, α-FeSi when occlusion and release of lithium ions by the Fe—Si-based compound particles is achieved.₂And β-FeSi₂Is small, the volume change of the Fe-Si-based compound particles is suppressed, and β-FeSi₂In addition, the amount of occluded and released lithium ions increases due to the presence of the Si film and the like. As a result, even if the number of repetitions of occlusion and release of lithium ions by the Fe-Si-based compound particles increases, the amount of lithium ions occluded and released by the Fe-Si-based compound particles can be maintained in a large state. Cycle characteristics and cycle life can be improved.
[0019]
The invention according to claim 7 is a non-aqueous electrolyte lithium secondary battery using the negative electrode according to claim 6.
The invention according to claim 8 is a lithium ion polymer secondary battery using the negative electrode according to claim 6.
In the non-aqueous electrolyte lithium secondary battery according to the seventh aspect or the lithium ion polymer secondary battery according to the eighth aspect, at the time of occlusion and release of lithium ions by the Fe-Si-based compound particles, α- FeSi₂And β-FeSi₂Is small, the volume change of the Fe-Si-based compound particles is suppressed, and β-FeSi₂In addition, the amount of occluded and released lithium ions increases due to the presence of the Si film and the like. As a result, even if the number of repetitions of occlusion and release of lithium ions by the Fe-Si-based compound particles increases, the amount of occlusion and release of lithium ions by the Fe-Si-based compound particles can be maintained in a large state. The cycle characteristics and cycle life of the battery can be improved.
[0020]
According to a ninth aspect of the present invention, there is provided the formula (1) FeSi according to the second aspect._{2 + x}(0.01 ≦ x ≦ 4.0) and α-FeSi₂And β-FeSi₂For a lithium secondary battery, comprising a step of performing a heat treatment in which Fe-Si-based compound particles mainly composed of polycrystalline particles composed of a mixed phase polycrystalline material are kept at 600 to 900 ° C. for 20 to 80 hours in a non-oxidizing atmosphere. This is a method for producing a negative electrode material.
In the method for producing a negative electrode material for a lithium secondary battery according to the ninth aspect, the heat treatment is performed on the Fe-Si-based compound particles so that β-FeSi₂Thus, Fe-Si-based compound particles mainly composed of polycrystalline particles composed of a single-phase polycrystalline material can be obtained.
[0021]
The invention according to claim 10 is a step of preparing a mixture by mixing 0.05 to 1900% by weight of carbonaceous particles with 100% by weight of the Fe—Si-based compound particles according to any one of claims 1 to 4, Forming the mixture to form a molded body; and holding the molded body at a temperature of 300 to 1000 ° C. in an inert gas atmosphere or vacuum for 2 to 10 hours to form a sintered body. And a step of pulverizing the sintered body.
In the method for producing a negative electrode material for a lithium secondary battery according to claim 10, the Fe-Si-based compound particles are mixed with carbonaceous particles, sintered, and then pulverized, so that the Fe-Si-based compound particles are ground. Since the carbonaceous particles are integrated, a voltage drop during charging and discharging of the secondary battery can be reduced, and charging and discharging at a high current density can be performed.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, an embodiment of the present invention will be described with reference to the drawings.
[1] Negative electrode material
(1) Composition of negative electrode material
As shown in FIG. 1, the negative electrode material of the non-aqueous electrolyte lithium secondary battery or the lithium ion polymer secondary battery is α-FeSi₂12a and β-FeSi₂It is composed of Fe—Si-based compound particles 11 mainly composed of polycrystalline particles 12 made of a mixed phase polycrystalline material 12b. α-FeSi₂12a is a tetragonal crystal, β-FeSi₂12b is an orthorhombic crystal. Part or all of the surface of the Fe-Si-based compound particles 11 is made of Si or SiO.₂Covered with a Si-based film 12c made of The Si-based film 12c made of Si has a diamond structure,₂Has an amorphous structure or an amorphous structure including a small amount of crystal parts. Further, the Fe-Si-based compound particles 11 are a negative electrode active material that stores or releases lithium ions.
[0023]
The polycrystalline particles are α-FeSi₂And β-FeSi₂Β-FeSi instead of polycrystalline particles consisting of multiphase polycrystalline₂May be polycrystalline particles composed of a single-phase polycrystalline material. Further, Si atoms may be dissolved in the crystal grains of the polycrystalline particles. In this case, the surface of the polycrystalline particles is covered with the Si-based film, and the Si atoms are dissolved in the crystal grains of the polycrystalline particles, or the surface of the polycrystalline particles is not covered with the Si-based film. Si atoms may be dissolved in the crystal grains of the crystal grains.
[0024]
On the other hand, the Fe-Si-based compound particles 11 are preferably a compound represented by the formula (1).
FeSi_{2 + x}            ...... (1)
Here, in the formula (1), it is in the range of 0.01 ≦ x ≦ 4.0, preferably 0.02 ≦ x ≦ 2.0. The reason why x in the above formula (1) is limited to the range of 0.01 ≦ x ≦ 4.0 is that when the value is less than 0.01, the charge / discharge capacity of the Fe—Si-based compound particles is low, and the value exceeds 4.0. This is because the Fe-Si-based compound particles undergo severe expansion and contraction during charge and discharge, and the cycle life of the material is shortened.
[0025]
The Fe-Si-based compound particles may be a compound represented by the following formula (2).
Fe_1-yM_ySi_{2 + x}      …… (2)
Here, in the formula (2), x is in the range of 0.01 ≦ x ≦ 4.0, preferably 0.02 ≦ x ≦ 2.0, and y is 0.01 ≦ y ≦ 0.05, preferably Is in the range of 0.01 ≦ y ≦ 0.04, and M is one or more elements selected from the group consisting of Co, Cr, Mn and Al. The reason that y in the above formula (2) is limited to the range of 0.01 ≦ y ≦ 0.05 is that if it is less than 0.01, sufficient electron conductivity cannot be obtained. This is because charging and discharging are hindered.
[0026]
The average particle diameter of the Fe-Si-based compound particles 11 is 10 nm to 10 μm, preferably 30 nm to 8 μm. The reason why the average particle diameter of the particles 11 is limited to the range of 10 nm to 10 μm is that when the particle diameter is less than 10 nm, the Fe—Si compound particles are extremely difficult to handle when producing a negative electrode of a secondary battery. This is because it is difficult to prepare a suitable negative electrode mixture and it is difficult to discharge at a high current density.
[0027]
Further, the surface of the Fe-Si-based compound particles 11 is preferably covered with a carbonaceous film having a thickness of 1 nm to 1 μm, preferably 10 nm to 700 nm. The carbonaceous film is formed of acetylene black, ketchin black, or the like. The reason why the thickness of the carbonaceous film is limited to the range of 1 nm to 1 μm is that if the thickness is less than 1 nm, the coating with the carbonaceous film is not sufficient and sufficient electric conductivity cannot be obtained. Is too large to limit the charge / discharge capacity of the negative electrode material.
[0028]
(2) Manufacturing method of negative electrode material
First, the Si particles or the lump and the Fe particles or the lump are separated by an atomic ratio of Si / Fe of (2.01 / 1.00) to (6.00 / 1.00), preferably (2.02 / 1/1). (0.000) to (4.00 / 1.00) to prepare a mixture. Next, this mixture is put into a crucible, and is kept at 1400 to 1550 ° C. for 0.5 to 5 hours in an inert gas atmosphere, and then gradually cooled to room temperature to produce an ingot. Further, the ingot is pulverized using a universal pulverizer, and then pulverized using a wet ball mill, a planetary ball mill, or the like to produce Fe-Si-based compound particles having an average particle diameter of 10 nm to 10 μm. The Fe-Si-based compound particles are composed of α-FeSi₂And β-FeSi₂The main component is polycrystalline particles composed of a mixed phase polycrystalline material, and a part or all of the surface of the polycrystalline particles is Si or SiO.₂And a Si atom solid-dissolved in the crystal grains of the polycrystalline particles.
[0029]
Here, the reason why the mixing ratio in the atomic ratio of Si / Fe is limited to the range of (2.01 / 1.00) to (6.00 / 1.00) is that the mixing ratio is less than 2.01 / 1.00 or 6 / 1.00. If it exceeds 0.000 / 1.00, the above formula (1)_{2 + x}This is because Fe—Si-based compound particles represented by (0.01 ≦ x ≦ 4.0) cannot be obtained. Examples of the inert gas atmosphere include an argon gas atmosphere and a nitrogen gas atmosphere. The reason why the ingot is kept in the inert gas atmosphere is to prevent oxidation of the ingot.
[0030]
The pulverized Fe-Si-based compound particles may be added with an element consisting of M, that is, one or more elements selected from the group consisting of Co, Cr, Mn, and Al. As a result, the Fe—Si-based compound particles become Fe 2_1-yM_ySi_{2 + x}The compound represented by In order to add the above-mentioned M to the Fe-Si-based compound particles, when a mixture of the above-mentioned Si particles or lumps and Fe particles or lumps is prepared, a predetermined amount may be added to the mixture.
[0031]
In addition, the above formula (1)_{2 + x}(0.01 ≦ x ≦ 4.0) and α-FeSi₂And β-FeSi₂Fe-Si-based compound particles mainly composed of polycrystalline particles composed of a mixed phase polycrystalline material in a non-oxidizing atmosphere at 600 to 900 ° C, preferably 800 to 900 ° C for 20 to 80 hours, preferably 20 to 80 hours. When a heat treatment of holding for 50 hours is performed, the polycrystalline particles become β-FeSi₂Of a single-phase polycrystal.
[0032]
Here, examples of the non-oxidizing atmosphere include an inert gas atmosphere such as an argon gas and a nitrogen gas, and a carbon dioxide atmosphere. The Fe-Si-based compound particles are kept in a non-oxidizing atmosphere by the Fe-Si-based compound. This is to prevent oxidation of the particles. The reason why the heat treatment temperature is limited to the range of 600 to 900 ° C. is that β-FeSi₂Is difficult to proceed, and if the temperature exceeds 900 ° C., β-FeSi₂Is α-FeSi₂Because it will change to Further, the heat treatment time was limited to the range of 20 to 80 hours because the β-FeSi₂This is because the production amount is small, and if it exceeds 80 hours, the production efficiency decreases.
[0033]
Further, it is preferable that carbonaceous particles are mixed and integrated with the Fe-Si-based compound particles as follows. First, carbonaceous particles having an average particle diameter of 1 nm to 1 μm are mixed with 100% by weight of the Fe—Si-based compound particles in an amount of 0.05 to 1900% by weight, preferably 0.1 to 1000% by weight to prepare a mixture. Next, this mixture is molded to produce a molded article such as a pellet. Next, after the formed body is formed into a predetermined shape, it is kept at a temperature of 300 to 1000 ° C., preferably 400 to 800 ° C. in an inert gas atmosphere or vacuum for 2 to 10 hours, preferably 2 to 5 hours. To produce a sintered body. Further, the sintered body is pulverized by a wet ball mill, a planetary ball mill, or the like.
[0034]
The reason why the mixing ratio of the carbonaceous particles is limited to the range of 0.05 to 1900% by weight is that if the content is less than 0.05% by weight, sufficient conductivity is not obtained in the fired sintered body, and 1900% by weight is not obtained. %, The proportion of carbon is too high and the charge / discharge capacity per unit weight of the negative electrode material decreases. Examples of the inert gas atmosphere include an argon gas atmosphere and a carbon dioxide gas atmosphere. The reason why the heat treatment temperature is limited to the range of 300 to 1000 ° C. is that the carbon does not react with the Fe—Si compound at a temperature lower than 300 ° C., and the β-FeSi compound in the Fe—Si compound at a temperature higher than 1000 ° C.₂Is α-FeSi₂Because it will change to The reason why the heat treatment time is limited to the range of 2 to 10 hours is that if the heat treatment time is less than 2 hours, the carbon and the Fe-Si compound do not sufficiently react, and if the heat treatment time exceeds 10 hours, the production cost is unnecessarily increased. is there.
[0035]
Further, the Fe-Si-based compound particles are preferably coated with a carbonaceous film using the following mechanical method. First, carbonaceous particles having an average particle diameter of 1 nm to 1 μm are mixed with 100% by weight of the Fe—Si-based compound particles in an amount of 0.05 to 1900% by weight, preferably 0.1 to 1000% by weight to prepare a mixture. Next, the mixture is mixed in an inert gas atmosphere by a ball mill using a stainless steel container and balls for 5 to 100 hours. Thus, Fe-Si-based compound particles covered with a carbonaceous film having an average thickness of 10 to 700 nm are obtained. The reason why the mixing time of the mixture is limited to the range of 5 to 100 hours is that the carbon particles and the Fe-Si-based compound particles are not uniformly mixed for less than 5 hours, and the production cost is unnecessary for more than 100 hours. This is because there is a problem of causing a great increase. As a method for coating the Fe-Si-based compound particles with a carbonaceous film, there are gas-phase methods such as a CVD method and a PVD method. However, since these methods may generate SiC, the above-mentioned mechanical method is used. Is preferred.
[0036]
In the negative electrode material thus manufactured, as shown in FIG. 1, a change in the volume of the Fe—Si-based compound particles 11 during insertion and extraction of lithium ions by the Fe—Si-based compound particles 11 is suppressed. This is because α-FeSi in the polycrystalline particles 12₂12a and β-FeSi₂This is because the change in volume of 12b is small. For this reason, since cracks do not occur in the Fe-Si-based compound particles 11, it is possible to prevent the Fe-Si-based compound particles 11 from being cracked and becoming fine.
[0037]
Α-FeSi₂Although 12a has a small lithium ion occlusion and release amount, β-FeSi₂12b has a relatively large amount of occlusion and release of lithium ions, and the Si-based film 12c and the polycrystalline particles 12 having a solid solution of Si atoms have a large amount of occlusion and release of lithium ions. As a result, even if the number of repetitions of occlusion and release of lithium ions by the Fe-Si-based compound particles 11 increases, the amount of occlusion and release of lithium ions by the Fe-Si-based compound particles 11 can be maintained in a large state.
[0038]
In addition, if an element consisting of M is added to the Fe-Si-based compound particles, the Fe-Si-based compound particles are integrated with the carbonaceous particles, or the Fe-Si-based compound particles are coated with a carbonaceous film. Since the conductivity of the Fe-Si-based compound particles, that is, the conductivity of the negative electrode material, can be increased, the voltage drop during charging and discharging of the secondary battery is reduced, and charging and discharging at a high current density can be performed. .
[0039]
[2] Negative electrode
(1) Configuration of negative electrode
A negative electrode is manufactured by mixing a binder such as polyvinylidene fluoride into the negative electrode material composed of the Fe-Si-based compound particles. 0.5 to 40% by weight, preferably 1 to 30% by weight of a binder is mixed with 100% by weight of the negative electrode material.
The reason why the mixing ratio of the binder is limited to the range of 0.5 to 40% by weight is that when the content is less than 0.5% by weight, the binding force is insufficient, and when it exceeds 40% by weight, the charge / discharge per unit weight of the secondary battery is performed. This is because the capacity decreases.
[0040]
(2) Method for manufacturing negative electrode
First, 100% by weight of the negative electrode material obtained in the above [1] and [2] and 0.5 to 40% by weight, preferably 1 to 30% by weight of a binder such as polyvinylidene fluoride (PVdF) are mixed and mixed. After forming the body, the mixture is mixed with a solvent such as N-methyl-2-pyrrolidone (NMP) to prepare a negative electrode slurry. Next, the negative electrode slurry is applied to the upper surface of the negative electrode current collector foil by a screen printing method, a doctor blade method, or the like, and dried to prepare a negative electrode. The negative electrode slurry was applied on a glass substrate, dried, and then separated from the glass substrate to produce a negative electrode film. The negative electrode film was further laminated on the negative electrode current collector and press-molded at a predetermined pressure to form the negative electrode film. May be produced.
[0041]
In the negative electrode thus manufactured, α-FeSi at the time of occlusion and release of lithium ions by the Fe-Si-based compound particles is used.₂And β-FeSi₂Is small, the volume change of the Fe-Si-based compound particles is suppressed, and β-FeSi₂In addition, the amount of occluded and released lithium ions increases due to the presence of the Si film and the like.
As a result, even if the number of repetitions of occlusion and release of lithium ions by the Fe-Si-based compound particles increases, the amount of lithium ions occluded and released by the Fe-Si-based compound particles can be maintained in a large state. Cycle characteristics and cycle life can be improved.
[0042]
[3] Manufacturing method of secondary battery
(1) Non-aqueous electrolyte lithium secondary battery
First, a negative electrode prepared according to the above [2] [2], a non-aqueous electrolyte [for example, a mixed solvent of ethylene carbonate (EC) and diethylene carbonate (DEC) (mixing weight ratio 1: 1) and lithium perchlorate] Is dissolved at 1 mol / liter) and a positive electrode slurry comprising a binder, a positive electrode material, and a conductive additive is applied on a positive electrode current collector by a doctor blade method, and dried. Prepare a positive electrode. Next, the negative electrode, the electrolyte layer and the positive electrode are laminated. Thus, a non-aqueous electrolyte lithium secondary battery is obtained.
[0043]
(2) Lithium ion polymer secondary battery
First, a negative electrode obtained by the above [2] [2], a polymer electrolyte layer made of polyethylene oxide, polyvinylidene fluoride or the like, and a positive electrode slurry made of a binder, a positive electrode material and a conductive auxiliary on a positive electrode current collector A positive electrode formed by applying and drying by a blade method is prepared. Next, the negative electrode, the electrolyte layer and the positive electrode are laminated. Thereby, a lithium ion polymer secondary battery is obtained.
[0044]
In the non-aqueous electrolyte lithium secondary battery and the lithium ion polymer secondary battery manufactured as described above, α-FeSi when absorbing and releasing lithium ions by Fe-Si-based compound particles is used.₂And β-FeSi₂Is small, the volume change of the Fe-Si-based compound particles is suppressed, and β-FeSi₂In addition, the amount of occluded and released lithium ions increases due to the presence of the Si film and the like. As a result, even if the number of repetitions of occlusion and release of lithium ions by the Fe-Si-based compound particles increases, the amount of occlusion and release of lithium ions by the Fe-Si-based compound particles can be maintained in a large state. The cycle characteristics and cycle life of the battery can be improved, the initial charge / discharge efficiency can be improved, and charge / discharge at a high current density can be performed.
[0045]
【Example】
Next, examples of the present invention will be described in detail together with comparative examples.
<Example 1>
(1) Production of Fe-Si-based compound particles
First, a mixture was prepared by mixing Si powder and Fe powder so that the atomic ratio of Si / Fe became 72/28. This mixture was put into a quartz crucible, kept at 1500 ° C. for 1 hour in an argon gas atmosphere, and then gradually cooled to room temperature to produce an ingot. Next, this ingot was pulverized using a universal pulverizer, and then pulverized using a wet ball mill or the like to prepare a negative electrode material composed of Fe-Si-based compound particles having an average particle diameter of 1 μm. The Fe-Si-based compound particles are α-FeSi₂And β-FeSi₂, A Si-based film covering a part of the surface of the polycrystalline particles with Si, and Si atoms dissolved in the crystal grains of the polycrystalline particles.
[0046]
(2) Production of negative electrode
First, 70% by weight of the above negative electrode material, 15% by weight of acetylene black (conductive agent), and 15% by weight of polyvinylidene fluoride (binder) were mixed, and then the mixture was mixed with NMP (solvent) to form a negative electrode. A slurry was prepared. Next, the negative electrode slurry was applied on a glass substrate, dried, and then peeled off to produce a negative electrode film having a thickness of 0.09 cm. This negative electrode film was cut into squares each measuring 1.2 cm × 1.2 cm in length and width to obtain two square negative electrode films. Next, these negative electrode films were arranged on both surfaces of a copper mesh negative electrode current collector having a length, width, and thickness of 1 cm × 1 cm × 0.1 cm, respectively, to produce a laminate. Further, the laminate was press-bonded by applying a pressure of 0.5 to 3 MPa with a press machine heated to 110 to 130 ° C.
Thus, a negative electrode was obtained. This negative electrode was used as Example 1.
[0047]
<Example 2>
A negative electrode was produced in the same manner as in Example 1, except that a Si powder and an Fe powder were mixed so that the atomic ratio of Si / Fe became 75/25 to produce a mixture. This negative electrode was designated as Example 2.
<Example 3>
A negative electrode was produced in the same manner as in Example 1, except that a Si powder and an Fe powder were mixed so that the atomic ratio of Si / Fe became 80/20 to produce a mixture. This negative electrode was designated as Example 3.
[0048]
<Example 4>
Except that Si powder, Fe powder and Co powder were mixed so that the atomic ratio of Si / Fe / Co was 72 / 27.72 / 0.28 to prepare a mixture, the same as in Example 1 was performed. To produce a negative electrode. This negative electrode was designated as Example 4.
<Example 5>
Except that Si powder, Fe powder and Mn powder were mixed so that the atomic ratio of Si / Fe / Mn became 72 / 27.72 / 0.28 to prepare a mixture, the same as in Example 1 was performed. To produce a negative electrode. This negative electrode was designated as Example 5.
[0049]
<Example 6>
The Fe-Si-based compound particles having an average particle diameter of 1 μm produced in the same manner as in Example 1 were subjected to a heat treatment at 800 ° C. for 50 hours in an argon gas atmosphere to convert the polycrystalline particles into β-FeSi.₂Thus, a single-phase polycrystalline Fe-Si-based compound particle, that is, a negative electrode material was obtained. Using this negative electrode material, a negative electrode was produced in the same manner as in Example 1. This negative electrode was designated as Example 6.
[0050]
<Example 7>
A mixture was prepared by mixing 233% by weight of carbonaceous particles having an average particle diameter of 100 nm with 100% by weight of Fe-Si-based compound particles having an average particle diameter of 1 μm prepared in the same manner as in Example 1. It was formed into a pellet having a diameter and a height of 5 mm and 10 mm, respectively. Next, the pellet-like mixture was maintained at a temperature of 800 ° C. for 4 hours in an argon gas atmosphere to produce a sintered body. Further, after sintering this sintered body in a mortar, Fe-Si-based compound particles having an average particle size of 10 μm were classified. A negative electrode was manufactured in the same manner as in Example 1 using the classified Fe—Si-based compound particles, that is, the negative electrode material. This negative electrode was designated as Example 7. When the negative electrode material was observed with a powder X-ray diffractometer (XRD), no peak of SiC as a foreign substance was observed.
[0051]
<Example 8>
Example 7 After mixing 100% by weight of carbonaceous particles having an average particle diameter of 100 nm with 100% by weight of Fe—Si-based compound particles having an average particle diameter of 1 μm prepared in the same manner as in Example 1, a mixture was prepared. In the same manner as in the above, a negative electrode was produced. This negative electrode was designated as Example 8. When the negative electrode material was observed with a powder X-ray diffractometer (XRD), no peak of SiC as a foreign substance was observed.
<Example 9>
After mixing 43% by weight of carbonaceous particles having an average particle diameter of 100 nm with 100% by weight of Fe—Si-based compound particles having an average particle diameter of 1 μm prepared in the same manner as in Example 1, a mixture was prepared. In the same manner as in the above, a negative electrode was produced. This negative electrode was designated as Example 9. When the negative electrode material was observed with a powder X-ray diffractometer (XRD), no peak of SiC as a foreign substance was observed.
[0052]
<Example 10>
A mixture was prepared by mixing 70% by weight of carbonaceous particles having an average particle diameter of 100 nm with 100% by weight of Fe-Si-based compound particles having an average particle diameter of 1 μm and produced in the same manner as in Example 1. It is put into a stainless steel container together with a stainless steel ball, and the inside of the container is set to an argon gas atmosphere. In this state, the container was rotated for 20 hours to cover the Fe-Si-based compound particles with the Si-based film. A negative electrode was manufactured in the same manner as in Example 1 using the Fe—Si-based compound particles, that is, the negative electrode material. This negative electrode was designated as Example 10. The average thickness of the Si-based film was 0.2 μm. The average thickness of the Si-based film was measured by dividing the Fe-Si-based compound particles and observing the cross section with a high-resolution scanning electron microscope (SEM).
[0053]
<Comparative Example 1>
A negative electrode was produced in the same manner as in Example 1 except that a Si powder and an Fe powder were mixed so that the atomic ratio of Si / Fe became 66.3 / 33.3 to produce a mixture. This negative electrode was used as Comparative Example 1.
<Comparative Example 2>
A negative electrode was produced in the same manner as in Example 1, except that a Si powder and an Fe powder were mixed so that the atomic ratio of Si / Fe became 90/10 to produce a mixture. This negative electrode was used as Comparative Example 2.
<Comparative Example 3>
A negative electrode was produced in the same manner as in Example 1, except that single crystal silicon particles having an average particle size of 5 μm were used as the negative electrode material. This negative electrode was designated as Comparative Example 3.
[0054]
<Comparison test and evaluation>
As shown in FIG. 2, the negative electrodes 21 of Examples 1 to 10 and Comparative Examples 1 to 3 were attached to a charge / discharge cycle test device 31. In this apparatus 31, an electrolytic solution 33 (a solution obtained by dissolving a lithium salt in an organic solvent) is stored in a container 32, and the negative electrode 21 is connected to a positive electrode 22 (a metal having a length, width, and thickness of 2 cm × 2 cm × 0.2 cm, respectively). A lithium plate: a counter electrode and a reference electrode 23 (a metal lithium plate having a length of 1 cm × 1 cm × 0.2 cm each in length × width × 0.2 cm) are immersed in the electrolytic solution 33, and the negative electrode 21, the positive electrode 22 and the reference electrode 23 The configuration is such that they are electrically connected to the osstat 34 (potentiometer). A charge / discharge cycle test was performed using this device, and the initial discharge capacity (mA · h / g), the initial charge / discharge efficiency (%), and the discharge capacity retention rate (%) of each negative electrode were measured. With the composition of the Fe—Si-based compound particles and the formula (1)_{2 + x}X in the formula or Fe of the formula (2)_1-yM_ySi_{2 + x}X and y, the mixing ratio of the carbonaceous particles with respect to 100% by weight of the Fe-Si-based compound particles, and the phase of the Fe-Si-based compound particles (α + β is α-FeSi₂And β-FeSi₂Β is β-FeSi₂Is a single-phase polycrystal. ) Are shown in Table 1.
[0055]
In the charge / discharge test, the current density during charging and discharging was set to 0.5 mA / cm.²Lithium was absorbed in the negative electrode by the CVCC method from the initial voltage during charging to 0.1 V, and lithium was released from the negative electrode by the CC method to 2 V during discharging. The initial discharge capacity is the capacity at the time of the first discharge, and the initial charge / discharge efficiency was calculated from [(initial discharge capacity / initial charge capacity) × 100%]. Further, the discharge capacity retention ratio (%) was calculated from the following equation (3).
Discharge capacity retention ratio = (discharge capacity at 20th cycle / initial discharge capacity) × 100 (3)
[0056]
[Table 1]

[0057]
As is clear from Table 1, the initial discharge capacity was as low as 420 mA · h / g in Comparative Example 1 having a small Si content, and was 2100 mA · h / g and 1300 mA · h in Comparative Examples 2 and 3 having a large Si content. / G, whereas Examples 1 to 3 in which the Si content was the above-mentioned intermediate value showed an intermediate value of 650 to 940 mA · h / g, the initial charge / discharge efficiency was comparatively higher. In Examples 1 to 3, it was as low as 62 to 75%, whereas in Examples 1 to 3, it was as high as 83 to 87%. Further, the discharge capacity retention ratio was as low as 15 to 58% in Comparative Examples 1 to 3, and as high as 85 to 92% in Examples 1 to 3. As a result, it was found that when the Si content was too large, the initial discharge capacity was large, but the cycle characteristics deteriorated. This is presumably because the excessive amount of the Si component has caused a decrease in the initial charge / discharge efficiency and cycle characteristics, which are inherent problems of Si.
[0058]
In Example 1 in which Co or Mn was not added, the initial discharge capacity was 650 mA · h / g, and the initial charge / discharge efficiency was 87%. In Nos. 4 and 5, the initial discharge capacity was slightly higher at 690 mA · h / g and 680 mA · h / g, and the initial charge / discharge efficiency was slightly higher at 89% and 88%. In Examples 4 and 5 to which Co or Mn was added, the conductivity of the negative electrode was improved, the voltage drop during charging and discharging was reduced, and charging and discharging at a high current density became possible.
[0059]
Also, α-FeSi₂And β-FeSi₂In Example 1 including Fe—Si-based compound particles composed of polycrystalline particles composed of a mixed phase polycrystalline material, the initial discharge capacity was 650 mA · h / g, the initial charge / discharge efficiency was 87%, and the discharge was While the capacity retention ratio was 92%, β-FeSi₂In Example 6 including Fe-Si-based compound particles composed of polycrystalline particles composed of a single-phase polycrystalline material, the initial discharge capacity was as high as 680 mA · h / g, and the initial charge and discharge efficiency was as high as 91%. The discharge capacity retention ratio was as high as 94%. This is because α-FeSi₂And β-FeSi₂Β-FeSi from polycrystalline particles consisting of multiphase polycrystalline₂It is considered that the polycrystalline particles composed of the single-phase polycrystalline material have a larger occlusion amount and release amount of lithium ions.
[0060]
In Example 1 in which the carbonaceous particles were not integrated with the Fe—Si-based compound particles, the initial discharge capacity was 650 mA · h / g, the initial charge / discharge efficiency was 87%, and the discharge capacity retention rate was 92%. %, On the other hand, in Examples 7 and 8 in which the carbonaceous particles were integrated with the Fe-Si-based compound particles, although the initial discharge capacity was as low as 390 to 540 mAh / g, the initial charge and discharge efficiency was low. The discharge capacity retention ratio increased to 89 to 95%, and the discharge capacity retention ratio increased to 93 to 98%. In Examples 7 and 8 in which the carbonaceous particles were integrated with the Fe-Si-based compound particles, the conductivity of the negative electrode was improved, the voltage drop during charging and discharging was reduced, and the charging and discharging at a high current density was also improved. It is now possible.
[0061]
Further, in Example 1 in which the Fe—Si-based compound particles were not covered with the carbonaceous film, the initial discharge capacity was 650 mA · h / g, the initial charge / discharge efficiency was 87%, and the discharge capacity retention rate was 92%. %, On the other hand, in Example 10 in which the Fe—Si-based compound particles were covered with the carbonaceous film, the initial discharge capacity was as low as 53 mA · hour / g, but the initial charge and discharge efficiency was as high as 93%. And the discharge capacity retention ratio was as high as 95%.
[0062]
【The invention's effect】
As described above, according to the present invention, α-FeSi₂And β-FeSi₂Consisting of a mixed phase polycrystal or β-FeSi₂The negative electrode material is composed of Fe-Si-based compound particles whose main component is polycrystalline particles composed of a single-phase polycrystalline body, and the Fe-Si-based compound particles form a part or all of the surface of the polycrystalline particles with Si or SiO₂And / or Si atoms dissolved in the crystal grains of the polycrystalline particles, so that Fe-Si-based compound particles absorb and release lithium ions at the time of occlusion and release of lithium ions. The change in volume of the Si-based compound particles is suppressed, and β-FeSi₂And the amount of lithium ions absorbed and released by the Si film and the like. As a result, even if the number of repetitions of insertion and extraction of lithium ions by the Fe-Si-based compound particles increases, the amount of occlusion and release of lithium ions by the Fe-Si-based compound particles can be maintained in a large state.
[0063]
Further, the Fe—Si-based compound particles are represented by the formula (1)_{2 + x}If the compound is represented by the formula, Fe-Si-based compound particles having the polycrystalline particles and a Si-based film or solid solution Si atoms can be obtained.
Further, the Fe—Si-based compound particles are represented by the formula (2)_1-yM_ySi_{2 + x}If the compound is represented by α-FeSi₂Or β-FeSi₂Since some of the Fe therein is replaced by M, the conductivity of the Fe—Si-based compound particles, that is, the conductivity of the negative electrode material can be improved. If the surface of the Fe-Si-based compound particles is covered with a carbonaceous film having a thickness of 1 nm to 1 µm, the conductivity of the Fe-Si-based compound particles, that is, the conductivity of the negative electrode material can be increased. As a result, a voltage drop during charging and discharging of the secondary battery is reduced, and charging and discharging at a high current density can be performed.
[0064]
Further, if a binder is mixed with the Fe-Si-based compound particles to form a negative electrode, a change in volume of the Fe-Si-based compound particles during insertion and extraction of lithium ions by the Fe-Si-based compound particles is suppressed. Together with β-FeSi₂In addition, the amount of occluded and released lithium ions increases due to the presence of the Si film and the like. As a result, even if the number of repetitions of occlusion and release of lithium ions by the Fe-Si-based compound particles increases, the amount of lithium ions occluded and released by the Fe-Si-based compound particles can be maintained in a large state. Cycle characteristics and cycle life can be improved.
[0065]
In addition, in the nonaqueous electrolyte lithium secondary battery or lithium ion polymer secondary battery using the above-described negative electrode, the volume change of the Fe-Si-based compound particles during the occlusion and release of lithium ions by the Fe-Si-based compound particles is suppressed. And β-FeSi₂In addition, the amount of occluded and released lithium ions increases due to the presence of the Si film and the like. As a result, even if the number of repetitions of occlusion and release of lithium ions by the Fe-Si-based compound particles increases, the amount of occlusion and release of lithium ions by the Fe-Si-based compound particles can be maintained in a large state. The cycle characteristics and cycle life of the battery can be improved.
[0066]
The formula (1) FeSi_{2 + x}And α-FeSi₂And β-FeSi₂By subjecting Fe-Si-based compound particles mainly composed of polycrystalline particles composed of a mixed phase polycrystalline material to a predetermined heat treatment in a non-oxidizing atmosphere, β-FeSi₂Thus, Fe-Si-based compound particles mainly composed of polycrystalline particles composed of a single-phase polycrystalline material can be obtained.
Further, a mixture is prepared by mixing the carbonaceous particles with the Fe-Si-based compound particles, and after molding the mixture, the mixture is heated in a predetermined atmosphere to produce a sintered body, and the sintered body is pulverized. For example, since the carbonaceous particles are integrated with the Fe-Si-based compound particles, a voltage drop during charging and discharging of the secondary battery can be reduced, and charging and discharging at a high current density can be performed.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of Fe—Si-based compound particles constituting a negative electrode material for a lithium secondary battery according to an embodiment of the present invention.
FIG. 2 shows an apparatus used for a charge / discharge cycle test of negative electrodes for lithium secondary batteries of Examples and Comparative Examples.
[Explanation of symbols]
11 Fe-Si based compound particles
12 polycrystalline particles
12a α-FeSi₂
12b β-FeSi₂
12c Si-based film

Claims

Fe-Si composed mainly of polycrystalline particles (12) composed of a mixed-phase polycrystal of α-FeSi ₂ (12a) and β-FeSi ₂ (12b) or a single-phase polycrystal of β-FeSi ₂ Composed of system compound particles (11),
The Fe-Si-based compound particles (11), wherein the polycrystalline particles Si-based film part or all of the surface (12) is covered with Si or SiO ₂ (12c), or crystal grains of the polycrystalline grains A negative electrode material for a lithium secondary battery, comprising one or both of Si atoms solid-dissolved in a lithium secondary battery.

The negative electrode material for a lithium secondary battery according to claim 1, wherein the Fe-Si-based compound particles are a compound represented by the formula (1).
FeSi _{2 + x} (1)
Here, in Expression (1), 0.01 ≦ x ≦ 4.0.

The negative electrode material for a lithium secondary battery according to claim 1, wherein the Fe-Si-based compound particles are a compound represented by the formula (2).
_{_{_{Fe 1-y M y Si 2}}} + x ...... (2)
Here, in equation (2),
0.01 ≦ x ≦ 4.0,
0.01 ≦ y ≦ 0.05,
M is one or more elements selected from the group consisting of Co, Cr, Mn and Al.

The negative electrode material for a lithium secondary battery according to any one of claims 1 to 3, wherein the average particle diameter of the Fe-Si-based compound particles is 10 nm to 10 m.

The negative electrode material for a lithium secondary battery according to any one of claims 1 to 4, wherein the surface of the Fe-Si-based compound particles is coated with a carbonaceous film made of carbonaceous particles having a thickness of 1 nm to 1 m.

A negative electrode for a lithium secondary battery produced by mixing 0.5 to 40% by weight of a binder with 100% by weight of the Fe-Si-based compound particles according to any one of claims 1 to 5.

A non-aqueous electrolyte lithium secondary battery using the negative electrode according to claim 6.

A lithium ion polymer secondary battery using the negative electrode according to claim 6.

Polycrystalline particles represented by the formula (1) FeSi _{2 + x} (0.01 ≦ x ≦ 4.0) according to claim ₂ and composed of a mixed phase polycrystal of α-FeSi ₂ and β-FeSi ₂ as a main component. A method for producing a negative electrode material for a lithium secondary battery, comprising a step of performing a heat treatment of maintaining the Fe-Si-based compound particles at 600 to 900 ° C for 20 to 80 hours in a non-oxidizing atmosphere.

A step of mixing 0.05 to 1900% by weight of carbonaceous particles with 100% by weight of the Fe-Si-based compound particles according to any one of claims 1 to 4, to produce a mixture.
Forming the mixture to form a molded body,
A step of producing a sintered body by holding the molded body at a temperature of 300 to 1000 ° C. for 2 to 10 hours in an inert gas atmosphere or vacuum;
Pulverizing the sintered body to produce a negative electrode material for a lithium secondary battery.