JP2004214054A - Negative electrode active material for lithium secondary battery, its manufacturing method, and lithium secondary battery - Google Patents

Negative electrode active material for lithium secondary battery, its manufacturing method, and lithium secondary battery Download PDF

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Publication number
JP2004214054A
JP2004214054A JP2003000446A JP2003000446A JP2004214054A JP 2004214054 A JP2004214054 A JP 2004214054A JP 2003000446 A JP2003000446 A JP 2003000446A JP 2003000446 A JP2003000446 A JP 2003000446A JP 2004214054 A JP2004214054 A JP 2004214054A
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negative electrode
active material
electrode active
lithium secondary
secondary battery
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JP3827642B2 (en
Inventor
Keiko Matsubara
恵子 松原
Toshiaki Tsuno
利章 津野
Teru Takakura
輝 高椋
Kiin Chin
揆允 沈
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Samsung SDI Co Ltd
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Samsung SDI Co Ltd
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Priority to JP2003000446A priority Critical patent/JP3827642B2/en
Priority to KR1020040000262A priority patent/KR100570639B1/en
Priority to US10/752,300 priority patent/US20040214085A1/en
Priority to CNB200410005090XA priority patent/CN100452493C/en
Publication of JP2004214054A publication Critical patent/JP2004214054A/en
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/134—Electrodes based on metals, Si or alloys
    • E—FIXED CONSTRUCTIONS
    • E03—WATER SUPPLY; SEWERAGE
    • E03C—DOMESTIC PLUMBING INSTALLATIONS FOR FRESH WATER OR WASTE WATER; SINKS
    • E03C1/00—Domestic plumbing installations for fresh water or waste water; Sinks
    • E03C1/02—Plumbing installations for fresh water
    • E03C1/021—Devices for positioning or connecting of water supply lines
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/139—Processes of manufacture
    • H01M4/1395—Processes of manufacture of electrodes based on metals, Si or alloys
    • E—FIXED CONSTRUCTIONS
    • E03—WATER SUPPLY; SEWERAGE
    • E03C—DOMESTIC PLUMBING INSTALLATIONS FOR FRESH WATER OR WASTE WATER; SINKS
    • E03C1/00—Domestic plumbing installations for fresh water or waste water; Sinks
    • E03C1/02—Plumbing installations for fresh water
    • E03C2001/028—Alignment aids for plumbing installations
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Abstract

<P>PROBLEM TO BE SOLVED: To provide a negative electrode active material completely preventing particle size reduction caused by expansion and contraction of an alloy volume in charge-discharge and separation from a current collector, to provide the manufacturing method of the negative active material, and to provide a lithium secondary battery. <P>SOLUTION: The negative electrode active material for the lithium secondary battery comprises aggregates of a porous particles comprising Si, many voids having a mean particle size of 10 nm to 10 μm are formed on the inside of the porous particles , and a mean particle size of the aggregate is 1 μm to 100 μm. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、リチウム二次電池用負極活物質及びその製造方法並びにリチウム二次電池に関するものである。
【０００２】
【従来の技術】
リチウム二次電池の負極活物質の高容量化の研究は、現在の負極活物質を炭素とする電池システムが実用化される以前から行われ、現在もＳｉやＳｎ、Ａｌ等の金属材料を中心に活発に行われているものの、未だ実用化には至っていない。これは主として、充放電する際にＳｉやＳｎ、Ａｌ等の金属がリチウムと合金化することのよる体積の膨張収縮が生じ、これが金属の微粉化を招き、サイクル特性が低下するといった不具合を解決できないことによるものである。
【０００３】
そこで、この問題を解決すべく、下記特許文献１に示されているような非晶質合金や、下記非特許文献１または下記非特許文献２に示されているＮｉ−Ｓｉ系合金のように、リチウムと合金化が可能な金属及びリチウムと合金化しない金属からなる結晶質合金が検討されている。
【０００４】
【特許文献１】
特開２００２−２１６７４６号公報
【非特許文献１】
「第４２回電池討論会予稿集」、社団法人電気化学会電池技術委員会、平成１３年１１月２１日、ｐ．２９６−２９７
【非特許文献２】
「第４３回電池討論会予稿集」、社団法人電気化学会電池技術委員会、平成１４年１０月１２日、ｐ．３２６−３２７
【０００５】
【発明が解決しようとする課題】
しかし、上記の結晶質合金または非晶質合金は、リチウムと合金化しない金属、あるいは合金化しても容量の低い金属間化合物を含むために合金質量あたりの充放電容量が低下するといった問題があった。また、これら合金を粉体にして使用する場合には粉体の粒径が比較的大きくなり、充放電時の合金体積の膨張収縮による微粉化または集電体からの剥離や導電材との接触の欠如を完全に抑制できないといった問題があった。
【０００６】
本発明は、上記事情に鑑みてなされたものであって、充放電時の活物質体積の膨張収縮による微粉化及び集電体からの活物質の剥離や導電材との接触の欠如を完全に抑制できる負極活物質及びその製造方法並びにリチウム二次電池を提供することを目的とする。
【０００７】
【課題を解決するための手段】
上記の目的を達成するために、本発明は以下の構成を採用した。
本発明のリチウム二次電池用負極活物質は、Ｓｉからなる多孔質粒子の集合体からなり、前記多孔質粒子の内部に平均孔径が１０ｎｍ以上１０μｍ以下の範囲である多数のボイドが形成され、前記集合体の平均粒径が１μｍ以上１００μｍ以下の範囲であることを特徴する。
【０００８】
かかるリチウム二次電池用負極活物質によれば、前記多孔質粒子の内部に多数のボイドが形成されているので、多孔質粒子を構成するＳｉがリチウムと合金化して体積膨張する際に、ボイドの容積を圧縮しつつ膨張するので、多孔質粒子の体積が外観上あまり変化することがなく、これにより多孔質粒子の微粉化が防止される。
特に前記集合体の平均粒径が１μｍ以上１００μｍ以下の範囲であれば、多孔質粒子の体積が見かけ上ほとんど変化することがない。
更に前記多孔質粒子の内部に多数のボイドが形成されているので、リチウム二次電池の負極活物質として用いた場合に当該ボイドに非水電解液を含侵させることができ、これによりリチウムイオンを多孔質粒子の内部まで侵入させてリチウムイオンの拡散を効率よく行うことができ、高率充放電が可能になる。
【０００９】
また本発明のリチウム二次電池用負極活物質は、先に記載のリチウム二次電池用負極活物質であって、前記ボイドの平均孔径をｎとし、前記集合体の平均粒径をＮとしたとき、ｎ／Ｎ比が０．００１以上０．２以下の範囲であることを特徴とする。
【００１０】
かかるリチウム二次電池用負極活物質によれば、ｎ／Ｎ比が０．００１以上０．２以下の範囲であり、多孔質粒子の粒径に対してボイドの孔径が極めて小さいので、多孔質粒子の強度が低下することなく、体積変化に伴う微粉化を防止できる。
【００１１】
また本発明のリチウム二次電池用負極活物質は、先に記載のリチウム二次電池用負極活物質であって、前記多孔質粒子体積あたりの前記ボイドの空隙率が０．１％以上５０％以下の範囲であることを特徴とする。
【００１２】
かかるリチウム二次電池用負極活物質によれば、ボイドの空隙率が０．１％以上５０％以下の範囲なので、リチウムとの合金化に伴うＳｉの体積膨張をボイドによって十分に吸収することができ、多孔質粒子の体積が外観上ほとんど変化することがなく、また多孔質粒子の強度が低下しないために微粉化を防止できる。
【００１３】
また本発明のリチウム二次電池用負極活物質は、先に記載のリチウム二次電池用負極活物質であって、前記多孔質粒子の組織の一部が非晶質相であり、残部が結晶質相であることを特徴とする。
【００１４】
かかるリチウム二次電池用負極活物質によれば、多孔質粒子の組織の一部が非晶質相なので、当該負極活物質を用いた電池のサイクル特性を高めることができる。
【００１５】
また本発明のリチウム二次電池用負極活物質は、先に記載のリチウム二次電池用負極活物質であって、前記多孔質粒子は、Ｓｎ、Ａｌ、Ｐｂ、Ｉｎ、Ｎｉ、Ｃｏ、Ａｇ、Ｍｎ、Ｃｕ、Ｇｅ、Ｃｒ、Ｔｉ、Ｆｅのうちの少なくとも１種以上の元素Ｍ及びＳｉを含む合金溶湯が急冷されて急冷合金とされ、該急冷合金に含まれる前記元素Ｍが酸またはアルカリによって溶出除去されることにより形成されたものであることを特徴とする。
【００１６】
かかるリチウム二次電池用負極活物質によれば、前記多孔質粒子が、Ｓｉと元素Ｍからなる急冷合金から元素Ｍを溶出除去させることにより得られたものであり、急冷合金において元素Ｍが除去された部分がボイドとなるので、極めて微細なボイドを有するものとなる。
【００１７】
また本発明のリチウム二次電池用負極活物質は、先に記載のリチウム二次電池用負極活物質であって、前記合金溶湯における元素Ｍの含有率が０．０１質量％以上７０質量％以下の範囲であることを特徴とする。
【００１８】
かかるリチウム二次電池用負極活物質によれば、前記合金溶湯における元素Ｍの含有率が上記の範囲なので、ボイドの平均孔径並びにボイドの空隙率を上記の範囲内とすることができる。
【００１９】
次に本発明のリチウム二次電池は、先のいずれかに記載のリチウム二次電池用負極活物質を具備してなることを特徴とする。
【００２０】
かかるリチウム二次電池によれば、上記の負極活物質を具備しており、負極活物質が微粉化したり、集電体から脱落するおそれがなく、また導電材との接触も維持され、充放電容量を向上できるとともにサイクル特性を向上できる。
【００２１】
更に本発明のリチウム二次電池用負極活物質の製造方法は、Ｓｎ、Ａｌ、Ｐｂ、Ｉｎ、Ｎｉ、Ｃｏ、Ａｇ、Ｍｎ、Ｃｕ、Ｇｅ、Ｃｒ、Ｔｉ、Ｆｅのうちの少なくとも１種以上の元素Ｍ及びＳｉを含む合金溶湯を急冷することにより急冷合金を形成し、該急冷合金に含まれる前記元素Ｍを、前記元素Ｍが可溶な酸またはアルカリによって溶出除去することにより、Ｓｉからなる多孔質粒子の集合体を得ることを特徴とする。
【００２２】
かかるリチウム二次電池用負極活物質の製造方法によれば、Ｓｉ及び元素Ｍからなる急冷合金から元素Ｍを溶出除去させることにより、元素Ｍが除去された部分をボイドとするＳｉからなる多孔質粒子を形成することができる。形成されたボイドは、平均孔径が極めて微小であってしかも多孔質粒子の全体に渡って均一に分布しているので、Ｓｉがリチウムと合金化して体積膨張する際にボイドの容積を圧縮しつつ膨張させることが可能となり、体積が外観上あまり変化することのない多孔質粒子を得ることができる。
また、急冷合金から元素Ｍを除去することによって、多孔質粒子の組織の大部分をリチウムと合金化しやすいＳｉのみにすることができ、重量あたりのエネルギー密度が高い負極活物質を得ることができる。
更に、合金溶湯を急冷することにより、得られた急冷合金の組織の少なくとも一部を、リチウムと合金化しやすい非晶質相にすることができ、これによりサイクル特性を向上させることができる。
更にまた、合金溶湯を急冷することにより、得られた急冷合金の組織中に微小な結晶粒からなる結晶質相が形成される場合もあり、この場合には結晶質相に含まれる元素Ｍのみを容易に溶出除去させることができる。このように結晶粒の小さい結晶質相や非晶質相から元素Ｍを溶出除去させることにより得られるボイドは、大きな結晶粒からなる結晶質相から元素Ｍを除去した場合と比べ平均孔径が小さく、また粒子全体に均一に分布する。ボイドの平均孔径が大きく、また粒子全体に不均一に存在すると、充電によりＳｉが体積膨張する際、その影響を粒子全体にわたって均等に分散させることが難しくなるとともに、粒子の強度も低下するためサイクル劣化を引き起こすため好ましくない。
【００２３】
また本発明のリチウム二次電池用負極活物質の製造方法は、先に記載のリチウム二次電池用負極活物質の製造方法であって、前記合金溶湯をガスアトマイズ法、水アトマイズ法、ロール急冷法のうちのいずれかの方法で急冷することを特徴とする。
【００２４】
かかるリチウム二次電池用負極活物質の製造方法によれば、上記のいずれかの急冷方法を採用することで、急冷合金を容易に得ることができる。
【００２５】
また本発明のリチウム二次電池用負極活物質の製造方法は、先に記載のリチウム二次電池用負極活物質の製造方法であって、前記合金溶湯の急冷速度が１００Ｋ／秒以上であることを特徴とする。
【００２６】
かかるリチウム二次電池用負極活物質の製造方法によれば、合金溶湯の急冷速度を１００Ｋ／秒以上にすることで、組織の少なくとも一部が結晶質相である急冷合金を容易に得ることができる。
また、合金溶湯の急冷速度を上記の範囲にすることで、組織中に結晶質相が形成される場合があり、この場合には結晶質相を構成する結晶粒を小さくすることができる。
【００２７】
また本発明のリチウム二次電池用負極活物質の製造方法は、先に記載のリチウム二次電池用負極活物質の製造方法であって、前記急冷合金を、前記元素Ｍが可溶な酸またはアルカリの溶液に浸積させて前記元素Ｍを溶出させた後に、洗浄及び乾燥することにより、前記急冷合金中の前記元素Ｍを溶出除去することを特徴とする。
【００２８】
かかるリチウム二次電池用負極活物質の製造方法によれば、急冷合金を、元素Ｍが可溶な酸またはアルカリの溶液に浸積させて元素Ｍを溶出させるので、元素Ｍの溶出除去を容易に行える。
【００２９】
また本発明のリチウム二次電池用負極活物質の製造方法は、先に記載のリチウム二次電池用負極活物質の製造方法であって、前記合金溶湯における元素Ｍの含有率が０．０１質量％以上７０質量％以下の範囲であることを特徴とする。
【００３０】
かかるリチウム二次電池用負極活物質の製造方法によれば、急冷溶湯における元素Ｍの含有率が上記の範囲なので、元素Ｍが少なすぎてボイドの数が少なくなったり、元素Ｍが過剰になってボイドの平均孔径が過大になるおそれがない。
【００３１】
【発明の実施の形態】
以下、本発明の実施の形態を図面を参照して説明する。
本発明のリチウム二次電池用の負極活物質は、Ｓｉからなる多孔質粒子の集合体であって、多孔質粒子の内部に平均孔径が１０ｎｍ以上１０μｍ以下の範囲の多数のボイドが形成されてなり、かつ多孔質粒子の集合体の平均粒径が１μｍ以上１００μｍ以下の範囲のものである。
【００３２】
この負極活物質はリチウム二次電池の負極に備えられる。リチウム二次電池が充電されると、リチウムが正極から負極に移行するが、この時に負極においてリチウムが多孔質粒子を構成するＳｉと合金化する。この合金化に伴ってＳｉの体積膨張が起こる。また放電時にはＳｉからリチウムが脱離して正極側に移行する。この脱離に伴って膨張状態のＳｉが元の体積に収縮する。このように、充放電の繰り返しに伴ってＳｉの膨張収縮が起きる。
【００３３】
この負極活物質によれば、多孔質粒子の内部に多数のボイドが形成されているので、多孔質粒子を構成するＳｉがリチウムと合金化して体積膨張する際に、ボイドの容積を圧縮しつつ膨張するので、多孔質粒子の大きさが外観上ほとんど変化することがなく、これにより多孔質粒子の微粉化が防止される。
【００３４】
また、本実施形態の負極活物質を構成する多孔質粒子は、Ｓｎ、Ａｌ、Ｐｂ、Ｉｎ、Ｎｉ、Ｃｏ、Ａｇ、Ｍｎ、Ｃｕ、Ｇｅ、Ｃｒ、Ｔｉ、Ｆｅのうちの少なくとも１種以上の元素Ｍ及びＳｉを含む合金溶湯が急冷されて急冷合金とされ、該急冷合金に含まれる前記元素Ｍが酸またはアルカリによって溶出除去されることにより形成されたものである。即ち、本実施形態の多孔質粒子は、Ｓｉと元素Ｍを含む急冷合金から元素Ｍを溶出除去させることによって得られたものであり、急冷合金において元素Ｍが除去された部分がボイドとなり、極めて微細なボイドを有するものとなる。
【００３５】
図１は、多孔質粒子の一例を示す断面模式図である。
図１に示すように、この一例の多孔質粒子１の内部には、多数のボイド２…が形成されている。各ボイド２…の断面形状は比較的均一な形状になっている。
また図２は、多孔質粒子の別の例を示す断面模式図である。
図２に示すように、この別の例の多孔質粒子１１の内部には、多数のボイド１２…が形成されている。この各ボイド１２…の断面形状はそれぞれ不揃いで不均一な形状になっている。
【００３６】
また、図１及び図２に示す多孔質粒子１，１１は、組織の一部がＳｉの非晶質相であり、残部がＳｉの結晶質相から構成されている。尚、これらの多孔質粒子１、１１は組織の全部がＳｉの結晶質相から構成される場合もある。このような組織の相違は、後述するように、主として負極活物質の製造の際に予め形成する急冷合金の結晶組織の違いに由来するものである。
多孔質粒子１，１１の組織の一部に非晶質相が含まれれば、負極活物質のサイクル特性を高めることができる。
【００３７】
また、多孔質粒子１，１１の平均粒径は１μｍ以上１００μｍ以下であることが好ましい。平均粒径が１μｍ未満であると、多孔質粒子１，１１に占めるボイド２，１２の割合が相対的に増加して多孔質粒子１，１１の強度が低下してしまうので好ましくない。また、平均粒径が１００μｍを越えると、多孔質粒子１，１１自体の体積変化が大きくなって微粉化が進行してしまうので好ましくない。
【００３８】
上記の多孔質粒子１，１１の内部にあるボイド２…、１２…は、平均孔径が１０ｎｍ以上１０μｍ以下の範囲のものである。
特に、図１の多孔質粒子１に含まれるボイド２…は、平均孔径が１０ｎｍ以上０．５μｍ以下の範囲のものである。また、図２の多孔質粒子１１に含まれるボイド１２…は、平均孔径が２００ｎｍ以上２μｍ以下の範囲のものであって図１に示すボイド２より孔径が大きいものである。
【００３９】
ボイド２…、１２…の平均孔径が１０ｎｍ未満であると、ボイド２…、１２…の容積が極端に小さくなり、Ｓｉがリチウムと合金化して体積膨張した際にこの膨張分を吸収しきれず、多孔質粒子１，１１の大きさが外観上変化してしまい、多孔質粒子１，１１が割れて微粉化するおそれがあるので好ましくない。また、ボイド２…、１２…の平均孔径が１０μｍを越えると、ボイドの容積が増大して多孔質粒子自体の強度が低下してしまうので好ましくない。
【００４０】
また、ボイド２、１２の平均孔径をｎとし、多孔質粒子１、１１の平均粒径をＮとしたとき、ｎ／Ｎ比が０．００１以上０．２以下の範囲であることが好ましい。ｎ／Ｎ比がこの範囲だと、多孔質粒子１、１１の粒径に対するボイド２，１２の相対孔径が極めて小さくなり、多孔質粒子の強度が低下することなく、体積変化に伴う微粉化を防止できる。
ｎ／Ｎ比が０．００１未満だと、ボイド２，１２の相対孔径が小さくなりすぎ、リチウムとＳｉとの合金化に伴う体積膨張を吸収できなくなるので好ましくない。また、ｎ／Ｎ比が０．２をこえると、多孔質粒子１，１１の強度が低下し、微粉化が進行するので好ましくない。
【００４１】
また、多孔質粒子１，１１の体積あたりのボイド２，１２の空隙率が０．１％以上５０％以下の範囲であることが好ましい。ボイドの空隙率がこの範囲であれば、リチウムとの合金化に伴うＳｉの体積膨張をボイドによって十分に吸収することができ、多孔質粒子の体積が外観上ほとんど変化することがなく、また多孔質粒子の強度が低下しないために微粉化を防止できる。
空隙率が０．１％未満だと、リチウムとＳｉとの合金化に伴う体積膨張を吸収できなくなるので好ましくない。また空隙率が５０％を越えると、多孔質粒子１，１１の強度が低下し、微粉化が進行するので好ましくない。
【００４２】
次に、本実施形態のリチウム二次電池は、上記の負極活物質を備えた負極と、正極と、電解質を少なくとも具備してなるものである。
【００４３】
リチウム二次電池の負極は、例えば、多孔質粒子の集合体からなる負極活物質が、多孔質粒子同士を相互に結着する結着材によってシート状に固化成形されたものを例示できる。
また、上記のシート状に固化成形されたものに限るものではなく、円柱状、円盤状、板状若しくは柱状に固化成形されたペレットであっても良い。
【００４４】
結着材は、有機質または無機質のいずれでも良いが、多孔質粒子と共に溶媒に分散あるいは溶解し、更に溶媒を除去することにより多孔質粒子同士を結着させるものであればどのようなものでもよい。また、多孔質粒子と共に混合し、加圧成形等の固化成形を行うことにより多孔質粒子同士を結着させるものでもよい。このような結着材としてたとえば、ビニル系樹脂、セルロース系樹脂、フェノール樹脂、熱可塑性樹脂、熱硬化性樹脂などが使用でき、たとえばポリフッ化ビニリデン、ポリビニルアルコール、カルボキシメチルセルロース、スチレンブタジエンラバー、等の樹脂を例示できる。
また、本発明に係る負極においては、負極活物質及び結着材の他に、導電助材としてカーボンブラック、黒鉛粉末、金属粉末等を添加しても良い。
【００４５】
次に正極としては例えば、ＬｉＭｎ２Ｏ４、ＬｉＣｏＯ２、ＬｉＮｉＯ２、ＬｉＦｅＯ２、Ｖ２Ｏ５、ＴｉＳ、ＭｏＳ等、及び有機ジスルフィド化合物や有機ポリスルフィド化合物等のリチウムを吸蔵、放出が可能な正極活物質を含むものを例示できる。
また、上記の正極には、上記正極活物質の他に、ポリフッ化ビニリデン等の結着材や、カーボンブラック等の導電助材を添加しても良い。
正極及び負極の具体例として、上記の正極または負極を金属箔若しくは金属網からなる集電体に塗布してシート状に成形したものを例示できる。
【００４６】
更に電解質としては、例えば、非プロトン性溶媒にリチウム塩が溶解されてなる有機電解液を例示できる。
非プロトン性溶媒としては、プロピレンカーボネート、エチレンカーボネート、ブチレンカーボネート、ベンゾニトリル、アセトニトリル、テトラヒドロフラン、２−メチルテトラヒドロフラン、γ−ブチロラクトン、ジオキソラン、４−メチルジオキソラン、Ｎ、Ｎ−ジメチルホルムアミド、ジメチルアセトアミド、ジメチルスルホキシド、ジオキサン、１，２−ジメトキシエタン、スルホラン、ジクロロエタン、クロロベンゼン、ニトロベンゼン、ジメチルカーボネート、メチルエチルカーボネート、ジエチルカーボネート、メチルプロピルカーボネート、メチルイソプロピルカーボネート、エチルブチルカーボネート、ジプロピルカーボネート、ジイソプロピルカーボネート、ジブチルカーボネート、ジエチレングリコール、ジメチルエーテル等の非プロトン性溶媒、あるいはこれらの溶媒のうちの二種以上を混合した混合溶媒を例示でき、特にプロピレンカーボネート（ＰＣ）、エチレンカーボネート（ＥＣ）、ブチレンカーボネート（ＢＣ）のいずれか１つを必ず含むとともにジメチルカーボネート（ＤＭＣ）、メチルエチルカーボネート（ＭＥＣ）、ジエチルカーボネート（ＤＥＣ）のいずれか１つを必ず含むことが好ましい。
【００４７】
また、リチウム塩としては、ＬｉＰＦ６、ＬｉＢＦ４、ＬｉＳｂＦ６、ＬｉＡｓＦ６、ＬｉＣｌＯ４、ＬｉＣＦ３ＳＯ３、Ｌｉ（ＣＦ３ＳＯ２）２Ｎ、ＬｉＣ４Ｆ９ＳＯ３、ＬｉＳｂＦ６、ＬｉＡｌＯ４、ＬｉＡｌＣｌ４、ＬｉＮ（ＣｘＦ２ｘ＋１ＳＯ２）（ＣｙＦ２ｙ十１ＳＯ２）（ただしｘ、ｙは自然数）、ＬｉＣｌ、ＬｉＩ等のうちの１種または２種以上のリチウム塩を混合させてなるものを例示でき、特にＬｉＰＦ６、ＬｉＢＦ４のいずれか１つを含むものが好ましい。
またこの他に、リチウム二次電池の有機電解液として従来から知られているものを用いることもできる。
【００４８】
また電解質の別の例として、ＰＥＯ、ＰＶＡ等のポリマーに上記記載のリチウム塩のいずれかを混合させたものや、膨潤性の高いポリマーに有機電解液を含浸させたもの等、いわゆるポリマー電解質を用いても良い。
更に、本発明のリチウム二次電池は、正極、負極、電解質のみに限られず、必要に応じて他の部材等を備えていても良く、例えば正極と負極を隔離するセパレータを具備しても良い。
【００４９】
かかるリチウム二次電池によれば、上記の負極活物質を具備しており、負極活物質が微粉化したり、集電体から脱落するおそれがなく、また導電材との接触も維持され、充放電容量を向上できるとともにサイクル特性を向上できる。
また、多孔質粒子の内部に多数のボイドが形成されているので、リチウム二次電池の負極活物質として用いた場合に当該ボイドに非水電解液を含侵させることができ、これによりリチウムイオンを多孔質粒子の内部まで侵入させてリチウムイオンの拡散を効率よく行うことができ、高率充放電が可能になる。
【００５０】
次に、本発明のリチウム二次電池用負極活物質の製造方法を説明する。
リチウム二次電池用負極活物質の製造方法は、Ｓｉと元素Ｍとを含有する急冷合金を製造する工程と、得られた急冷合金を溶出処理する工程とから概略構成されている。以下、各工程を順に説明する
【００５１】
まず、急冷合金を製造する工程では、Ｓｉと元素Ｍとを含む合金溶湯を急冷して急冷合金とする。合金溶湯は、Ｓｎ、Ａｌ、Ｐｂ、Ｉｎ、Ｎｉ、Ｃｏ、Ａｇ、Ｍｎ、Ｃｕ、Ｇｅ、Ｃｒ、Ｔｉ、Ｆｅのうちの少なくとも１種以上の元素Ｍと、Ｓｉとを含むものであり、これらの単体あるいは合金を例えば高周波誘導加熱法により同時に溶解することによって得られる。
合金溶湯における元素Ｍの含有率は０．０１質量％以上７０質量％以下の範囲であることが好ましい。合金溶湯における元素Ｍの含有率が前記の範囲であれば、元素Ｍが少なすぎてボイドが少なくなったり、元素Ｍが過剰になってボイドの平均孔径が過大になるおそれがない。
【００５２】
合金溶湯を急冷する方法としては、例えば、ガスアトマイズ法、水アトマイズ法、ロール急冷法等を用いることができる。ガスアトマイズ法及び水アトマイズ法では粉末状の急冷合金が得られ、ロール急冷法では薄帯状の急冷合金が得られる。薄帯状の急冷合金は更に粉砕して粉末にする。こうして得られた粉末状の急冷合金の平均粒径が、最終的に得ようとする多孔質粒子の集合体の平均粒径となる。従って、急冷合金の粉末を得る際には、その平均粒径を１μｍ以上１００μｍ以下の範囲に調整することが必要である。
【００５３】
合金溶湯から得られた急冷合金は、組織全体が非晶質相である急冷合金、若しくは一部が非晶質相であるとともに残部が微結晶粒からなる結晶質相である急冷合金、若しくは組織全体が微結晶粒からなる結晶質相である急冷合金となる。
非晶質相には、主としてＳｉと元素Ｍとの合金相が含まれる。一方、結晶質相が存在する場合には、元素ＭとＳｉを含む合金相、Ｓｉ単相、元素Ｍの単相のうちの一つ以上の相が含まれる。従って急冷合金には、非晶質相としてのＳｉと元素Ｍとの合金相、結晶質相としての元素ＭとＳｉの合金相、結晶質相としてのＳｉ単相、結晶質相としての元素Ｍの単相のうちの一つ以上の相が含まれる。Ｓｉと元素Ｍは一定の割合で合金相を形成するが、合金溶湯に含まれるＳｉ量が過剰だと合金相の他にＳｉ単相が形成されやすくなり、また元素Ｍが過剰だと合金相の他に元素Ｍからなる相が形成されやすくなる。また結晶質相は、平均粒径が数〜数十ｎｍ程度の微結晶粒により構成される。このような微細な結晶粒は合金溶湯を急冷することによってはじめて得られるものである。
【００５４】
尚、急冷の際の急冷速度は、１００Ｋ／秒以上であることが好ましい。急冷速度が１００Ｋ／秒未満では、結晶質相に含まれる結晶粒が肥大化するおそれがあり、この後の溶出工程において平均孔径の大きなボイドが形成されてしまう場合があるので好ましくない。
【００５５】
次に、急冷合金を溶出処理する工程では、急冷合金に含まれる前記元素Ｍを、前記元素Ｍが可溶な酸またはアルカリによって溶出除去する。
具体的には、粉末状の急冷合金を、元素Ｍが可溶な酸またはアルカリの溶液に浸積させて前記元素Ｍを溶出させ、次いで、洗浄及び乾燥を行う。溶出させる際には３０〜６０℃で加熱しつつ１〜５時間程度の攪拌を行うと良い。
元素Ｍの溶出に用いる酸は、元素Ｍの種類にもよるが、塩酸若しくは硫酸などが好ましい。また元素Ｍの溶出に用いるアルカリは、元素Ｍの種類にもよるが、水酸化ナトリウムや水酸化カリウムなどが好ましい。尚、これらの酸またはアルカリはＳｉを腐食しないものが好ましい。
【００５６】
急冷合金から元素Ｍを溶出させることにより、元素Ｍが除去された部分をボイドとするＳｉからなる多孔質粒子が得られる。
急冷合金には、上述したように、非晶質相としてのＳｉと元素Ｍとの金属相、結晶質相としての合金相、結晶質相としてのＳｉ単相、結晶質相としての元素Ｍの単相のうちのいずれか一つ以上の相が含まれている。
このような組織構造を有する急冷合金から元素Ｍを溶出除去させると、上記合金相がＳｉ単相となり、また元素Ｍ単相は全部が除かれる。このようにして、溶出後の急冷合金粉末には、非晶質相としてのＳｉ単相、結晶質相としてのＳｉ単相のうちのいずれか一方若しくは両方の相が含まれることになる。
【００５７】
非晶質相な合金相から元素Ｍが除去されて形成されたＳｉ単相は、図１に示したような、ほぼ均一な断面形状であって孔径が揃ったボイド２…を有するものとなる。一方、結晶質相から元素Ｍ単相の全部が除かれた場合は、図２に示したような、個々に不均一な断面形状を有して孔径が不揃いなボイド１２…を有するものとなる。こうして得られたボイド２，１２は、平均孔径が１０ｎｍ以上１０μｍ以下の範囲のものとなる。
【００５８】
本実施形態のリチウム二次電池用負極活物質の製造方法によれば、Ｓｉ及び元素Ｍからなる急冷合金から元素Ｍを溶出除去させることにより、元素Ｍが除去された部分をボイドとするＳｉからなる多孔質粒子を形成することができる。形成されたボイドは、平均孔径が極めて微小であってしかも多孔質粒子の全体に渡って均一に分布しているので、Ｓｉがリチウムと合金化して体積膨張する際にボイドの容積を圧縮しつつ膨張させることが可能となり、大きさが外観上あまり変化することのない多孔質粒子を得ることができる。
また、急冷合金から元素Ｍを除去することによって、多孔質粒子の組織の大部分をリチウムと合金化しやすいＳｉのみにすることができ、重量あたりのエネルギー密度が高い負極活物質を得ることができる。
更に、合金溶湯を急冷することにより、得られた急冷合金の組織の少なくとも一部を非晶質相にすることができ、これによりサイクル特性を向上させることができる。
更にまた、合金溶湯を急冷することにより、得られた急冷合金の組織中に微小な結晶粒からなる結晶質相が形成される場合もあり、この場合には結晶質相に含まれる元素Ｍ相のみを容易に溶出除去させることができる。
【００５９】
【実施例】
［負極活物質の製造］
（実施例１）
５ｍｍ角程度の大きさの塊状のＳｉを５０重量部と、Ｎｉ粉末を５０重量部とを用意し、これらを混合してからＡｒ雰囲気中において高周波加熱法により溶解して合金溶湯とした。この合金溶湯を８０ｋｇ／ｃｍ２の圧力のヘリウムガスを用いたガスアトマイズ法によって急冷することにより、平均粒径９μｍの急冷合金からなる粉末を得た。このときの急冷速度は１×１０５Ｋ／秒であった。得られた粉末に対してＸ線回折を行ったところ、ＮｉＳｉ２なる組成の結晶質相と非晶質相が混在した合金相の存在が確認された。
次に、得られた急冷合金粉末を希硝酸中に入れ、５０℃で１時間攪拌したのち十分に洗浄しながら濾過し、１００℃の乾燥炉で２時間乾燥した。このようにして、実施例１の負極活物質を製造した。
【００６０】
（実施例２）
Ｓｉを８０重量部と、Ｎｉを２０重量部を用いたこと以外は上記実施例１と同様にして実施例２の負極活物質を製造した。
なお、このときの急冷合金粉末には、結晶質相としてのＳｉ単相と、結晶質相および非晶質相のＮｉＳｉ２なる組成の合金相が観察された。
急冷合金粉末の組織中にＳｉ単相とＮｉＳｉ２合金相が検出されたのは、Ｓｉ量がＮｉ量よりも多いためにＳｉの一部がＮｉと合金化できず、この一部のＳｉがＳｉ単相として析出したためと思われる。
【００６１】
（実施例３）
５ｍｍ角程度の大きさの塊状のＳｉを７０重量部と、Ａｌ粉末を３０重量部とを用意し、これらを混合してからアルゴン雰囲気中において高周波加熱法により溶解して合金溶湯とした。この合金溶湯を８０ｋｇ／ｃｍ２の圧力のヘリウムガスを用いたガスアトマイズ法によって急冷することにより、平均粒径１０μｍの急冷合金からなる粉末を得た。得られた粉末に対してＸ線回折を行ったところ、結晶質相としてのＡｌ単相及びＳｉ単相の存在が確認された。
次に、得られた急冷合金粉末を塩酸水溶液中に入れ、５０℃で４時間攪拌したのち十分に洗浄しながら濾過し、１００℃の乾燥炉で２時間乾燥した。このようにして、実施例３の負極活物質を製造した。
【００６２】
（実施例４）
塩酸に代えて硫酸を用いたこと以外は上記実施例３と同様にして実施例４の負極活物質を製造した。
【００６３】
（比較例１）
５ｍｍ角程度の大きさの塊状のＳｉを５０重量部と、Ｎｉ粉末を５０重量部とを用意し、これらを混合してからアルゴン雰囲気中において高周波加熱法により溶解して合金溶湯とした。この合金溶湯を８０ｋｇ／ｃｍ２の圧力のヘリウムガスを用いたガスアトマイズ法によって急冷することにより、平均粒径９μｍの急冷合金からなる粉末を得た。この粉末を比較例１の負極活物質とした。得られた粉末に対してＸ線回折を行ったところ、ＮｉＳｉ２なる組成の結晶質相と非晶質相が混在した合金相の存在が確認された。
【００６４】
（比較例２）
５ｍｍ角程度の大きさの塊状のＳｉを５０重量部と、Ｎｉ粉末を５０重量部とを用意し、これらを混合してからペレット状に固化成形し、電気炉に投入してアルゴン雰囲気中１６００℃で溶融し、自然冷却してインゴットを得た。このインゴットを粉砕して平均粒径２０μｍの粉末を得た。
次に、得られた粉末を希硝酸中に入れ、５０℃で１時間攪拌したのち十分に洗浄しながら濾過し、１００℃の乾燥炉で２時間乾燥した。このようにして、比較例２の負極活物質を製造した。
【００６５】
（比較例３）
平均粒径が１μｍのＳｉ粉末を比較例３の負極活物質とした。
【００６６】
（リチウム二次電池の製造）
実施例１〜４及び比較例１〜３の各々の負極活物質７０重量部と、導電材として平均粒径２μｍの黒鉛粉末２０重量部と、ポリフッ化ビニリデン１０重量部とを混合し、Ｎ−メチルピロリドンを加えてから攪拌してスラリーを作成した。次にこのスラリーを厚さ１４μｍの銅箔上に塗布してから乾燥し、これを圧延して厚さ８０μｍの負極電極を作成した。作成した負極電極を直径１３ｍｍの円形に打ち抜き、この負極電極に多孔質ポリプロピレン製のセパレータを挟んで対極として金属リチウムを重ね、更に容積比でＥＣ：ＤＭＣ：ＤＥＣ＝３：３：１の混合溶媒にＬｉＰＦ６を１モル／Ｌの濃度で添加してなる電解液を注液することにより、コイン型のリチウム二次電池を製造した。
得られたリチウム二次電池に対して、電池電圧０Ｖ〜１．５Ｖの範囲で０．２Ｃの電流密度による充放電を３０サイクル繰り返し行った。
【００６７】
（実施例１〜４の負極活物質の物性）
実施例１の負極活物質を電子顕微鏡により観察したところ、図１に示したような断面形状がほぼ均一なボイドを有する多孔質粒子が得られていることが判明した。ボイドの平均孔径は、２００〜５００ｎｍの程度であった。更にこの多孔質粒子をエネルギー分散型Ｘ線分析装置による元素分析を行ったところ、多孔質粒子の表面、断面のいずれともにＮｉが観察されなかった。
従って、前記の塩酸による溶出理によってＮｉが溶出除去され、そのあとに均一なボイドが形成されたことがわかった。
【００６８】
次に、実施例２の負極活物質を電子顕微鏡により観察したところ、図２に示したような、断面形状が不揃いで不均一な孔径のボイドを有する多孔質粒子が得られていることが判明した。ボイドの平均孔径は、２００ｎｍ〜２μｍ程度と実施例１のボイドより大きかった。更にこの多孔質粒子をエネルギー分散型Ｘ線分析装置による元素分析を行ったところ、多孔質粒子の表面、断面のいずれともにＮｉが観察されなかった。
なお、ボイドの形状が不揃いになったのは、急冷合金粉末が組成の異なる複数の組織によって形成されており、急冷合金粉末に含まれていたＳｉ単相及びＮｉＳｉ２合金相から、ＮｉＳｉ２合金相のみに含まれていたＮｉが溶出除去されたためと考えられる。
【００６９】
次に、実施例３の負極活物質を電子顕微鏡により観察したところ、図２に示したような、断面形状が不揃いで不均一な孔径のボイドを有する多孔質粒子が得られていることが判明した。ボイドの平均孔径は、３００ｎｍ〜２μｍ程度と実施例１のボイドより大きかった。更にこの多孔質粒子をエネルギー分散型Ｘ線分析装置による元素分析を行ったところ、多孔質粒子の表面、断面のいずれともにＡｌが観察されなかった。
なお、ボイドの形状が不揃いになったのは、急冷合金粉末に含まれていたＳｉ単相及びＡｌ単相から、Ａｌ単相のみが溶出除去されたためと考えられる。
【００７０】
次に、実施例４の負極活物質については、実施例３と同様に、断面形状が不揃いで不均一な孔径のボイドを有していた。ボイドの平均孔径も実施例３と同様であった。また、元素分析の結果、Ａｌは検出されず、硫酸による処理でもＡｌを除去できることが分かった。
【００７１】
（リチウム二次電池の特性）
表１に、１サイクル目の放電容量に対する３０サイクル目の放電容量の容量維持率を示す。
【００７２】
【表１】

Figure 2004214054
【００７３】
実施例１〜４のリチウム二次電池については、容量維持率が８３〜９５％と良好であることが分かる。一方、比較例１〜３は、容量維持率が２０〜４５％と低いことが分かる。
【００７４】
比較例１の負極活物質ではＮｉの溶出処理が行われなかったため、負極活物質の粉末を構成する粒子にボイドが形成されず、このため充放電の繰り返しにより負極活物質の体積変化が大きくなり、負極活物質の微粉化が進んで容量維持率が低くなったと思われる。
【００７５】
また比較例２の負極活物質では合金溶湯の急冷を行わず、合金溶湯を自然放冷したため、放冷後の合金の組織中の結晶粒が肥大化し、これに伴ってボイドの孔径が大きくなった。このため、負極活物質の粉末を構成する粒子の強度が低下し、充放電の繰り返しにより負極活物質の微粉化が進んで容量維持率が低くなったと思われる。
【００７６】
更に比較例３の負極活物質は単なるＳｉの粉末なので、比較例１と同様に充放電の繰り返しにより負極活物質の体積変化が大きくなり、負極活物質の微粉化が進んで容量維持率が低くなったと思われる。
【００７７】
以上説明したように、ガスアトマイズ法による急冷合金の形成とその後の溶出除去処理によって得られた実施例１〜４の負極活物質は、比較例１〜３に比べサイクル特性が向上する。しかし、実施例１〜４の負極活物質では、溶出除去前の急冷合金の組織の状態が、ボイドの形状や最終的な電池特性に大きく影響する。
すなわち、急冷凝固により組織中の結晶質相の微細化が起こっている場合に、除去したい元素ＭとＳｉとが合金化すると均一で細かいボイドが形成され、充放電における体積変化を柔軟に吸収することができる。ボイドの大きさが大きいと、粒子の強度が低下するためやや低くなる。
また、多孔質粒子とすることにより電解液の含浸がスムーズになることがリチウムイオンの拡散を助けて、電池特性の向上に寄与するものとなる。
【００７８】
【発明の効果】
以上、詳細に説明したように、本発明のリチウム二次電池用負極活物質によれば、前記多孔質粒子の内部に多数のボイドが形成されているので、多孔質粒子を構成するＳｉがリチウムと合金化して体積膨張する際に、ボイドの容積を圧縮しつつ膨張するので、多孔質粒子の体積が外観上あまり変化することがなく、これにより多孔質粒子の微粉化が防止される。
特に前記集合体の平均粒径が１μｍ以上１００μｍ以下の範囲であれば、多孔質粒子の体積が見かけ上ほとんど変化することがない。
更に前記多孔質粒子の内部に多数のボイドが形成されているので、リチウム二次電池の負極活物質として用いた場合に当該ボイドに非水電解液を含侵させることができ、これによりリチウムイオンを多孔質粒子の内部まで侵入させてリチウムイオンの拡散を効率よく行うことができ、高率充放電が可能になる。
【図面の簡単な説明】
【図１】本発明の実施形態であるリチウム二次電池用の負極活物質を構成する多孔質粒子の一例を示す断面模式図。
【図２】本発明の実施形態であるリチウム二次電池用の負極活物質を構成する多孔質粒子の別の例を示す断面模式図。
【符号の説明】
１、１１…多孔質粒子、２，１２…ボイド[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a negative electrode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery.
[0002]
[Prior art]
Research on increasing the capacity of the negative electrode active material of lithium secondary batteries has been conducted before the battery system using the current negative electrode active material as carbon has been put into practical use, and even now, metal materials such as Si, Sn, and Al have been mainly used. Although it has been actively conducted, it has not yet been put to practical use. This mainly solves the problem that the metal such as Si, Sn, and Al alloys with lithium during charge / discharge causes volume expansion and contraction, which causes the metal to be pulverized and the cycle characteristics to be reduced. It is due to the inability to do so.
[0003]
Therefore, in order to solve this problem, an amorphous alloy such as that described in Patent Literature 1 below, or a Ni—Si alloy shown in Non Patent Literature 1 or Non Patent Literature 2 described below is used. A crystalline alloy comprising a metal that can be alloyed with lithium and a metal that cannot be alloyed with lithium has been studied.
[0004]
[Patent Document 1]
JP 2002-216746 A
[Non-patent document 1]
"The 42nd Battery Symposium Proceedings," The Institute of Electrical Engineers of Japan, Battery Technology Committee, November 21, 2001, p. 296-297
[Non-patent document 2]
"The 43rd Battery Symposium Proceedings," The Institute of Electrical Engineers of Japan, Battery Technology Committee, October 12, 2002, p. 326-327
[0005]
[Problems to be solved by the invention]
However, the above-mentioned crystalline alloy or amorphous alloy has a problem that the charge / discharge capacity per alloy mass is reduced due to the inclusion of a metal that does not alloy with lithium or an intermetallic compound having a low capacity even when alloyed. Was. In addition, when these alloys are used as powders, the particle size of the powders becomes relatively large, so that the volume of the alloys during charging / discharging becomes finer due to expansion and contraction, peeling from the current collector, and contact with conductive materials. However, there was a problem that the lack of the information cannot be completely suppressed.
[0006]
The present invention has been made in view of the above circumstances, and completely eliminates pulverization due to expansion and contraction of the active material volume during charge and discharge, peeling of the active material from the current collector, and lack of contact with the conductive material. An object is to provide a negative electrode active material that can be suppressed, a method for producing the same, and a lithium secondary battery.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the present invention employs the following configurations.
The negative electrode active material for a lithium secondary battery of the present invention is formed of an aggregate of porous particles made of Si, and a number of voids having an average pore diameter in a range of 10 nm to 10 μm are formed inside the porous particles, The average particle diameter of the aggregate is in a range of 1 μm or more and 100 μm or less.
[0008]
According to such a negative electrode active material for a lithium secondary battery, since a large number of voids are formed inside the porous particles, when Si constituting the porous particles alloys with lithium and expands in volume, the voids are formed. Is expanded while compressing the volume of the porous particles, so that the volume of the porous particles does not change much in appearance, thereby preventing the porous particles from being pulverized.
In particular, if the average particle diameter of the aggregate is in the range of 1 μm or more and 100 μm or less, the volume of the porous particles hardly changes apparently.
Further, since a large number of voids are formed inside the porous particles, when used as a negative electrode active material of a lithium secondary battery, the voids can be impregnated with a non-aqueous electrolyte, thereby increasing the lithium ion Can penetrate into the interior of the porous particles to efficiently diffuse lithium ions, thereby enabling high-rate charging and discharging.
[0009]
Further, the negative electrode active material for a lithium secondary battery of the present invention is the negative electrode active material for a lithium secondary battery described above, wherein the average pore diameter of the void is n, and the average particle diameter of the aggregate is N. At this time, the n / N ratio is in the range of 0.001 or more and 0.2 or less.
[0010]
According to such a negative electrode active material for a lithium secondary battery, the n / N ratio is in the range of 0.001 or more and 0.2 or less, and the pore size of the void is extremely small with respect to the particle size of the porous particles. The pulverization accompanying the volume change can be prevented without reducing the strength of the particles.
[0011]
Further, the negative electrode active material for a lithium secondary battery according to the present invention is the negative electrode active material for a lithium secondary battery described above, wherein the porosity of the voids per volume of the porous particles is 0.1% or more and 50% or more. It is characterized by the following range.
[0012]
According to such a negative electrode active material for a lithium secondary battery, since the void porosity is in the range of 0.1% or more and 50% or less, the volume expansion of Si accompanying alloying with lithium can be sufficiently absorbed by the voids. Since the appearance of the porous particles hardly changes and the strength of the porous particles does not decrease, pulverization can be prevented.
[0013]
Further, the negative electrode active material for a lithium secondary battery of the present invention is the negative electrode active material for a lithium secondary battery described above, wherein a part of the structure of the porous particles is an amorphous phase, and the rest is crystalline. It is characterized by being a hue.
[0014]
According to such a negative electrode active material for a lithium secondary battery, since a part of the structure of the porous particles is an amorphous phase, the cycle characteristics of a battery using the negative electrode active material can be improved.
[0015]
Further, the negative electrode active material for a lithium secondary battery of the present invention is the negative electrode active material for a lithium secondary battery described above, wherein the porous particles include Sn, Al, Pb, In, Ni, Co, Ag, An alloy melt containing at least one or more elements M and Si of Mn, Cu, Ge, Cr, Ti, and Fe is quenched to form a quenched alloy, and the element M contained in the quenched alloy is acid or alkaline. It is characterized by being formed by elution and removal.
[0016]
According to such a negative electrode active material for a lithium secondary battery, the porous particles are obtained by eluting and removing the element M from a quenched alloy composed of Si and the element M, and the element M is removed in the quenched alloy. Since the portion formed becomes a void, it has an extremely fine void.
[0017]
The negative electrode active material for a lithium secondary battery of the present invention is the negative electrode active material for a lithium secondary battery described above, wherein the content of the element M in the molten alloy is 0.01% by mass or more and 70% by mass or less. It is characterized by being within the range.
[0018]
According to such a negative electrode active material for a lithium secondary battery, since the content of the element M in the molten alloy is in the above range, the average pore diameter of voids and the porosity of voids can be in the above ranges.
[0019]
Next, a lithium secondary battery of the present invention is characterized by comprising the negative electrode active material for a lithium secondary battery described in any one of the above.
[0020]
According to such a lithium secondary battery, the negative electrode active material is provided, and there is no fear that the negative electrode active material is pulverized or falls off the current collector, the contact with the conductive material is maintained, and the charge and discharge are performed. The capacity can be improved and the cycle characteristics can be improved.
[0021]
Further, the method for producing a negative electrode active material for a lithium secondary battery according to the present invention includes the method of producing at least one of Sn, Al, Pb, In, Ni, Co, Ag, Mn, Cu, Ge, Cr, Ti, and Fe. A quenched alloy is formed by quenching the molten alloy containing the elements M and Si, and the element M contained in the quenched alloy is eluted and removed with an acid or alkali in which the element M is soluble, thereby comprising Si. It is characterized by obtaining an aggregate of porous particles.
[0022]
According to such a method for producing a negative electrode active material for a lithium secondary battery, the element M is eluted and removed from the quenched alloy composed of Si and the element M, so that the porous body made of Si having a portion where the element M is removed as a void is formed. Particles can be formed. Since the formed voids have an extremely small average pore size and are evenly distributed throughout the porous particles, when the Si is alloyed with lithium and expands in volume, the voids are compressed. It becomes possible to expand, and the porous particle whose volume does not change much in appearance can be obtained.
In addition, by removing the element M from the quenched alloy, most of the structure of the porous particles can be made only of Si that easily alloys with lithium, and a negative electrode active material having a high energy density per weight can be obtained. .
Furthermore, by quenching the molten alloy, at least a part of the structure of the obtained quenched alloy can be made to be an amorphous phase that is easily alloyed with lithium, thereby improving cycle characteristics.
Furthermore, by rapidly cooling the molten alloy, a crystalline phase composed of fine crystal grains may be formed in the structure of the obtained rapidly cooled alloy. In this case, only the element M contained in the crystalline phase is formed. Can be easily eluted and removed. The voids obtained by eluting and removing the element M from the crystalline phase and the amorphous phase having small crystal grains have a smaller average pore size than the case where the element M is removed from the crystalline phase including large crystal grains. , And are evenly distributed throughout the particle. If the average pore diameter of the voids is large and non-uniformly present throughout the particles, it becomes difficult to uniformly disperse the influence of Si when the volume expands due to charging, and the strength of the particles is also reduced. It is not preferable because it causes deterioration.
[0023]
The method for producing a negative electrode active material for a lithium secondary battery according to the present invention is the method for producing a negative electrode active material for a lithium secondary battery described above, wherein the alloy melt is subjected to a gas atomization method, a water atomization method, a roll quenching method. Quenching by any one of the methods described above.
[0024]
According to such a method for producing a negative electrode active material for a lithium secondary battery, a quenched alloy can be easily obtained by employing any of the quenching methods described above.
[0025]
Further, the method for producing a negative electrode active material for a lithium secondary battery according to the present invention is the method for producing a negative electrode active material for a lithium secondary battery described above, wherein the quenching rate of the molten alloy is 100 K / sec or more. It is characterized by.
[0026]
According to the method for producing a negative electrode active material for a lithium secondary battery, by setting the quenching rate of the molten alloy to 100 K / sec or more, it is possible to easily obtain a quenched alloy in which at least a part of the structure is a crystalline phase. it can.
Further, by setting the quenching speed of the molten alloy to the above range, a crystalline phase may be formed in the structure, and in this case, the crystal grains constituting the crystalline phase can be reduced.
[0027]
Further, the method for producing a negative electrode active material for a lithium secondary battery of the present invention is the method for producing a negative electrode active material for a lithium secondary battery described above, wherein the quenched alloy is an acid or After the element M is eluted by being immersed in an alkali solution, the element M in the quenched alloy is eluted and removed by washing and drying.
[0028]
According to the method for producing a negative electrode active material for a lithium secondary battery, the quenched alloy is immersed in a solution of an acid or an alkali in which the element M is soluble to elute the element M. Can be done.
[0029]
The method for producing a negative electrode active material for a lithium secondary battery according to the present invention is the method for producing a negative electrode active material for a lithium secondary battery described above, wherein the content of the element M in the molten alloy is 0.01 mass. % To 70% by mass or less.
[0030]
According to the method for producing a negative electrode active material for a lithium secondary battery, since the content of the element M in the quenched molten metal is in the above range, the element M is too small, the number of voids is reduced, or the element M is excessive. Therefore, there is no possibility that the average pore diameter of the voids becomes excessively large.
[0031]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
The negative electrode active material for a lithium secondary battery of the present invention is an aggregate of porous particles made of Si, in which a number of voids having an average pore size of 10 nm or more and 10 μm or less are formed inside the porous particles. And the average particle size of the aggregate of porous particles is in the range of 1 μm or more and 100 μm or less.
[0032]
This negative electrode active material is provided for a negative electrode of a lithium secondary battery. When the lithium secondary battery is charged, lithium moves from the positive electrode to the negative electrode. At this time, lithium is alloyed with Si constituting the porous particles in the negative electrode. The volume expansion of Si occurs with this alloying. At the time of discharge, lithium is desorbed from Si and moves to the positive electrode side. With this desorption, the expanded Si contracts to its original volume. As described above, the expansion and contraction of Si occurs with the repetition of charging and discharging.
[0033]
According to this negative electrode active material, since a large number of voids are formed inside the porous particles, when Si constituting the porous particles is alloyed with lithium to expand the volume, the void volume is reduced. Since it expands, the size of the porous particles hardly changes in appearance, thereby preventing the porous particles from being pulverized.
[0034]
In addition, the porous particles constituting the negative electrode active material of the present embodiment include at least one of Sn, Al, Pb, In, Ni, Co, Ag, Mn, Cu, Ge, Cr, Ti, and Fe. The molten alloy containing the elements M and Si is quenched to form a quenched alloy, and the element M contained in the quenched alloy is formed by elution and removal with an acid or alkali. That is, the porous particles of the present embodiment are obtained by eluting and removing the element M from the quenched alloy containing Si and the element M, and the portion of the quenched alloy from which the element M has been removed becomes voids. It has fine voids.
[0035]
FIG. 1 is a schematic sectional view showing an example of the porous particles.
As shown in FIG. 1, a large number of voids 2 are formed inside the porous particles 1 of this example. The cross-sectional shape of each void 2 is relatively uniform.
FIG. 2 is a schematic sectional view showing another example of the porous particles.
As shown in FIG. 2, a large number of voids 12 are formed inside the porous particles 11 of this other example. The cross-sectional shapes of the voids 12 are irregular and non-uniform.
[0036]
In the porous particles 1 and 11 shown in FIGS. 1 and 2, a part of the structure is an amorphous phase of Si, and the rest is composed of a crystalline phase of Si. In some cases, these porous particles 1 and 11 may be entirely composed of a crystalline phase of Si. As will be described later, such a difference in the structure is mainly caused by a difference in the crystal structure of the quenched alloy formed in advance in the production of the negative electrode active material.
If a part of the structure of the porous particles 1 and 11 contains an amorphous phase, the cycle characteristics of the negative electrode active material can be improved.
[0037]
The average particle diameter of the porous particles 1 and 11 is preferably 1 μm or more and 100 μm or less. If the average particle size is less than 1 μm, the proportion of the voids 2 and 12 in the porous particles 1 and 11 is relatively increased, and the strength of the porous particles 1 and 11 is undesirably reduced. On the other hand, if the average particle size exceeds 100 μm, the volume change of the porous particles 1 and 11 themselves becomes large, and the pulverization proceeds, which is not preferable.
[0038]
The voids 2,..., 12... In the porous particles 1, 11 have an average pore diameter of 10 nm or more and 10 μm or less.
In particular, the voids 2 included in the porous particles 1 shown in FIG. 1 have an average pore diameter of 10 nm or more and 0.5 μm or less. The voids 12 contained in the porous particles 11 in FIG. 2 have an average pore diameter in a range of 200 nm or more and 2 μm or less, and have a larger pore diameter than the void 2 shown in FIG.
[0039]
If the average pore size of the voids 2 ..., 12 ... is less than 10 nm, the volume of the voids 2 ..., 12 ... becomes extremely small, and when Si is alloyed with lithium and volume-expanded, this expansion cannot be absorbed. Since the size of the porous particles 1 and 11 changes in appearance, there is a possibility that the porous particles 1 and 11 may be cracked and pulverized, which is not preferable. If the average pore size of the voids 2,..., 12 exceeds 10 μm, the void volume increases and the strength of the porous particles themselves decreases, which is not preferable.
[0040]
When the average pore diameter of the voids 2 and 12 is n and the average particle diameter of the porous particles 1 and 11 is N, the n / N ratio is preferably in the range of 0.001 to 0.2. When the n / N ratio is in this range, the relative pore size of the voids 2 and 12 with respect to the particle size of the porous particles 1 and 11 becomes extremely small, and the pulverization accompanying the volume change can be performed without reducing the strength of the porous particles. Can be prevented.
If the n / N ratio is less than 0.001, the relative pore diameters of the voids 2 and 12 become too small, so that the volume expansion accompanying the alloying of lithium and Si cannot be absorbed. On the other hand, if the n / N ratio exceeds 0.2, the strength of the porous particles 1 and 11 decreases, and the pulverization proceeds, which is not preferable.
[0041]
Further, the porosity of the voids 2, 12 per volume of the porous particles 1, 11 is preferably in the range of 0.1% or more and 50% or less. When the void porosity is within this range, the volume expansion of Si caused by alloying with lithium can be sufficiently absorbed by the void, and the volume of the porous particles hardly changes in appearance, and Since the strength of the porous particles does not decrease, pulverization can be prevented.
If the porosity is less than 0.1%, it is not preferable because volume expansion accompanying the alloying of lithium and Si cannot be absorbed. On the other hand, if the porosity exceeds 50%, the strength of the porous particles 1 and 11 is reduced, and pulverization proceeds, which is not preferable.
[0042]
Next, the lithium secondary battery of the present embodiment includes at least a negative electrode including the above-described negative electrode active material, a positive electrode, and an electrolyte.
[0043]
As the negative electrode of the lithium secondary battery, for example, a negative electrode active material formed of an aggregate of porous particles is solidified and formed into a sheet by using a binder for binding the porous particles to each other.
Further, the pellets are not limited to those solidified and formed into the above-mentioned sheet shape, and may be pellets solidified and formed into a column shape, a disk shape, a plate shape or a column shape.
[0044]
The binder may be either organic or inorganic, but may be any material that disperses or dissolves in the solvent together with the porous particles and further binds the porous particles by removing the solvent. . Alternatively, the porous particles may be mixed with the porous particles and solidified by pressure molding or the like to bind the porous particles together. As such a binder, for example, a vinyl resin, a cellulose resin, a phenol resin, a thermoplastic resin, a thermosetting resin, and the like can be used. For example, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, styrene butadiene rubber, and the like can be used. Resin can be exemplified.
Further, in the negative electrode according to the present invention, in addition to the negative electrode active material and the binder, carbon black, graphite powder, metal powder, or the like may be added as a conductive auxiliary.
[0045]
Next, as the positive electrode, for example, LiMn 2 O 4 , LiCoO 2 , LiNiO 2 , LiFeO 2 , V 2 O 5 , TiS, MoS, etc., and those containing a positive electrode active material capable of inserting and extracting lithium, such as organic disulfide compounds and organic polysulfide compounds.
Further, in addition to the positive electrode active material, a binder such as polyvinylidene fluoride or a conductive auxiliary material such as carbon black may be added to the positive electrode.
Specific examples of the positive electrode and the negative electrode include those obtained by applying the positive electrode or the negative electrode to a current collector made of a metal foil or a metal net and forming the same into a sheet.
[0046]
Furthermore, examples of the electrolyte include an organic electrolyte obtained by dissolving a lithium salt in an aprotic solvent.
Examples of aprotic solvents include propylene carbonate, ethylene carbonate, butylene carbonate, benzonitrile, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, dioxolan, 4-methyldioxolan, N, N-dimethylformamide, dimethylacetamide, dimethylacetate Sulfoxide, dioxane, 1,2-dimethoxyethane, sulfolane, dichloroethane, chlorobenzene, nitrobenzene, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, ethyl butyl carbonate, dipropyl carbonate, diisopropyl carbonate, dibutyl carbonate , Diethylene glycol, dimethyl Aprotic solvents such as ether, or a mixed solvent obtained by mixing two or more of these solvents, and in particular, any one of propylene carbonate (PC), ethylene carbonate (EC), and butylene carbonate (BC) And preferably one of dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC).
[0047]
As the lithium salt, LiPF 6 , LiBF 4 , LiSbF 6 , LiAsF 6 , LiClO 4 , LiCF 3 SO 3 , Li (CF 3 SO 2 ) 2 N, LiC 4 F 9 SO 3 , LiSbF 6 , LiAlO 4 , LiAlCl 4 , LiN (C x F 2x + 1 SO 2 ) (C y F 2y11 SO 2 ) (Where x and y are natural numbers), LiCl, LiI, etc., and a mixture of one or more lithium salts. 6 , LiBF 4 Those containing any one of the above are preferred.
In addition, other known organic electrolytes for lithium secondary batteries can also be used.
[0048]
Further, as another example of the electrolyte, a so-called polymer electrolyte such as a mixture of any of the above-described lithium salts in a polymer such as PEO or PVA, or a polymer obtained by impregnating an organic electrolyte with a polymer having a high swelling property is used. May be used.
Furthermore, the lithium secondary battery of the present invention is not limited to the positive electrode, the negative electrode, and the electrolyte, and may include other members and the like as necessary, and may include, for example, a separator that separates the positive electrode and the negative electrode. .
[0049]
According to such a lithium secondary battery, the negative electrode active material is provided, and there is no fear that the negative electrode active material is pulverized or falls off the current collector, the contact with the conductive material is maintained, and the charge and discharge are performed. The capacity can be improved and the cycle characteristics can be improved.
In addition, since a large number of voids are formed inside the porous particles, when used as a negative electrode active material of a lithium secondary battery, the voids can be impregnated with a non-aqueous electrolyte, thereby increasing lithium ion Can penetrate into the interior of the porous particles to efficiently diffuse lithium ions, thereby enabling high-rate charging and discharging.
[0050]
Next, a method for producing the negative electrode active material for a lithium secondary battery of the present invention will be described.
The method for producing a negative electrode active material for a lithium secondary battery generally includes a step of producing a quenched alloy containing Si and an element M, and a step of eluting the obtained quenched alloy. Hereinafter, each step will be described in order.
[0051]
First, in the step of manufacturing a quenched alloy, a molten alloy containing Si and the element M is quenched to form a quenched alloy. The alloy melt contains at least one element M of Sn, Al, Pb, In, Ni, Co, Ag, Mn, Cu, Ge, Cr, Ti, and Fe, and Si. Can be obtained by simultaneously melting a simple substance or an alloy thereof by, for example, a high frequency induction heating method.
The content of the element M in the molten alloy is preferably in the range of 0.01% by mass to 70% by mass. When the content of the element M in the molten alloy is in the above range, there is no possibility that the amount of the element M is too small to reduce the number of voids, or that the element M becomes excessive and the average pore diameter of the voids becomes excessively large.
[0052]
As a method of rapidly cooling the molten alloy, for example, a gas atomizing method, a water atomizing method, a roll quenching method, or the like can be used. A powder quenched alloy is obtained by the gas atomizing method and the water atomizing method, and a thin strip quenched alloy is obtained by the roll quenching method. The strip-shaped quenched alloy is further pulverized into a powder. The average particle size of the powdered quenched alloy thus obtained is the average particle size of the aggregate of porous particles to be finally obtained. Therefore, when obtaining a quenched alloy powder, it is necessary to adjust the average particle size to a range of 1 μm or more and 100 μm or less.
[0053]
The quenched alloy obtained from the molten alloy is a quenched alloy in which the entire structure is an amorphous phase, or a quenched alloy in which a part is an amorphous phase and the remainder is a crystalline phase composed of fine crystal grains, or a structure. It becomes a quenched alloy which is a crystalline phase composed entirely of fine crystal grains.
The amorphous phase mainly includes an alloy phase of Si and the element M. On the other hand, when a crystalline phase exists, one or more phases of an alloy phase containing the element M and Si, a single phase of Si, and a single phase of the element M are included. Therefore, the quenched alloy includes an alloy phase of Si and element M as an amorphous phase, an alloy phase of element M and Si as a crystalline phase, a single phase of Si as a crystalline phase, and an element M as a crystalline phase. One or more of the following single phases: Si and the element M form an alloy phase at a constant ratio. However, if the amount of Si contained in the molten alloy is excessive, a single Si phase in addition to the alloy phase is easily formed, and if the element M is excessive, the alloy phase is excessive. In addition, a phase composed of the element M is easily formed. The crystalline phase is composed of fine crystal grains having an average particle size of several to several tens nm. Such fine crystal grains can be obtained only by rapidly cooling the molten alloy.
[0054]
In addition, it is preferable that the rapid cooling speed at the time of rapid cooling is 100 K / sec or more. If the quenching rate is less than 100 K / sec, the crystal grains contained in the crystalline phase may be enlarged, and voids having a large average pore diameter may be formed in the subsequent elution step, which is not preferable.
[0055]
Next, in the step of eluting the quenched alloy, the element M contained in the quenched alloy is eluted and removed with an acid or alkali in which the element M is soluble.
Specifically, the powdered quenched alloy is immersed in a solution of an acid or an alkali in which the element M is soluble to elute the element M, followed by washing and drying. When eluting, it is preferable to perform stirring for about 1 to 5 hours while heating at 30 to 60 ° C.
The acid used to elute the element M depends on the type of the element M, but hydrochloric acid or sulfuric acid is preferred. The alkali used to elute the element M depends on the type of the element M, but is preferably sodium hydroxide or potassium hydroxide. Preferably, these acids or alkalis do not corrode Si.
[0056]
By eluting the element M from the quenched alloy, porous particles made of Si having a portion where the element M is removed as a void can be obtained.
As described above, the quenched alloy includes a metal phase of Si and an element M as an amorphous phase, an alloy phase as a crystalline phase, a single phase of Si as a crystalline phase, and an element M as a crystalline phase. Any one or more of the single phases may be included.
When the element M is eluted and removed from the quenched alloy having such a structure, the alloy phase becomes a Si single phase, and the entire element M single phase is removed. In this way, the quenched alloy powder after elution contains one or both of the Si single phase as the amorphous phase and the Si single phase as the crystalline phase.
[0057]
The Si single phase formed by removing the element M from the amorphous alloy phase has voids 2 having a substantially uniform cross-sectional shape and a uniform hole diameter as shown in FIG. . On the other hand, when all of the element M single phase is removed from the crystalline phase, voids 12 having uneven cross-sectional shapes and irregular pore diameters as shown in FIG. 2 are obtained. . The voids 2 and 12 thus obtained have an average pore diameter of 10 nm or more and 10 μm or less.
[0058]
According to the method for producing a negative electrode active material for a lithium secondary battery of the present embodiment, the element M is eluted and removed from the quenched alloy composed of Si and the element M, so that the part where the element M has been removed has a void. Porous particles can be formed. Since the formed voids have an extremely small average pore size and are evenly distributed throughout the porous particles, when the Si is alloyed with lithium and expands in volume, the voids are compressed. It is possible to expand the porous particles and to obtain porous particles whose size does not change much in appearance.
In addition, by removing the element M from the quenched alloy, most of the structure of the porous particles can be made only of Si that easily alloys with lithium, and a negative electrode active material having a high energy density per weight can be obtained. .
Furthermore, by quenching the molten alloy, at least a part of the structure of the obtained quenched alloy can be made into an amorphous phase, and thereby the cycle characteristics can be improved.
Furthermore, by rapidly cooling the molten alloy, a crystalline phase composed of fine crystal grains may be formed in the structure of the obtained rapidly cooled alloy. In this case, the element M phase contained in the crystalline phase may be formed. Alone can be easily eluted and removed.
[0059]
【Example】
[Production of negative electrode active material]
(Example 1)
50 parts by weight of massive Si having a size of about 5 mm square and 50 parts by weight of Ni powder were prepared, mixed, and then melted in a Ar atmosphere by a high-frequency heating method to obtain a molten alloy. 80kg / cm of this alloy melt 2 By quenching by a gas atomizing method using helium gas at a pressure of, a powder of a quenched alloy having an average particle size of 9 μm was obtained. The rapid cooling rate at this time is 1 × 10 5 K / sec. When X-ray diffraction was performed on the obtained powder, NiSi 2 The existence of an alloy phase having a mixture of a crystalline phase and an amorphous phase having the following composition was confirmed.
Next, the obtained quenched alloy powder was placed in dilute nitric acid, stirred at 50 ° C. for 1 hour, filtered while sufficiently washing, and dried in a drying oven at 100 ° C. for 2 hours. Thus, the negative electrode active material of Example 1 was manufactured.
[0060]
(Example 2)
A negative electrode active material of Example 2 was manufactured in the same manner as in Example 1 except that 80 parts by weight of Si and 20 parts by weight of Ni were used.
The quenched alloy powder at this time includes a single phase of Si as a crystalline phase and NiSi of a crystalline phase and an amorphous phase. 2 Alloy phases of different compositions were observed.
Si single phase and NiSi in the structure of quenched alloy powder 2 It is considered that the reason why the alloy phase was detected was that a part of Si could not be alloyed with Ni because the amount of Si was larger than the amount of Ni, and a part of this Si was precipitated as a Si single phase.
[0061]
(Example 3)
70 parts by weight of massive Si having a size of about 5 mm square and 30 parts by weight of Al powder were prepared, mixed, and melted by a high frequency heating method in an argon atmosphere to obtain a molten alloy. 80kg / cm of this alloy melt 2 By quenching by a gas atomization method using helium gas having a pressure of, a powder of a quenched alloy having an average particle size of 10 µm was obtained. When the obtained powder was subjected to X-ray diffraction, the existence of an Al single phase and a Si single phase as crystalline phases was confirmed.
Next, the obtained quenched alloy powder was placed in an aqueous hydrochloric acid solution, stirred at 50 ° C. for 4 hours, then filtered while sufficiently washing, and dried in a drying oven at 100 ° C. for 2 hours. Thus, the negative electrode active material of Example 3 was manufactured.
[0062]
(Example 4)
A negative electrode active material of Example 4 was produced in the same manner as in Example 3 except that sulfuric acid was used instead of hydrochloric acid.
[0063]
(Comparative Example 1)
50 parts by weight of massive Si having a size of about 5 mm square and 50 parts by weight of Ni powder were prepared, mixed, and then melted by a high frequency heating method in an argon atmosphere to obtain a molten alloy. 80kg / cm of this alloy melt 2 By quenching by a gas atomizing method using helium gas at a pressure of, a powder of a quenched alloy having an average particle size of 9 μm was obtained. This powder was used as a negative electrode active material of Comparative Example 1. When X-ray diffraction was performed on the obtained powder, NiSi 2 The existence of an alloy phase having a mixture of a crystalline phase and an amorphous phase having the following composition was confirmed.
[0064]
(Comparative Example 2)
50 parts by weight of bulk Si having a size of about 5 mm square and 50 parts by weight of Ni powder are prepared, mixed, solidified and formed into pellets, put into an electric furnace, and placed in an electric furnace at 1600 in an argon atmosphere. C. and melted naturally to obtain an ingot. This ingot was pulverized to obtain a powder having an average particle diameter of 20 μm.
Next, the obtained powder was placed in dilute nitric acid, stirred at 50 ° C. for 1 hour, filtered while sufficiently washing, and dried in a drying oven at 100 ° C. for 2 hours. Thus, the negative electrode active material of Comparative Example 2 was manufactured.
[0065]
(Comparative Example 3)
The Si powder having an average particle size of 1 μm was used as the negative electrode active material of Comparative Example 3.
[0066]
(Manufacture of lithium secondary batteries)
70 parts by weight of each of the negative electrode active materials of Examples 1 to 4 and Comparative Examples 1 to 3, 20 parts by weight of graphite powder having an average particle diameter of 2 μm as a conductive material, and 10 parts by weight of polyvinylidene fluoride were mixed. After adding methylpyrrolidone and stirring, a slurry was prepared. Next, this slurry was applied on a copper foil having a thickness of 14 μm, dried, and then rolled to form a negative electrode having a thickness of 80 μm. The prepared negative electrode was punched into a circle having a diameter of 13 mm. Metal lithium was stacked on the negative electrode as a counter electrode with a porous polypropylene separator interposed therebetween, and a mixed solvent of EC: DMC: DEC = 3: 3: 1 by volume ratio. LiPF 6 Was added at a concentration of 1 mol / L to thereby produce a coin-type lithium secondary battery.
The obtained lithium secondary battery was repeatedly charged and discharged at a current density of 0.2 C for 30 cycles at a battery voltage of 0 V to 1.5 V.
[0067]
(Physical Properties of Negative Electrode Active Materials of Examples 1 to 4)
Observation of the negative electrode active material of Example 1 with an electron microscope revealed that porous particles having voids having a substantially uniform cross-sectional shape as shown in FIG. 1 were obtained. The average pore size of the voids was on the order of 200-500 nm. Further, when the porous particles were subjected to elemental analysis using an energy dispersive X-ray analyzer, Ni was not observed on either the surface or the cross section of the porous particles.
Therefore, it was found that Ni was eluted and removed by the above-mentioned elution with hydrochloric acid, and thereafter, a uniform void was formed.
[0068]
Next, when the negative electrode active material of Example 2 was observed with an electron microscope, it was found that porous particles having irregular cross-sectional shapes and having voids of non-uniform pore diameter were obtained as shown in FIG. did. The average pore diameter of the void was about 200 nm to 2 μm, which was larger than the void of Example 1. Further, when the porous particles were subjected to elemental analysis using an energy dispersive X-ray analyzer, Ni was not observed on either the surface or the cross section of the porous particles.
The irregular shape of the voids was caused by the fact that the quenched alloy powder was formed by a plurality of structures having different compositions, and the Si single phase and the NiSi 2 From the alloy phase, NiSi 2 It is considered that Ni contained only in the alloy phase was eluted and removed.
[0069]
Next, when the negative electrode active material of Example 3 was observed with an electron microscope, it was found that porous particles having a non-uniform cross-sectional shape and having voids having a non-uniform pore size were obtained as shown in FIG. did. The average pore diameter of the void was about 300 nm to 2 μm, which was larger than the void of Example 1. Further, when the porous particles were subjected to elemental analysis using an energy dispersive X-ray analyzer, no Al was observed on any of the surface and the cross section of the porous particles.
The irregular shape of the voids is considered to be because only the Al single phase was eluted and removed from the Si single phase and the Al single phase contained in the quenched alloy powder.
[0070]
Next, as in Example 3, the negative electrode active material of Example 4 had voids of irregular cross-sectional shapes and non-uniform pore diameters. The average pore diameter of the voids was the same as in Example 3. As a result of elemental analysis, Al was not detected, and it was found that Al could be removed even by treatment with sulfuric acid.
[0071]
(Characteristics of lithium secondary battery)
Table 1 shows the capacity retention ratio of the discharge capacity at the 30th cycle to the discharge capacity at the 1st cycle.
[0072]
[Table 1]
Figure 2004214054
[0073]
It can be seen that the lithium secondary batteries of Examples 1 to 4 had good capacity retention rates of 83 to 95%. On the other hand, it is understood that Comparative Examples 1 to 3 have a low capacity retention ratio of 20 to 45%.
[0074]
In the negative electrode active material of Comparative Example 1, no Ni elution treatment was performed, so that voids were not formed in the particles constituting the powder of the negative electrode active material, and therefore the volume change of the negative electrode active material increased due to repeated charging and discharging. It is considered that the pulverization of the negative electrode active material progressed and the capacity retention ratio decreased.
[0075]
In addition, in the negative electrode active material of Comparative Example 2, the alloy melt was allowed to cool naturally without quenching the alloy melt, so that the crystal grains in the structure of the alloy after cooling were enlarged, and the pore size of the voids was accordingly increased. Was. For this reason, it is considered that the strength of the particles constituting the powder of the negative electrode active material decreased, and the repetition of charge and discharge promoted the pulverization of the negative electrode active material, resulting in a lower capacity retention rate.
[0076]
Further, since the negative electrode active material of Comparative Example 3 is a mere Si powder, the volume change of the negative electrode active material increases due to repetition of charge and discharge, as in Comparative Example 1, and the fineness of the negative electrode active material proceeds, resulting in a low capacity retention rate. It seems to have become.
[0077]
As described above, the negative electrode active materials of Examples 1 to 4 obtained by the formation of the quenched alloy by the gas atomizing method and the subsequent elution removal treatment have improved cycle characteristics as compared with Comparative Examples 1 to 3. However, in the negative electrode active materials of Examples 1 to 4, the state of the structure of the quenched alloy before elution and removal greatly affects the shape of voids and final battery characteristics.
In other words, when the crystalline phase in the structure is refined by rapid solidification, uniform and fine voids are formed when the element M to be removed and Si are alloyed, and the volume change in charge and discharge is flexibly absorbed. be able to. When the size of the voids is large, the strength of the particles is reduced, so that the size is slightly reduced.
In addition, the porous particles facilitate the impregnation of the electrolytic solution, which facilitates diffusion of lithium ions and contributes to improvement of battery characteristics.
[0078]
【The invention's effect】
As described above in detail, according to the negative electrode active material for a lithium secondary battery of the present invention, since a large number of voids are formed inside the porous particles, Si constituting the porous particles is lithium. When the alloy is alloyed with and expands in volume, the void expands while compressing the volume of the void, so that the volume of the porous particles does not change much in appearance, thereby preventing the porous particles from being pulverized.
In particular, if the average particle diameter of the aggregate is in the range of 1 μm or more and 100 μm or less, the volume of the porous particles hardly changes apparently.
Further, since a large number of voids are formed inside the porous particles, when used as a negative electrode active material of a lithium secondary battery, the voids can be impregnated with a non-aqueous electrolyte, thereby increasing the lithium ion Can penetrate into the interior of the porous particles to efficiently diffuse lithium ions, thereby enabling high-rate charging and discharging.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing an example of porous particles constituting a negative electrode active material for a lithium secondary battery according to an embodiment of the present invention.
FIG. 2 is a schematic cross-sectional view showing another example of porous particles constituting a negative electrode active material for a lithium secondary battery according to an embodiment of the present invention.
[Explanation of symbols]
1, 11: porous particles, 2, 12: void

Claims (12)

Ｓｉからなる多孔質粒子の集合体からなり、前記多孔質粒子の内部に平均孔径が１０ｎｍ以上１０μｍ以下の範囲である多数のボイドが形成され、前記集合体の平均粒径が１μｍ以上１００μｍ以下の範囲であることを特徴するリチウム二次電池用負極活物質。It is composed of an aggregate of porous particles made of Si, and a number of voids having an average pore size in a range of 10 nm or more and 10 μm or less are formed inside the porous particles, and the average particle size of the aggregate is 1 μm or more and 100 μm or less A negative electrode active material for a lithium secondary battery, wherein the negative electrode active material has a range. 前記ボイドの平均孔径をｎとし、前記集合体の平均粒径をＮとしたとき、ｎ／Ｎ比が０．００１以上０．２以下の範囲であることを特徴とする請求項１に記載のリチウム二次電池用負極活物質。The n / N ratio is in a range of 0.001 or more and 0.2 or less, where n is an average pore diameter of the void and N is an average particle diameter of the aggregate. Negative electrode active material for lithium secondary batteries. 前記多孔質粒子体積あたりの前記ボイドの空隙率が０．１％以上５０％以下の範囲であることを特徴とする請求項１または請求項２に記載のリチウム二次電池用負極活物質。3. The negative electrode active material for a lithium secondary battery according to claim 1, wherein a porosity of the voids per volume of the porous particles is in a range of 0.1% or more and 50% or less. 4. 前記多孔質粒子の組織の一部が非晶質相であり、残部が結晶質相であることを特徴とする請求項１ないし請求項３のいずれかに記載のリチウム二次電池用負極活物質。4. The negative electrode active material for a lithium secondary battery according to claim 1, wherein a part of the structure of the porous particles is an amorphous phase, and the remaining part is a crystalline phase. 5. . 前記多孔質粒子は、Ｓｎ、Ａｌ、Ｐｂ、Ｉｎ、Ｎｉ、Ｃｏ、Ａｇ、Ｍｎ、Ｃｕ、Ｇｅ、Ｃｒ、Ｔｉ、Ｆｅのうちの少なくとも１種以上の元素Ｍ及びＳｉを含む合金溶湯が急冷されて急冷合金とされ、該急冷合金に含まれる前記元素Ｍが酸またはアルカリによって溶出除去されることにより形成されたものであることを特徴とする請求項１ないし請求項４のいずれかに記載のリチウム二次電池用負極活物質。The porous particles are formed by quenching a molten alloy containing at least one element M and Si of Sn, Al, Pb, In, Ni, Co, Ag, Mn, Cu, Ge, Cr, Ti, and Fe. 5. The quenched alloy as claimed in claim 1, wherein said element M contained in said quenched alloy is formed by elution and removal with an acid or an alkali. Negative electrode active material for lithium secondary batteries. 前記合金溶湯における元素Ｍの含有率が０．０１質量％以上７０質量％以下の範囲であることを特徴とする請求項５に記載のリチウム二次電池用負極活物質。The negative electrode active material for a lithium secondary battery according to claim 5, wherein the content of the element M in the molten alloy is in a range of 0.01% by mass to 70% by mass. 請求項１ないし請求項６のいずれかに記載のリチウム二次電池用負極活物質を具備してなることを特徴とするリチウム二次電池。A lithium secondary battery comprising the negative electrode active material for a lithium secondary battery according to claim 1. Ｓｎ、Ａｌ、Ｐｂ、Ｉｎ、Ｎｉ、Ｃｏ、Ａｇ、Ｍｎ、Ｃｕ、Ｇｅ、Ｃｒ、Ｔｉ、Ｆｅのうちの少なくとも１種以上の元素Ｍ及びＳｉを含む合金溶湯を急冷することにより急冷合金を形成し、該急冷合金に含まれる前記元素Ｍを、前記元素Ｍが可溶な酸またはアルカリによって溶出除去することにより、Ｓｉからなる多孔質粒子の集合体を得ることを特徴とするリチウム二次電池用負極活物質の製造方法。A quenched alloy is formed by quenching a molten alloy containing at least one element M and Si of at least one of Sn, Al, Pb, In, Ni, Co, Ag, Mn, Cu, Ge, Cr, Ti, and Fe. A lithium secondary battery characterized by obtaining an aggregate of porous particles made of Si by eluting and removing the element M contained in the quenched alloy with an acid or alkali in which the element M is soluble. Of producing a negative electrode active material for use. 前記合金溶湯をガスアトマイズ法、水アトマイズ法、ロール急冷法のうちのいずれかの方法で急冷することを特徴とする請求項８に記載のリチウム二次電池用負極活物質の製造方法。The method for producing a negative electrode active material for a lithium secondary battery according to claim 8, wherein the molten alloy is quenched by any one of a gas atomization method, a water atomization method, and a roll quenching method. 前記合金溶湯の急冷速度が１００Ｋ／秒以上であることを特徴とする請求項８または請求項９に記載のリチウム二次電池用負極活物質の製造方法。The method for producing a negative electrode active material for a lithium secondary battery according to claim 8 or 9, wherein the quenching rate of the molten alloy is 100 K / sec or more. 前記急冷合金を、前記元素Ｍが可溶な酸またはアルカリの溶液に浸積させて前記元素Ｍを溶出させた後に、洗浄及び乾燥することにより、前記急冷合金中の前記元素Ｍを溶出除去することを特徴とする請求項８に記載のリチウム二次電池用負極活物質の製造方法。The quenched alloy is immersed in an acid or alkali solution in which the element M is soluble to elute the element M, and then washed and dried to elute and remove the element M in the quenched alloy. The method for producing a negative electrode active material for a lithium secondary battery according to claim 8, wherein: 前記合金溶湯における元素Ｍの含有率が０．０１質量％以上７０質量％以下の範囲であることを特徴とする請求項８に記載のリチウム二次電池用負極活物質の製造方法。The method for producing a negative electrode active material for a lithium secondary battery according to claim 8, wherein the content of the element M in the molten alloy is in a range of 0.01% by mass to 70% by mass.
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