JP4963330B2 - Lithium iron composite oxide for positive electrode active material of lithium secondary battery, method for producing the same, and lithium secondary battery using the same - Google Patents
Lithium iron composite oxide for positive electrode active material of lithium secondary battery, method for producing the same, and lithium secondary battery using the same Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は、リチウムの吸蔵・脱離現象を利用したリチウム二次電池を構成することのできる正極活物質用リチウム鉄複合酸化物、その製造方法およびそれを正極活物質として構成されるリチウム二次電池に関する。
【０００２】
【従来の技術】
リチウムの吸蔵・脱離現象を利用したリチウム二次電池は、高エネルギー密度であることから、携帯電話、パソコン等の小型化に伴い、通信機器、情報関連機器の分野では、リチウム二次電池が広く普及するに至っている。また、資源問題、環境問題から、自動車の分野でも電気自動車に対する要望が高まり、安価であってかつ容量が大きく、サイクル特性が良好なリチウム二次電池の開発が急がれている。
【０００３】
現在、リチウム二次電池の正極活物質には、４Ｖ級の二次電池を構成できるものとして、ＬｉＣｏＯ2、ＬｉＮｉＯ2、ＬｉＭｎ2Ｏ4等のリチウム遷移金属複合酸化物が好んで用いられており、特に、ＬｉＣｏＯ2は、合成が容易でかつ取り扱いも比較的容易であることに加え、充放電サイクル特性において優れることから、ＬｉＣｏＯ2を正極活物質に使用する二次電池が主流となっている。
【０００４】
ところが、コバルト等は資源量として少なく、ＬｉＣｏＯ2等を正極活物質に使用した二次電池では、電気自動車用電池をにらんだ将来の量産化、大型化に対応しにくく、また価格的にも極めて高価なものにならざるを得ない。そこでコバルト等に代えて、資源として豊富であり、かつ安価な鉄を構成元素として含む、リチウム鉄複合酸化物を正極活物質に採用する試みがされている。
【０００５】
リチウム鉄複合酸化物を正極活物質として用いたリチウム二次電池としては、例えば、特開平９−１３４７２５号公報に、オリビン構造を有するＬｉＦｅＰＯ4、ＬｉＦｅＶＯ4等を正極活物質として用いたリチウム二次電池が示されている。
【０００６】
しかしながら、本発明者が追試したところ、上述の公報に示されているようなオリビン構造リチウム鉄複合酸化物を正極活物質として用いたリチウム二次電池では、充分な容量を得ることができなかった。すなわち、実用的な充放電密度で充放電した場合に、活物質放電容量としては１００ｍＡｈ／ｇ以上が必要とされているが、上記リチウム鉄複合酸化物を正極活物質として用いた場合には、その活物質放電容量を得ることは困難であった。
【０００７】
このことは、特開平１１−２５９８３号公報に示されているように、結晶構造がＬｉＦｅＰＯ4と同様のオリビン構造であるＬｉＣｏＰＯ4の活物質放電容量が８０ｍＡｈ／ｇであり、理論容量の約半分となっていることからも推認される。
【０００８】
【発明が解決しようとする課題】
上記問題を解決すべく種々の実験、検討を行った結果、本発明者は、上記問題は、リチウム鉄複合酸化物粒子におけるリチウムイオンの吸蔵・脱離の反応が遅いことが原因であるとの知見を得た。一般に、リチウム鉄複合酸化物を正極活物質として用いた場合には、正極は粉末状のリチウム鉄複合酸化物に導電材および結着剤を混合し、ペースト状の正極合材としたものを、集電体表面に塗布等することによって形成される。従来のリチウム鉄複合酸化物を正極活物質として用いたリチウム二次電池は、正極の上記反応に関与する面積、すなわち、リチウム鉄複合酸化物の単位重量あたりの上記反応に関与する面積が小さいために、リチウムイオンの吸蔵・脱離の反応が充分に行われないと考えられる。
【０００９】
本発明は、上記問題を解決するためになされたものであり、リチウム鉄複合酸化物におけるリチウムイオンの吸蔵・脱離の反応に関与する表面積を大きくし、反応を活性化することで、活物質放電容量が大きく、かつ、サイクル後においてもその容量を維持できるといったサイクル劣化の少ないリチウム二次電池正極活物質用リチウム鉄複合酸化物を提供することを目的とする。
【００１０】
また、本発明は、上記リチウム二次電池正極活物質用リチウム鉄複合酸化物を、簡便に製造する方法を提供することを目的とし、さらに本発明は、上記リチウム鉄複合酸化物を正極活物質として用いることにより、安価で、放電容量が大きく、サイクルを繰り返しても容量低下の少ないサイクル特性に優れたリチウム二次電池を提供することを目的とする。
【００１１】
【課題を解決するための手段】
本発明によって提供される好ましい一つの態様のリチウム二次電池正極活物質用リチウム鉄複合酸化物は、基本組成をＬｉＦｅＰＯ4とするオリビン構造リチウム鉄複合酸化物であって、リチウム源と鉄源とリン源との混合物を焼成して得られた焼成体を解砕することにより製造された、略単独に存在する粒子の粒径の平均値（即ち平均粒径）が１μｍ以下であることを特徴とする。
【００１２】
リチウム鉄複合酸化物の粒子の大きさを、平均粒径が１μｍ以下となるようにすることで、正極におけるリチウムイオンの吸蔵・脱離の反応に関与する表面積が増加し、反応が活性化するため、リチウムイオンの吸蔵・脱離がスムーズに行われると考えられる。その結果、本発明のリチウム二次電池正極活物質用リチウム鉄複合酸化物は、活物質放電容量が大きく、かつ、サイクル後においてもその容量を維持できるといったサイクル劣化の少ないリチウム鉄複合酸化物となる。
【００１３】
本発明のリチウム鉄複合酸化物は、その製造方法を特に限定するものではないが、以下の方法によれば、より簡便に製造することができる。すなわち、本発明によって提供される好ましい一つの態様のリチウム鉄複合酸化物の製造方法は、リチウム源と、鉄源と、リン源（好適にはリン含有アンモニウム塩）とを混合して混合物を得る原料混合工程と、該混合物を６００℃以上７００℃以下の温度で焼成する焼成工程と、該焼成により得られた焼成体を解砕する解砕工程とを含んでなる、略単独に存在する粒子の粒径の平均値が１μｍ以下であるリチウム二次電池正極活物質用リチウム鉄複合酸化物の製造方法である。
【００１４】
焼成を６００℃以上７００℃以下という比較的低い温度で行うことにより、焼成における粒子の成長を抑制し、該得られた焼成体を解砕することにより、略単独に存在する粒子の粒径の平均値が１μｍ以下と小さいリチウム鉄複合酸化物を得ることができる。
【００１５】
したがって、本発明のリチウム鉄複合酸化物の製造方法は、上記活物質放電容量が大きく、かつ、サイクル劣化の少ないリチウム鉄複合酸化物を簡便に製造する方法となる。
【００１６】
また、本発明のリチウム二次電池は、上記本発明のリチウム鉄複合酸化物を正極活物質として用いることを特徴とするものである。本発明のリチウム二次電池は、安価であることに加え、リチウムイオンの吸蔵・脱離がスムーズに行われ、反応が活性化するという上記リチウム鉄複合酸化物の作用を有効に発揮でき、その結果、放電容量が大きく、サイクルを繰り返しても容量低下の少ないサイクル特性に優れたリチウム二次電池となる。
【００１７】
【発明の実施の形態】
以下に、本発明のリチウム二次電池正極活物質用リチウム鉄複合酸化物、その製造方法およびそれを用いたリチウム二次電池について、それらの好適な実施形態を詳細に説明する。
【００１８】
〈リチウム鉄複合酸化物〉
本発明のリチウム二次電池正極活物質用リチウム鉄複合酸化物は、基本組成をＬｉＦｅＰＯ4とするオリビン構造リチウム鉄複合酸化物であって、その粒子の平均粒径が１μｍ以下であるリチウム鉄複合酸化物である。
【００１９】
「基本組成を〜とする」とは、その組成式で表される組成のものだけでなく、例えば、結晶構造におけるＦｅのサイトの一部をＣｏ、Ｎｉ、Ｍｎ、Ｍｇ等の他の元素で置換したものをも含むことを意味する。さらに、化学量論組成のものだけでなく、一部の元素が欠損等した非化学量論組成のものをも含むことを意味する。
【００２０】
本発明のリチウム鉄複合酸化物は、その結晶構造が斜方晶系のオリビン構造となるものであり、その空間群はＰｍｎｂで表される。つまり、オリビン構造とは酸素の六方最密充填を基本とし、その四面体サイトにリンが、八面体サイトにリチウムと鉄がともに位置する構造である。
【００２１】
本発明のリチウム鉄複合酸化物は、その粒子の平均粒径が１μｍ以下となるものである。平均粒径が１μｍを超えると、正極におけるリチウム鉄複合酸化物粒子のリチウムイオンの吸蔵・脱離の反応に関与する表面積が小さくなり、リチウムイオンの吸蔵・脱離の反応がスムーズに行われず、実用的な充放電密度で充放電した場合に、充分な活物質放電容量を得ることができないからである。特に、電極の作製が容易であり、かつ、レート特性が良好であるという点を考慮すると、平均粒径を０．２μｍ以上０．８μｍ以下とすることが望ましい。
【００２２】
なお、リチウム鉄複合酸化物は、後に写真で示すように、略単独に存在する粒子から形成されるものである。したがって、平均粒径は、この略単独に存在する粒子の粒径の平均値であり、それぞれの粒径は、例えば、リチウム鉄複合酸化物の走査型電子顕微鏡（ＳＥＭ）写真を利用して測定することができる。すなわち、リチウム鉄複合酸化物のＳＥＭ写真を撮影し、その写真におけるリチウム鉄複合酸化物粒子の最長径と最短径を測定し、それら２つの値の平均値をその１つの粒子の粒径として採用することができる。
【００２３】
〈リチウム鉄複合酸化物の製造方法〉
本発明のリチウム鉄複合酸化物は、その製造方法を特に限定するものではないが、以下の方法によれば、より簡便に製造することができる。すなわち、本発明のリチウム鉄複合酸化物の製造方法は、原料を混合して混合物を得る原料混合工程と、該混合物を所定の温度で焼成する焼成工程とからなる方法である。
【００２４】
（１）原料混合工程
本発明のリチウム鉄複合酸化物の製造方法における原料混合工程は、リチウム化合物と、鉄化合物と、リン含有アンモニウム塩とを混合して混合物を得る工程である。
【００２５】
リチウム源となるリチウム化合物としては、Ｌｉ2ＣＯ3、Ｌｉ（ＯＨ）、Ｌｉ（ＯＨ）・Ｈ2Ｏ、ＬｉＮＯ3等を用いることができる。特に、吸湿性が低いという理由からＬｉ2ＣＯ3を用いることが望ましい。
【００２６】
鉄源となる鉄化合物としては、鉄の価数が２価である化合物として、ＦｅＣ2Ｏ4・２Ｈ2Ｏ、ＦｅＣｌ2等を用いることができる。特に、焼成時に発生するガスの腐食性が低いという理由からＦｅＣ2Ｏ4・２Ｈ2Ｏを用いることが望ましい。
【００２７】
リン源となるリン含有アンモニウム塩としては、ＮＨ4Ｈ2ＰＯ4、（ＮＨ4）2ＨＰＯ4、Ｐ2Ｏ5等を用いることができる。特に、比較的吸湿性が低く、腐食性ガスの発生量が少ないという理由からＮＨ4Ｈ2ＰＯ4を用いることが望ましい。
【００２８】
上記の原料は、いずれも粉末状のものを用いればよく、それらの混合は、通常の粉体の混合に用いられている方法で行えばよい。具体的には、例えば、ボールミル、ミキサー、乳鉢等を用いて混合すればよい。なお、それぞれの原料の混合割合は、製造しようとするリチウム鉄複合酸化物の組成に応じた割合とすればよい。
【００２９】
また、平均粒径が１μｍ以下となるリチウム鉄複合酸化物を得るためには、上記原料の平均粒径を制御することが望ましく、特に、各原料とも平均粒径が１μｍ以下のものを用いることが望ましい。
【００３０】
（２）焼成工程
焼成工程は、原料混合工程で得られた混合物を６００℃以上７００℃以下の温度で焼成する工程である。焼成は、鉄が３価に酸化されるのを防ぐため、不活性雰囲気下、または還元雰囲気下、具体的には、例えば、アルゴン気流中あるいは窒素気流中等にて行えばよい。
【００３１】
焼成温度は、６００℃以上７００℃以下とする。焼成温度が６００℃未満であると、反応が充分に進行せず、目的とする斜方晶のもの以外の副相が生成し、リチウム鉄複合酸化物の結晶性が悪くなるからである。反対に、７００℃を超えると、リチウム鉄複合酸化物の粒子が成長し、その粒径が大きくなるため、平均粒径が１μｍ以下となるリチウム鉄複合酸化物を得ることができないからである。特に、レート特性の向上という点を考慮すれば、６２０℃以上６７０℃以下とすることが望ましい。なお、焼成時間は焼成が完了するのに充分な時間であればよく、通常、１２時間程度行えばよい。
【００３２】
〈リチウム二次電池〉
本発明のリチウム二次電池は、上記本発明のリチウム鉄複合酸化物を正極活物質として用いたリチウム二次電池であり、正極活物質を除く他の構成については、特に限定するものではなく、既に公知のリチウム二次電池の構成に従えばよい。また、本発明のリチウム鉄複合酸化物は、その組成、粒子径等により種々のリチウム鉄複合酸化物が存在する。本発明のリチウム二次電池においては、それらの１種を正極活物質として用いるものであってもよく、また、２種以上を混合して用いるものであってもよい。さらに、本発明のリチウム鉄複合酸化物と既に公知の正極活物質材料とを混合して正極活物質とする構成を採用することもできる。
【００３３】
一般にリチウム二次電池は、リチウムイオンを吸蔵・放出する正極および負極と、この正極と負極との間に挟装されるセパレータと、正極と負極の間をリチウムイオンを移動させる非水電解液等を主要構成要素として構成することができる。
【００３４】
正極は、正極活物質となる上記本発明のリチウム鉄複合酸化物の粉状体に導電材および結着剤を混合し、適当な溶剤を加えてペースト状の正極合材としたものを、アルミニウム等の金属箔製の集電体表面に塗布乾燥し、必要に応じて電極密度を高めるべく圧縮して形成することができる。導電材は、正極の電気伝導性を確保するためのものであり、カーボンブラック、アセチレンブラック、黒鉛等の炭素物質粉状体の１種又は２種以上を混合したものを用いることができる。結着剤は、活物質粒子および導電材粒子を繋ぎ止める役割を果たすものでポリテトラフルオロエチレン、ポリフッ化ビニリデン、フッ素ゴム等の含フッ素樹脂、ポリプロピレン、ポリエチレン等の熱可塑性樹脂を用いることができる。これら活物質、導電材、結着剤を分散させる溶剤としては、Ｎ−メチル−２−ピロリドン等の有機溶剤を用いることができる。
【００３５】
負極は、負極活物質である金属リチウムを、シート状にして、あるいはシート状にしたものをニッケル、ステンレス等の集電体網に圧着して形成することができる。負極活物質には金属リチウムに代え、リチウム合金、またはリチウム化合物をも用いることができる。
【００３６】
また負極のもう一つの形態として、負極活物質にリチウムイオンを吸蔵・脱離できる炭素物質を用いて負極を構成させることもできる。使用できる炭素物質としては、天然あるいは人造の黒鉛、フェノール樹脂等の有機化合物焼成体、コークス等の紛状体が挙げられる。この場合は、負極活物質に結着剤を混合し、適当な溶媒を加えてペースト状にした負極合材を、銅等の金属箔集電体の表面に塗布、乾燥し、その後にプレスして形成することができる。この場合の塗布、乾燥、プレス等も通常の方法に従えばよい。炭素物質を負極活物質とした場合、正極同様、負極結着剤としてはポリフッ化ビニリデン等の含フッ素樹脂等を、溶剤としてはＮ−メチル−２−ピロリドン等の有機溶剤を用いることができる。
【００３７】
正極および負極に挟装させるセパレータは、正極と負極とを分離し電解液を保持するものであり、ポリエチレン、ポリプロピレン等の薄い微多孔膜を用いることができる。
【００３８】
非水電解液は、電解質としてのリチウム塩を有機溶媒に溶解させたものである。リチウム塩は有機溶媒に溶解することによって解離し、リチウムイオンとなって電解液中に存在する。使用できるリチウム塩としては、ＬｉＢＦ4、ＬｉＰＦ6、ＬｉＣｌＯ4、ＬｉＣＦ3ＳＯ3、ＬｉＡｓＦ6、ＬｉＮ（ＣＦ3ＳＯ2）2、ＬｉＮ（Ｃ2Ｆ5ＳＯ2）2等が挙げられる。これらのリチウム塩は、それぞれ単独で用いてもよく、また、これらのもののうち２種以上のものを併用することもできる。
【００３９】
リチウム塩を溶解させる有機溶媒には、非プロトン性の有機溶媒を用いる。例えば、環状カーボネート、鎖状カーボネート、環状エステル、環状エーテルあるいは鎖状エーテル等の１種または２種以上からなる混合溶媒を用いることができる。環状カーボネートの例示としてはエチレンカーボネート、プロピレンカーボネート、ブチレンカーボネート、ビニレンカーボネート等が、鎖状カーボネートの例示としてはジメチルカーボネート、ジエチルカーボネート、メチルエチルカーボネート等が、環状エステルの例示としてはガンマブチロラクトン、ガンマバレロラクトン等が、環状エーテルの例示としてはテトラヒドロフラン、２−メチルテトラヒドロフラン等が、鎖状エーテルの例示としてはジメトキシエタン、エチレングリコールジメチルエーテル等がそれぞれ挙げられる。これらのもののうちいずれか１種を単独で用いることも、また２種以上を混合させて用いることもできる。
【００４０】
なお、上記セパレータおよび非水電解液という構成に代えて、ポリエチレンオキシド等の高分子量ポリマーとＬｉＣｌＯ4やＬｉＮ（ＣＦ3ＳＯ2）2等のリチウム塩を使用した高分子固体電解質を用いることもでき、また、上記非水電解液をポリアクリロニトリル等の固体高分子マトリクスにトラップさせたゲル電解質を用いることもできる。
【００４１】
以上の構成要素によって構成される本発明のリチウム二次電池であるが、その形状は円筒型、積層型、コイン型等、種々のものとすることができる。いずれの形状を採る場合であっても、上記構成要素を電池ケースに収納し、正極集電体および負極集電体から外部に通ずる正極端子および負極端子までの間を集電用リード等を用いて接続し、電池ケースを密閉して電池系を外部と離隔し、リチウム二次電池が完成される。
【００４２】
〈その他の形態〉
以上、本発明のリチウム二次電池正極活物質用リチウム鉄複合酸化物、その製造方法、およびそれを用いたリチウム二次電池の実施形態について説明したが、上述した実施形態は一実施形態にすぎず、本発明のリチウム二次電池正極活物質用リチウム鉄複合酸化物、その製造方法、およびそれを用いたリチウム二次電池は、上記実施形態を始めとして、当業者の知識に基づいて種々の変更、改良を施した種々の形態で実施することができる。
【００４３】
【実施例】
上記実施形態に基づいて、平均粒径が１μｍ以下である本発明のリチウム鉄複合酸化物を実施例として製造した。また、これと比較すべく、平均粒径が１μｍを超えるリチウム鉄複合酸化物を比較例として製造した。そして、実施例および比較例のリチウム鉄複合酸化物をそれぞれ正極活物質としたリチウム二次電池を作製し、それらの活物質放電容量を比較することによって、本発明のリチウム鉄複合酸化物の優秀性を確認した。以下、これらについて説明する。
【００４４】
〈実施例のリチウム鉄複合酸化物〉
リチウム源としてＬｉ2ＣＯ3を、鉄源としてＦｅＣ2Ｏ4・２Ｈ2Ｏを、リン源としてＮＨ4Ｈ2ＰＯ4をそれぞれ用い、Ｌｉ、Ｆｅ、Ｐがモル比で１：１：１となるように混合した。混合には自動乳鉢を用いた。この混合物を、アルゴン気流中、６２０℃で１２時間焼成した。そして、得られたリチウム鉄複合酸化物を解砕して、正極活物質となる粉末状のリチウム鉄複合酸化物とした。本リチウム鉄複合酸化物を実施例のリチウム鉄複合酸化物とした。
【００４５】
本実施例のリチウム鉄複合酸化物のＸ線回折パターンを図１に示す。図１のパターンより、本実施例のリチウム鉄複合酸化物は、その結晶構造が斜方晶系オリビン構造であるリチウム鉄複合酸化物であることが確認された。なお、組成分析より、本実施例のリチウム鉄複合酸化物の組成は、組成式ＬｉＦｅＰＯ4で表されることがわかった。
【００４６】
また、図２に本実施例のリチウム鉄複合酸化物を走査型電子顕微鏡（ＳＥＭ）で撮影した写真を示す。図２の写真より、本実施例のリチウム鉄複合酸化物は略単独の粒子からなることが確認でき、その粒子の平均粒径は約０．８μｍであった。
【００４７】
〈比較例のリチウム鉄複合酸化物〉
上記実施例のリチウム鉄複合酸化物の製造において、焼成温度を８７０℃に変更した以外は、実施例のリチウム鉄複合酸化物と同様に製造した。得られたリチウム鉄複合酸化物を比較例のリチウム鉄複合酸化物とした。
【００４８】
本比較例のリチウム鉄複合酸化物のＸ線回折パターンを図３に示す。図３のパターンより、本比較例のリチウム鉄複合酸化物は、その結晶構造が斜方晶系オリビン構造であるリチウム鉄複合酸化物であることが確認された。なお、組成分析より、本比較例のリチウム鉄複合酸化物の組成は、組成式ＬｉＦｅＰＯ4で表されることがわかった。
【００４９】
また、図４に本比較例のリチウム鉄複合酸化物を走査型電子顕微鏡（ＳＥＭ）で撮影した写真を示す。図４の写真より、本比較例のリチウム鉄複合酸化物は略単独の粒子からなることが確認でき、その粒子の平均粒径は約５μｍであった。
【００５０】
〈リチウム二次電池〉
上記実施例および比較例のリチウム鉄複合酸化物を正極活物質に用いてリチウム二次電池を作製した。正極は、まず、正極活物質となるそれぞれのリチウム鉄複合酸化物７０重量部に、導電材としてのカーボンブラックを２５重量部、結着剤としてのポリフッ化ビニリデンを５重量部混合し、溶剤として適量のＮ−メチル−２−ピロリドンを添加して、ペースト状の正極合材を調製した。次いで、このペースト状の正極合材をアルミニウム箔集電体に塗工して加圧し、正極合材の厚さを４５μｍとした後、直径１５ｍｍφの円盤状に打ち抜いて正極とした。
【００５１】
対向させる負極は、金属リチウムを活物質として用いた。金属リチウムをシート状にして、ニッケル集電体網に圧着し、厚さ１００μｍとした後、直径１７ｍｍφの円盤状に打ち抜いて負極とした。
【００５２】
セパレータにはポリエチレン製の微多孔膜を用い、セパレータに含浸させる非水電解液は、エチレンカーボネートとジエチルカーボネートとを体積比３：７に混合した混合溶媒にＬｉＰＦ6を１Ｍの濃度で溶解したものを用いた。上記正極および負極を、セパレータを介して対向させ、上記非水電解液を適量注入して含浸させた後、コイン型電池ケースに収納することによりリチウム二次電池を作製した。
【００５３】
なお、実施例のリチウム鉄複合酸化物を正極活物質に用いたリチウム二次電池を実施例のリチウム二次電池とし、同様に比較例のリチウム鉄複合酸化物を正極活物質に用いたリチウム二次電池を比較例のリチウム二次電池とした。
【００５４】
〈充放電サイクル試験〉
上記実施例および比較例のそれぞれのリチウム二次電池に対し、充放電サイクル試験を行った。充放電試験は２０℃の温度条件下で、５サイクルごとに電流密度を変えて合計３０サイクル行った。以下に、電流密度が同じである５サイクルを１回の充放電試験として、１〜６回の充放電試験の条件を説明する。
【００５５】
１回目の充放電試験は、電流密度０．２５ｍＡ／ｃｍ2の定電流で充電上限電圧４．０Ｖまで充電を行い、次いで電流密度０．２５ｍＡ／ｃｍ2の定電流で放電下限電圧３．０Ｖまで放電を行う充放電を１サイクルとし、このサイクルを５サイクル繰り返すものとした。そして、各サイクルごとに、それぞれのリチウム二次電池について放電容量を測定した。
【００５６】
以下、２〜６回目の充放電試験は、１回目の試験における充放電の際の電流密度をそれぞれ変更した以外は、１回目の試験と同様にして行った。そして、同様に各サイクルごとに、それぞれのリチウム二次電池について放電容量を測定した。すなわち、２回目の試験では、０．２５ｍＡ／ｃｍ2で充電し、０．５ｍＡ／ｃｍ2で放電し、３回目の試験では、０．５ｍＡ／ｃｍ2で充電し、０．５ｍＡ／ｃｍ2で放電し、４回目の試験では、０．５ｍＡ／ｃｍ2で充電し、１．０ｍＡ／ｃｍ2で放電し、５回目の試験では、１．０ｍＡ／ｃｍ2で充電し、１．０ｍＡ／ｃｍ2で放電し、６回目の試験では、１回目の試験と同様とし、０．２５ｍＡ／ｃｍ2で充電し、０．２５ｍＡ／ｃｍ2で放電した。
【００５７】
次に、実施例のリチウム二次電池について、上記６回の充放電試験を温度条件のみ変更して行った。すなわち、リチウム二次電池の実使用温度範囲の上限と目される６０℃の高温環境下で、他の条件は上記２０℃の場合と同様にして、上記６回、合計３０サイクルの充放電試験を行った。そして、各サイクルごとに放電容量を測定した。
【００５８】
〈リチウム二次電池の評価〉
実施例および比較例のそれぞれのリチウム二次電池について、上記充放電試験で測定した放電容量から、正極活物質１ｇあたりの放電容量、すなわち活物質放電容量を求め、その結果を図５および図６に示す。ここで図５は、実施例のリチウム二次電池の各サイクルごとの活物質放電容量を示し、◆印は２０℃下でのものを、○印は６０℃下でのものを示している。また、図６は、２０℃における比較例のリチウム二次電池の各サイクルごとの活物質放電容量を示す。
【００５９】
図５から明らかなように、実施例のリチウム二次電池は、サイクル初期に、２０℃下で、電流密度０．２５ｍＡ／ｃｍ2で放電した場合の活物質放電容量が１０５ｍＡｈ／ｇであり、１００ｍＡｈ／ｇを超える大きな活物質放電容量が得られた。この値は、図６に示す比較例のリチウム二次電池のものと比較すると、約３倍となっている。また、同様の電流密度で、６０℃下での活物質放電容量値は、約１３０ｍＡｈ／ｇであり、さらに大きな活物質放電容量が得られたことから、高温下では反応が活性化し、よりリチウムイオンの吸蔵・脱離がスムーズに行われると考えられる。したがって、平均粒径が１μｍ以下であるリチウム鉄複合酸化物を正極活物質に用いたリチウム二次電池は、大きな活物質放電容量を有することが確認できた。
【００６０】
なお、実施例のリチウム二次電池では、充放電の電流密度が大きくなるとともに、活物質放電容量は除々に減少している。これは、比較例のリチウム二次電池でも同様であるが、大きな電流密度で充放電すると、活物質により大きな負荷がかかり、内部抵抗が増加するためと考えられる。
【００６１】
しかし、図５において、電流密度を１回目のものと同様にした６回目の充放電試験（２６サイクル〜３０サイクル）における活物質放電容量は、１回目の充放電試験（１サイクル〜５サイクル）における活物質放電容量とほぼ同等の値となっている。すなわち、サイクルを繰り返してもその大きな活物質放電容量が維持されている。これは、２０℃下での試験に限られることなく、６０℃下での試験においても同様である。したがって、平均粒径が１μｍ以下であるリチウム鉄複合酸化物を正極活物質に用いたリチウム二次電池は、サイクルを繰り返しても容量低下が少なく、特に高温下でもサイクル特性に優れたリチウム二次電池となることが確認できた。
【００６２】
【発明の効果】
本発明のリチウム鉄複合酸化物は、その粒子の平均粒径が１μｍ以下と小さいため、正極におけるリチウムイオンの吸蔵・脱離の反応に関与する表面積が増加し、リチウムイオンの吸蔵・脱離がスムーズに行われることとなり、活物質放電容量が大きく、かつ、サイクル劣化の少ないリチウム鉄複合酸化物となる。
【００６３】
また、本発明のりチウム鉄複合酸化物の製造方法によれば、上記リチウム鉄複合酸化物を簡便に製造することができ、さらに、本発明のリチウム鉄複合酸化物を正極活物質として用いることにより、安価で、放電容量が大きく、サイクル特性に優れたリチウム二次電池となる。
【図面の簡単な説明】
【図１】 ６２０℃で焼成し、平均粒径が１μｍ以下である実施例のリチウム鉄複合酸化物のＸ線回折パターンである。
【図２】 実施例のリチウム鉄複合酸化物のＳＥＭ写真である。
【図３】 ８７０℃で焼成し、平均粒径が１μｍを超える比較例のリチウム鉄複合酸化物のＸ線回折パターンである。
【図４】 比較例のリチウム鉄複合酸化物のＳＥＭ写真である。
【図５】 実施例のリチウム二次電池の各サイクルごとの活物質放電容量を示すグラフである。
【図６】 比較例のリチウム二次電池の各サイクルごとの活物質放電容量を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a lithium iron composite oxide for a positive electrode active material capable of constituting a lithium secondary battery utilizing a lithium occlusion / desorption phenomenon, a method for producing the same, and a lithium secondary constituted using the same as a positive electrode active material It relates to batteries.
[0002]
[Prior art]
Lithium secondary batteries that use the lithium absorption / desorption phenomenon have a high energy density, so along with the downsizing of mobile phones, personal computers, etc. It has become widespread. Further, due to resource problems and environmental problems, there is an increasing demand for electric vehicles in the field of automobiles, and there is an urgent need to develop lithium secondary batteries that are inexpensive, have large capacity, and have good cycle characteristics.
[0003]
At present, as a positive electrode active material of a lithium secondary battery, LiCoO can be used as a secondary battery of 4V class. 2 LiNiO 2 , LiMn 2 O Four Lithium transition metal composite oxides such as LiCoO are particularly preferred. 2 In addition to being easy to synthesize and relatively easy to handle, LiCoO is excellent in charge / discharge cycle characteristics. 2 Rechargeable batteries using as the positive electrode active material are the mainstream.
[0004]
However, cobalt and other resources are small, and LiCoO 2 In the secondary battery using the positive electrode active material as a positive electrode active material, it is difficult to cope with future mass production and enlargement in view of the battery for electric vehicles, and the price must be extremely expensive. Therefore, in place of cobalt or the like, an attempt has been made to employ lithium iron composite oxide, which is abundant as a resource and contains inexpensive iron as a constituent element, as a positive electrode active material.
[0005]
As a lithium secondary battery using a lithium iron composite oxide as a positive electrode active material, for example, JP-A-9-134725 discloses LiFePO having an olivine structure. Four LiFeVO Four A lithium secondary battery using the above as a positive electrode active material is shown.
[0006]
However, as a result of further trial by the present inventors, a lithium secondary battery using the olivine-structure lithium iron composite oxide as shown in the above-mentioned publication as a positive electrode active material could not obtain a sufficient capacity. . That is, when charged and discharged at a practical charge / discharge density, an active material discharge capacity of 100 mAh / g or more is required, but when the lithium iron composite oxide is used as a positive electrode active material, It was difficult to obtain the active material discharge capacity.
[0007]
This is because the crystal structure is LiFePO as shown in Japanese Patent Application Laid-Open No. 11-25983. Four LiCoPO with the same olivine structure as Four The active material discharge capacity is 80 mAh / g, which is estimated to be about half of the theoretical capacity.
[0008]
[Problems to be solved by the invention]
As a result of various experiments and studies to solve the above problem, the present inventor said that the above problem is caused by a slow reaction of occlusion / desorption of lithium ions in lithium iron composite oxide particles. Obtained knowledge. In general, when lithium iron composite oxide is used as the positive electrode active material, the positive electrode is a powdered lithium iron composite oxide mixed with a conductive material and a binder to obtain a paste-like positive electrode mixture, It is formed by coating or the like on the current collector surface. A lithium secondary battery using a conventional lithium iron composite oxide as a positive electrode active material has a small area involved in the reaction of the positive electrode, that is, the area involved in the reaction per unit weight of the lithium iron composite oxide. In addition, it is considered that the lithium ion occlusion / desorption reaction is not sufficiently performed.
[0009]
The present invention has been made to solve the above problems, and by increasing the surface area involved in the lithium ion occlusion / desorption reaction in the lithium iron composite oxide and activating the reaction, the active material An object of the present invention is to provide a lithium iron composite oxide for a positive electrode active material for a lithium secondary battery that has a large discharge capacity and that can maintain the capacity even after cycling, and has little cycle deterioration.
[0010]
Another object of the present invention is to provide a method for easily producing the lithium iron composite oxide for a positive electrode active material of a lithium secondary battery, and the present invention further includes the above lithium iron composite oxide as a positive electrode active material. Therefore, it is an object of the present invention to provide a lithium secondary battery that is inexpensive, has a large discharge capacity, and is excellent in cycle characteristics with little decrease in capacity even after repeated cycles.
[0011]
[Means for Solving the Problems]
The present invention One preferred embodiment provided by Lithium iron composite oxide for lithium secondary battery positive electrode active material has a basic composition of LiFePO Four The olivine structure lithium iron composite oxide The average value of the particle diameters of particles that are produced by pulverizing a fired body obtained by firing a mixture of a lithium source, an iron source, and a phosphorus source (ie Average particle size ) Is 1 μm or less.
[0012]
By setting the average particle size of the lithium iron composite oxide particles to 1 μm or less, the surface area involved in the lithium ion occlusion / desorption reaction in the positive electrode is increased and the reaction is activated. Therefore, it is considered that the insertion / extraction of lithium ions is performed smoothly. As a result, the lithium iron composite oxide for the positive electrode active material of the lithium secondary battery of the present invention has a large active material discharge capacity and a lithium iron composite oxide with little cycle deterioration such that the capacity can be maintained even after cycling. Become.
[0013]
The method for producing the lithium iron composite oxide of the present invention is not particularly limited, but can be more easily produced by the following method. That is, the present invention One preferred embodiment provided by The method for producing lithium iron composite oxide is lithium source And iron source When, Phosphorus source (preferably Phosphorus-containing ammonium salt ) And a raw material mixing step for obtaining a mixture, and a firing step for firing the mixture at a temperature of 600 ° C. to 700 ° C. A crushing step of crushing the fired body obtained by the firing, Comprising A method for producing a lithium iron composite oxide for a positive electrode active material for a lithium secondary battery, wherein the average particle size of particles present almost alone is 1 μm or less. The
[0014]
By performing the firing at a relatively low temperature of 600 ° C. or more and 700 ° C. or less, the growth of particles in the firing is suppressed, By crushing the obtained fired body, the average value of the particle diameters of the particles present substantially alone Can be obtained as small as 1 μm or less.
[0015]
Therefore, the method for producing a lithium iron composite oxide of the present invention is a method for easily producing a lithium iron composite oxide having a large active material discharge capacity and little cycle deterioration.
[0016]
The lithium secondary battery of the present invention is characterized by using the lithium iron composite oxide of the present invention as a positive electrode active material. In addition to being inexpensive, the lithium secondary battery of the present invention can effectively exhibit the action of the lithium iron composite oxide, in which lithium ions are smoothly absorbed and desorbed and the reaction is activated. As a result, the lithium secondary battery has a large discharge capacity and excellent cycle characteristics with little decrease in capacity even after repeated cycles.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, preferred embodiments of the lithium iron composite oxide for a positive electrode active material of a lithium secondary battery according to the present invention, a production method thereof, and a lithium secondary battery using the same will be described in detail.
[0018]
<Lithium iron composite oxide>
The lithium iron composite oxide for a positive electrode active material of a lithium secondary battery of the present invention has a basic composition of LiFePO Four The lithium iron composite oxide having an average particle size of 1 μm or less.
[0019]
The “basic composition is assumed to be” means not only the composition represented by the composition formula but also, for example, a part of the Fe site in the crystal structure with other elements such as Co, Ni, Mn, and Mg. It is meant to include substituted ones. Furthermore, it means that not only a stoichiometric composition but also a non-stoichiometric composition in which some elements are deficient or the like is included.
[0020]
The lithium iron composite oxide of the present invention has an orthorhombic olivine structure in its crystal structure, and its space group is represented by Pmnb. In other words, the olivine structure is based on the hexagonal close-packed filling of oxygen, in which phosphorus is located at the tetrahedral site and both lithium and iron are located at the octahedral site.
[0021]
The lithium iron composite oxide of the present invention has an average particle size of 1 μm or less. When the average particle diameter exceeds 1 μm, the surface area involved in the lithium ion occlusion / desorption reaction of the lithium iron composite oxide particles in the positive electrode is reduced, and the lithium ion occlusion / desorption reaction is not smoothly performed. This is because sufficient active material discharge capacity cannot be obtained when charging / discharging at a practical charge / discharge density. In particular, it is desirable that the average particle size be 0.2 μm or more and 0.8 μm or less, considering that the electrode can be easily manufactured and the rate characteristics are good.
[0022]
The lithium iron composite oxide is formed from particles that exist substantially independently, as will be shown later in the photograph. Therefore, the average particle diameter is an average value of the particle diameters of the particles present almost alone, and each particle diameter is measured using, for example, a scanning electron microscope (SEM) photograph of the lithium iron composite oxide. can do. That is, a SEM photograph of lithium iron composite oxide is taken, the longest diameter and the shortest diameter of the lithium iron composite oxide particles in the photograph are measured, and the average value of these two values is adopted as the particle diameter of the one particle. can do.
[0023]
<Method for producing lithium iron composite oxide>
The method for producing the lithium iron composite oxide of the present invention is not particularly limited, but can be more easily produced by the following method. That is, the method for producing a lithium iron composite oxide of the present invention is a method comprising a raw material mixing step of mixing raw materials to obtain a mixture, and a firing step of firing the mixture at a predetermined temperature.
[0024]
(1) Raw material mixing process
The raw material mixing step in the method for producing a lithium iron composite oxide of the present invention is a step of obtaining a mixture by mixing a lithium compound, an iron compound, and a phosphorus-containing ammonium salt.
[0025]
Lithium compounds that serve as lithium sources include Li 2 CO Three , Li (OH), Li (OH) · H 2 O, LiNO Three Etc. can be used. In particular, because of its low hygroscopicity, Li 2 CO Three It is desirable to use
[0026]
As an iron compound as an iron source, a compound in which the valence of iron is divalent is FeC. 2 O Four ・ 2H 2 O, FeCl 2 Etc. can be used. In particular, FeC because of the low corrosiveness of the gas generated during firing. 2 O Four ・ 2H 2 It is desirable to use O.
[0027]
Examples of phosphorus-containing ammonium salts that serve as phosphorus sources include NH. Four H 2 PO Four , (NH Four ) 2 HPO Four , P 2 O Five Etc. can be used. In particular, NH is relatively low in hygroscopicity and has a low generation amount of corrosive gas. Four H 2 PO Four It is desirable to use
[0028]
Any of the above-mentioned raw materials may be used in the form of powder, and mixing thereof may be performed by a method used for normal powder mixing. Specifically, it may be mixed using, for example, a ball mill, a mixer, a mortar or the like. In addition, what is necessary is just to let the mixing ratio of each raw material be a ratio according to the composition of the lithium iron complex oxide to be manufactured.
[0029]
Further, in order to obtain a lithium iron composite oxide having an average particle size of 1 μm or less, it is desirable to control the average particle size of the above raw materials, and in particular, each raw material should have an average particle size of 1 μm or less. Is desirable.
[0030]
(2) Firing process
The firing step is a step of firing the mixture obtained in the raw material mixing step at a temperature of 600 ° C. or higher and 700 ° C. or lower. Firing may be performed in an inert atmosphere or a reducing atmosphere, specifically, for example, in an argon stream or a nitrogen stream to prevent iron from being oxidized to trivalent.
[0031]
The firing temperature is 600 ° C. or higher and 700 ° C. or lower. When the firing temperature is less than 600 ° C., the reaction does not proceed sufficiently, a subphase other than the intended orthorhombic crystal is generated, and the crystallinity of the lithium iron composite oxide is deteriorated. On the other hand, when the temperature exceeds 700 ° C., the lithium iron composite oxide particles grow and the particle size becomes large, so that a lithium iron composite oxide having an average particle size of 1 μm or less cannot be obtained. In particular, considering the improvement of rate characteristics, it is desirable that the temperature be 620 ° C. or higher and 670 ° C. or lower. Note that the firing time may be a time sufficient to complete the firing, and is usually performed for about 12 hours.
[0032]
<Lithium secondary battery>
The lithium secondary battery of the present invention is a lithium secondary battery using the lithium iron composite oxide of the present invention as a positive electrode active material, and other configurations excluding the positive electrode active material are not particularly limited, The configuration of a known lithium secondary battery may be followed. Further, the lithium iron composite oxide of the present invention includes various lithium iron composite oxides depending on the composition, particle diameter, and the like. In the lithium secondary battery of the present invention, one of them may be used as a positive electrode active material, or a mixture of two or more may be used. Furthermore, the lithium iron composite oxide of the present invention and an already known positive electrode active material can be mixed to form a positive electrode active material.
[0033]
Generally, a lithium secondary battery includes a positive electrode and a negative electrode that occlude and release lithium ions, a separator sandwiched between the positive electrode and the negative electrode, a non-aqueous electrolyte that moves lithium ions between the positive electrode and the negative electrode, and the like. Can be configured as main components.
[0034]
A positive electrode is a mixture of a conductive material and a binder mixed with a powder of the lithium iron composite oxide of the present invention, which becomes a positive electrode active material, and an appropriate solvent is added to form a paste-like positive electrode mixture. It can be formed by applying and drying on the surface of a current collector made of a metal foil, etc., and compressing it to increase the electrode density as necessary. The conductive material is for ensuring the electrical conductivity of the positive electrode, and a mixture of one or two or more carbon material powders such as carbon black, acetylene black, and graphite can be used. The binder plays a role of connecting the active material particles and the conductive material particles, and a fluororesin such as polytetrafluoroethylene, polyvinylidene fluoride, and fluororubber, and a thermoplastic resin such as polypropylene and polyethylene can be used. . An organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent for dispersing these active material, conductive material, and binder.
[0035]
The negative electrode can be formed by forming metal lithium, which is a negative electrode active material, into a sheet shape or by pressing a sheet into a current collector network such as nickel or stainless steel. Instead of metallic lithium, a lithium alloy or a lithium compound can also be used for the negative electrode active material.
[0036]
As another form of the negative electrode, the negative electrode can be configured using a carbon material capable of inserting and extracting lithium ions in the negative electrode active material. Examples of the carbon material that can be used include natural or artificial graphite, a fired organic compound such as phenol resin, and a powder such as coke. In this case, the negative electrode mixture prepared by mixing the negative electrode active material with a binder and adding an appropriate solvent to the paste is applied to the surface of a metal foil current collector such as copper, dried, and then pressed. Can be formed. Application, drying, pressing, and the like in this case may follow a normal method. When a carbon material is used as the negative electrode active material, a fluorine-containing resin such as polyvinylidene fluoride can be used as the negative electrode binder, and an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent.
[0037]
The separator sandwiched between the positive electrode and the negative electrode separates the positive electrode and the negative electrode and holds the electrolytic solution, and a thin microporous film such as polyethylene or polypropylene can be used.
[0038]
The nonaqueous electrolytic solution is obtained by dissolving a lithium salt as an electrolyte in an organic solvent. The lithium salt is dissociated by dissolving in an organic solvent, and becomes lithium ions and exists in the electrolytic solution. LiBF that can be used is LiBF Four , LiPF 6 LiClO Four , LiCF Three SO Three , LiAsF 6 , LiN (CF Three SO 2 ) 2 , LiN (C 2 F Five SO 2 ) 2 Etc. These lithium salts may be used alone, or two or more of these may be used in combination.
[0039]
As the organic solvent for dissolving the lithium salt, an aprotic organic solvent is used. For example, a mixed solvent composed of one or more of cyclic carbonate, chain carbonate, cyclic ester, cyclic ether, chain ether and the like can be used. Examples of cyclic carbonates are ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, etc., examples of chain carbonates are dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, etc., examples of cyclic esters are gamma butyrolactone, gamma valero. Examples of lactones include tetrahydrofuran, 2-methyltetrahydrofuran and the like as examples of cyclic ethers, and examples of chain ethers include dimethoxyethane and ethylene glycol dimethyl ether. Any one of these can be used alone, or two or more can be mixed and used.
[0040]
Instead of the separator and the non-aqueous electrolyte, a high molecular weight polymer such as polyethylene oxide and LiClO are used. Four And LiN (CF Three SO 2 ) 2 It is also possible to use a solid polymer electrolyte using a lithium salt such as a gel electrolyte, or a gel electrolyte obtained by trapping the non-aqueous electrolyte in a solid polymer matrix such as polyacrylonitrile.
[0041]
The lithium secondary battery of the present invention constituted by the above-described constituent elements can be various shapes such as a cylindrical type, a laminated type, and a coin type. Regardless of the shape, the above components are housed in a battery case, and a current collecting lead or the like is used between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal. The battery case is sealed and the battery system is separated from the outside to complete the lithium secondary battery.
[0042]
<Other forms>
As mentioned above, although the lithium iron composite oxide for lithium secondary battery positive electrode active material of the present invention, the manufacturing method thereof, and the embodiment of the lithium secondary battery using the same have been described, the above-described embodiment is only one embodiment. First, the lithium iron composite oxide for a positive electrode active material of a lithium secondary battery according to the present invention, a method for producing the same, and a lithium secondary battery using the same are variously based on the knowledge of those skilled in the art including the above-described embodiments. It can be implemented in various forms with changes and improvements.
[0043]
【Example】
Based on the said embodiment, the lithium iron complex oxide of this invention whose average particle diameter is 1 micrometer or less was manufactured as an Example. Moreover, in order to compare with this, the lithium iron complex oxide whose average particle diameter exceeds 1 micrometer was manufactured as a comparative example. Then, lithium secondary batteries using the lithium iron composite oxides of Examples and Comparative Examples as positive electrode active materials were produced respectively, and by comparing their active material discharge capacities, the lithium iron composite oxide of the present invention was excellent. The sex was confirmed. Hereinafter, these will be described.
[0044]
<Lithium iron composite oxide of Example>
Li as the lithium source 2 CO Three As an iron source 2 O Four ・ 2H 2 NH as a phosphorus source Four H 2 PO Four Were mixed so that the molar ratio of Li, Fe, and P was 1: 1: 1. An automatic mortar was used for mixing. This mixture was baked in an argon stream at 620 ° C. for 12 hours. And the obtained lithium iron complex oxide was crushed and it was set as the powdery lithium iron complex oxide used as a positive electrode active material. This lithium iron composite oxide was used as the lithium iron composite oxide of the examples.
[0045]
An X-ray diffraction pattern of the lithium iron composite oxide of this example is shown in FIG. From the pattern of FIG. 1, it was confirmed that the lithium iron composite oxide of this example was a lithium iron composite oxide whose crystal structure was an orthorhombic olivine structure. From the composition analysis, the composition of the lithium iron composite oxide of this example is Four It was found that
[0046]
Moreover, the photograph which image | photographed the lithium iron complex oxide of a present Example with the scanning electron microscope (SEM) in FIG. 2 is shown. From the photograph of FIG. 2, it was confirmed that the lithium iron composite oxide of this example was composed of substantially single particles, and the average particle size of the particles was about 0.8 μm.
[0047]
<Lithium iron composite oxide of comparative example>
In the production of the lithium iron composite oxide of the above example, it was produced in the same manner as the lithium iron composite oxide of the example except that the firing temperature was changed to 870 ° C. The obtained lithium iron composite oxide was used as a comparative example of lithium iron composite oxide.
[0048]
An X-ray diffraction pattern of the lithium iron composite oxide of this comparative example is shown in FIG. From the pattern of FIG. 3, it was confirmed that the lithium iron composite oxide of this comparative example was a lithium iron composite oxide whose crystal structure was an orthorhombic olivine structure. From the compositional analysis, the composition of the lithium iron composite oxide of this comparative example is the composition formula LiFePO 4. Four It was found that
[0049]
Moreover, the photograph which image | photographed the lithium iron complex oxide of this comparative example with the scanning electron microscope (SEM) in FIG. 4 is shown. From the photograph of FIG. 4, it was confirmed that the lithium iron composite oxide of this comparative example was composed of substantially single particles, and the average particle size of the particles was about 5 μm.
[0050]
<Lithium secondary battery>
A lithium secondary battery was fabricated using the lithium iron composite oxides of the above examples and comparative examples as the positive electrode active material. In the positive electrode, first, 70 parts by weight of each lithium iron composite oxide serving as a positive electrode active material is mixed with 25 parts by weight of carbon black as a conductive material and 5 parts by weight of polyvinylidene fluoride as a binder. An appropriate amount of N-methyl-2-pyrrolidone was added to prepare a paste-like positive electrode mixture. Next, the paste-like positive electrode mixture was applied to an aluminum foil current collector and pressed to make the thickness of the positive electrode mixture 45 μm, and then punched into a disk shape having a diameter of 15 mmφ to obtain a positive electrode.
[0051]
The negative electrode made to oppose used metallic lithium as an active material. Metal lithium was formed into a sheet shape and pressed onto a nickel current collector network to a thickness of 100 μm, and then punched into a disk shape having a diameter of 17 mmφ to form a negative electrode.
[0052]
The separator uses a polyethylene microporous membrane. The non-aqueous electrolyte impregnated in the separator is LiPF in a mixed solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 3: 7. 6 Was dissolved at a concentration of 1M. The positive electrode and the negative electrode were opposed to each other through a separator, and an appropriate amount of the non-aqueous electrolyte was injected and impregnated, and then stored in a coin-type battery case to produce a lithium secondary battery.
[0053]
The lithium secondary battery using the lithium iron composite oxide of the example as the positive electrode active material was used as the lithium secondary battery of the example, and similarly, the lithium secondary battery using the lithium iron composite oxide of the comparative example as the positive electrode active material was used. The secondary battery was a lithium secondary battery of a comparative example.
[0054]
<Charge / discharge cycle test>
A charge / discharge cycle test was performed on each of the lithium secondary batteries of the above Examples and Comparative Examples. The charge / discharge test was performed under a temperature condition of 20 ° C., changing the current density every 5 cycles, for a total of 30 cycles. The conditions of 1 to 6 charge / discharge tests will be described below with 5 cycles having the same current density as one charge / discharge test.
[0055]
In the first charge / discharge test, the current density was 0.25 mA / cm. 2 The battery is charged at a constant current of up to 4.0 V, and then the current density is 0.25 mA / cm. 2 Charging / discharging for discharging to a discharge lower limit voltage of 3.0 V with a constant current of 1 cycle was defined as 1 cycle, and this cycle was repeated 5 cycles. And the discharge capacity was measured about each lithium secondary battery for every cycle.
[0056]
Hereinafter, the second and sixth charge / discharge tests were performed in the same manner as the first test, except that the current density during charge / discharge in the first test was changed. Similarly, the discharge capacity of each lithium secondary battery was measured for each cycle. That is, in the second test, 0.25 mA / cm. 2 Charge at 0.5 mA / cm 2 In the third test, 0.5 mA / cm 2 Charge at 0.5 mA / cm 2 In the fourth test, 0.5 mA / cm 2 Charge at 1.0 mA / cm 2 In the fifth test, 1.0 mA / cm 2 Charge at 1.0 mA / cm 2 The sixth test was the same as the first test, 0.25 mA / cm 2 Charged at 0.25 mA / cm 2 Was discharged.
[0057]
Next, for the lithium secondary battery of the example, the above six charge / discharge tests were performed by changing only the temperature conditions. That is, under the high temperature environment of 60 ° C., which is regarded as the upper limit of the actual use temperature range of the lithium secondary battery, the other conditions are the same as in the case of 20 ° C., and the charge / discharge test of 30 times in total is performed 6 times. Went. And discharge capacity was measured for every cycle.
[0058]
<Evaluation of lithium secondary battery>
About each lithium secondary battery of an Example and a comparative example, the discharge capacity per 1g of positive electrode active materials, ie, active material discharge capacity, was calculated | required from the discharge capacity measured by the said charging / discharging test, and the result was shown in FIG. Shown in Here, FIG. 5 shows the active material discharge capacity for each cycle of the lithium secondary battery of the example. The ♦ marks indicate those at 20 ° C., and the ◯ marks indicate those at 60 ° C. Moreover, FIG. 6 shows the active material discharge capacity | capacitance for every cycle of the lithium secondary battery of the comparative example in 20 degreeC.
[0059]
As is clear from FIG. 5, the lithium secondary battery of the example has a current density of 0.25 mA / cm at 20 ° C. at the beginning of the cycle. 2 The active material discharge capacity when discharged at 105 mAh / g was 105 mAh / g, and a large active material discharge capacity exceeding 100 mAh / g was obtained. This value is about three times that of the lithium secondary battery of the comparative example shown in FIG. In addition, the active material discharge capacity value at 60 ° C. at the same current density was about 130 mAh / g, and a larger active material discharge capacity was obtained. It is considered that ions are absorbed and desorbed smoothly. Therefore, it was confirmed that a lithium secondary battery using a lithium iron composite oxide having an average particle size of 1 μm or less as a positive electrode active material has a large active material discharge capacity.
[0060]
In the lithium secondary battery of the example, the charge / discharge current density increases and the active material discharge capacity gradually decreases. This is also the case with the lithium secondary battery of the comparative example, but it is considered that when charging / discharging at a large current density, a large load is applied to the active material, and the internal resistance increases.
[0061]
However, in FIG. 5, the active material discharge capacity in the sixth charge / discharge test (26 cycles to 30 cycles) in the same manner as the first current density is the first charge / discharge test (1 cycle to 5 cycles). The value is almost equal to the active material discharge capacity at. That is, the large active material discharge capacity is maintained even when the cycle is repeated. This is not limited to the test at 20 ° C., and the same applies to the test at 60 ° C. Therefore, a lithium secondary battery using a lithium iron composite oxide having an average particle size of 1 μm or less as a positive electrode active material has little capacity reduction even when the cycle is repeated, and is particularly excellent in cycle characteristics even at high temperatures. It was confirmed that it would be a battery.
[0062]
【Effect of the invention】
Since the average particle size of the lithium iron composite oxide of the present invention is as small as 1 μm or less, the surface area involved in the lithium ion occlusion / desorption reaction in the positive electrode increases, and the lithium ion occlusion / desorption occurs. Therefore, the lithium iron composite oxide has a large active material discharge capacity and little cycle deterioration.
[0063]
Moreover, according to the manufacturing method of the lithium iron complex oxide of this invention, the said lithium iron complex oxide can be manufactured simply, and also by using the lithium iron complex oxide of this invention as a positive electrode active material. Therefore, the lithium secondary battery is inexpensive, has a large discharge capacity, and has excellent cycle characteristics.
[Brief description of the drawings]
FIG. 1 is an X-ray diffraction pattern of a lithium iron composite oxide of an example fired at 620 ° C. and having an average particle diameter of 1 μm or less.
FIG. 2 is a SEM photograph of lithium iron composite oxide of an example.
FIG. 3 is an X-ray diffraction pattern of a lithium iron composite oxide of a comparative example that is fired at 870 ° C. and whose average particle size exceeds 1 μm.
FIG. 4 is an SEM photograph of a lithium iron composite oxide of a comparative example.
FIG. 5 is a graph showing the active material discharge capacity for each cycle of the lithium secondary battery of the example.
FIG. 6 is a graph showing an active material discharge capacity for each cycle of a lithium secondary battery of a comparative example.
Claims (5)
リチウム源と鉄源とリン源とを混合して混合物を得る原料混合工程と、A raw material mixing step of mixing a lithium source, an iron source and a phosphorus source to obtain a mixture;
該混合物を６００℃以上７００℃以下の温度で焼成する焼成工程と、A firing step of firing the mixture at a temperature of 600 ° C. or higher and 700 ° C. or lower;
該焼成により得られた焼成体を解砕する解砕工程と、A crushing step of crushing the fired body obtained by the firing;
を含んでなる、略単独に存在する粒子の粒径の平均値が０．２μｍ以上０．８μｍ以下であることを特徴とするリチウム二次電池正極活物質用リチウム鉄複合酸化物を製造するための方法。In order to produce a lithium iron composite oxide for a positive electrode active material for a lithium secondary battery, characterized in that the average particle size of particles present substantially independently is 0.2 μm or more and 0.8 μm or less the method of.
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| JP4491946B2 (en) * | 2000-09-29 | 2010-06-30 | ソニー株式会社 | Method for producing positive electrode active material and method for producing non-aqueous electrolyte battery |
| JP4491947B2 (en) * | 2000-10-04 | 2010-06-30 | ソニー株式会社 | Method for producing positive electrode active material and method for producing non-aqueous electrolyte battery |
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| CA2411695A1 (en) | 2002-11-13 | 2004-05-13 | Hydro-Quebec | Electrode covered with a film obtained from an aqueous solution containing a water soluble binder, manufacturing process and usesthereof |
| ITRM20030048A1 (en) * | 2003-02-06 | 2004-08-07 | Cnr Consiglio Naz Delle Ricer Che | SYNTHESIS PROCEDURE OF A CATHODIC MATERIAL BASED ON METALLIC LITHIUM PHOSPHATE, CONTAINING INTRINSICALLY CARBON. |
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| JP3441107B2 (en) * | 1992-05-18 | 2003-08-25 | 三菱電線工業株式会社 | Lithium secondary battery |
| JP3218170B2 (en) * | 1995-09-06 | 2001-10-15 | キヤノン株式会社 | Lithium secondary battery and method of manufacturing lithium secondary battery |
| JP3523397B2 (en) * | 1995-11-07 | 2004-04-26 | 日本電信電話株式会社 | Non-aqueous electrolyte secondary battery |
| JP3484003B2 (en) * | 1995-11-07 | 2004-01-06 | 日本電信電話株式会社 | Non-aqueous electrolyte secondary battery |
| US5910382A (en) * | 1996-04-23 | 1999-06-08 | Board Of Regents, University Of Texas Systems | Cathode materials for secondary (rechargeable) lithium batteries |
| JP3906944B2 (en) * | 1997-05-12 | 2007-04-18 | 株式会社ジーエス・ユアサコーポレーション | Non-aqueous solid electrolyte battery |
| JPH11273735A (en) * | 1998-03-19 | 1999-10-08 | Yuasa Corp | Lithium-ion polymer secondary battery |
| JP4749551B2 (en) * | 1999-04-06 | 2011-08-17 | ソニー株式会社 | Positive electrode active material and non-aqueous electrolyte secondary battery |
| JP4949543B2 (en) * | 1999-04-06 | 2012-06-13 | ソニー株式会社 | Method for synthesizing LiFePO4 and method for producing nonaqueous electrolyte battery |
| JP4547748B2 (en) * | 1999-10-29 | 2010-09-22 | パナソニック株式会社 | Non-aqueous electrolyte battery |
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