JP4716072B2 - Lithium vanadium composite oxide for negative electrode active material of water based lithium secondary battery, method for producing the same, and lithium secondary battery using the same - Google Patents

Description

【００３３】
なかでも、六方晶系の層状岩塩構造を有するリチウムマンガン複合酸化物を用いることが望ましい。六方晶系の層状岩塩構造を有するリチウムマンガン複合酸化物は、充放電を繰り返しても、取り出せる容量の小さいスピネル構造への転移が生じないため、これを正極活物質として用いる態様の本発明のリチウム二次電池は、より大きな放電容量をより安定的に維持できる水系リチウム二次電池となる。
【００３４】
この六方晶系の層状岩塩構造を有するリチウムマンガン複合酸化物は、その製造方法が特に限定されるものではなく、いわゆる固相反応法によって焼成することができる。また、例えば、マンガン源となる二酸化マンガンと、リチウム源となる水酸化リチウム水溶液とを、Ｌｉ／Ｍｎがモル比で２以上１０以下となるような割合で混合して分散水溶液を得る分散水溶液調製工程と、該分散水溶液を１２０℃以上２５０℃以下の温度で加熱保持する水熱処理工程とを含んで構成される製造方法によって、製造することもできる。この製造方法は、一段階の水熱処理により目的とするリチウム複合酸化物を合成することが可能であるため、簡便、かつ工業的にも有利な製造方法となる。
【００３５】
また、オリビン構造のリチウム鉄複合リン酸化物も、その製造方法を特に限定するものではない。例えば、リチウム源となるＬｉＯＨ・Ｈ₂Ｏと、鉄源となるＦｅＣ₂Ｏ₄・２Ｈ₂Ｏと、リン源となる（ＮＨ₄）₂ＨＰＯ₄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を含んで構成される製造方法によって製造することができる。
【００３６】
（３）負極および正極
負極および正極は、ともに、粉末状のそれぞれの活物質に導電材および結着剤を混合し、それぞれ負極および正極合材としたものを、それぞれ金属製の集電体表面に圧着、または塗布乾燥して形成することができる。
【００３７】
導電材は、電極の電気伝導性を確保するためのものであり、例えば、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。また、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、例えば、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。
【００３８】
（４）水系電解液
本発明のリチウム二次電池で使用する電解液は、電解質としてのリチウム塩を水に溶解した水系電解液である。リチウム塩は水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。一般に、酸化物系の活物質材料は、中性からアルカリ性の水溶液中でより安定に存在する。また、リチウムイオンの吸蔵・脱離反応をより活性化させることをも考慮する場合には、使用する電解液は中性からアルカリ性であることが望ましい。なお、ここで中性とは、ｐＨの値でいえばｐＨ＝６〜８程度のことを意味する。
【００３９】
例えば、ｐＨ＝７の中性電解液を用いた場合、水の電気分解による水素発生電位は２．６２Ｖ、酸素発生電位は３．８５Ｖ（vs.Ｌｉ／Ｌｉ⁺）であり、ｐＨ＝１４のアルカリ性電解液を用いた場合は、水素発生電位は２．２１Ｖ、酸素発生電位は３．４４Ｖ（vs.Ｌｉ／Ｌｉ⁺）である。すなわち、中性の電解液を用いた場合には、水の電気分解による酸素発生電位が高いことから、上述したように、正極活物質がより多くのリチウムイオンを吸蔵・脱離可能となり、より大きな容量を取り出すことができる。したがって、より容量の大きな二次電池とする場合には、中性に近い電解液、具体的には、ｐＨ＝６〜１０である電解液を用いることが望ましい。
【００４０】
また一般に、水溶液は、非水溶液と比べて導電性が良好であり、例えば中性の水溶液は非水溶液の１０倍以上の導電率を有し、アルカリ性の水溶液は、非水溶液の１００倍以上の導電率を有する。そのため、電解液に水溶液を用いた二次電池は、非水系の二次電池と比較して、内部抵抗、特に反応抵抗が小さいものとなり、アルカリ性の電解液を用いた場合には、内部抵抗はより小さいものとなる。したがって、より出力特性やレート特性の良好な二次電池とする場合には、強アルカリ性の電解液、具体的には、ｐＨ＝１０〜１２である電解液を用いることが望ましい。
【００４１】
電解質として使用できるリチウム塩は、水に溶解するものであれば特に限定されるものではないが、正極活物質である酸化物の安定性等を考慮すると、溶解後、電解液が中性からアルカリ性となるようなリチウム塩を用いることが望ましい。具体的には、例えば、硝酸リチウム、水酸化リチウム、ヨウ化リチウム等を用いることが望ましい。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。特に、溶解度が高く、従って導電性も良いという理由から、中性の電解液とするためには硝酸リチウムを用いることが望ましく、また、強アルカリ性の電解液とするためには、硝酸リチウムと水酸化リチウムとを混合して用いることが望ましい。なお、電解液中のリチウム塩の濃度は、電解液の電気伝導度を高くして二次電池の内部抵抗を小さくできるという理由から、飽和濃度、あるいはそれに近い濃度とすることが望ましい。
【００４２】
（５）その他の構成要素等
本発明のリチウム二次電池では、上記正極と上記負極とを対向させることにより電極体を形成させる。正極と負極との間にはセパレータを挟装することが望ましい。このセパレータは、正極と負極とを分離し電解液を保持するものであり、セルロース系等のものを用いることができる。
【００４３】
本発明のリチウム二次電池は、その形状を特に限定するものではなく、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、電池形状に応じて形成させた上記電極体を、所定の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、この電極体に上記電解液を含浸させ電池ケースに密閉し、リチウム二次電池を完成することができる。
【００４４】
〈他の実施形態の許容〉
以上、本発明のリチウムバナジウム複合酸化物、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、本発明のリチウムバナジウム複合酸化物、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。
【００４５】
【実施例】
本発明のリチウム二次電池において負極活物質として使用されるリチウムバナジウム複合酸化物を実際に合成し、負極活物質としての特性を調査した。次いで、リチウムバナジウム複合酸化物を負極活物質として用いた本発明のリチウム二次電池を作製し、電池特性を評価した。また、比較のため、従来から検討されていたＬｉＶ₃Ｏ₈等を負極活物質として用いた水系リチウム二次電池を作製し、本発明のリチウム二次電池が優秀であることを確認した。
【００４６】
〈リチウムバナジウム複合酸化物の合成〉
（１）実施例のリチウムバナジウム複合酸化物
組成式ＬｉＶ₂Ｏ₄で表されるスピネル構造のリチウムバナジウム複合酸化物を合成した。まず、炭酸リチウム（Ｌｉ₂ＣＯ₃）と五酸化バナジウム（Ｖ₂Ｏ₅）とを、Ｌｉ：Ｖがモル比で１：２となるような割合で混合した。混合は自動乳鉢にて２０分間行った。次いで、その混合物１００重量部に対し、ケッチェンブラックを２重量部混合して原料混合物とした。混合は、自動乳鉢にて２０分間行った。原料混合物を、アルゴン気流中、７５０℃で２４時間焼成し、急冷してリチウムバナジウム複合酸化物を得た。なお、得られたリチウムバナジウム複合酸化物は、負極活物質として用いるため、乳鉢にて粉砕して粉末状のものとした。以下、本リチウムバナジウム複合酸化物を、便宜的に実施例のリチウムバナジウム複合酸化物ということにする。
【００４７】
（２）比較例のリチウムバナジウム複合酸化物
組成式ＬｉＶ₃Ｏ₈で表されるリチウムバナジウム複合酸化物を合成した。炭酸リチウム（Ｌｉ₂ＣＯ₃）と五酸化バナジウム（Ｖ₂Ｏ₅）とを、Ｌｉ：Ｖがモル比で１：３となるような割合で混合した。混合は自動乳鉢にて２０分間行った。次いで、その混合物を、アルゴン雰囲気にて７００℃まで昇温し、その温度で１２時間保持した後炉冷して、リチウムバナジウム複合酸化物を得た。なお、得られたリチウムバナジウム複合酸化物は、上記同様に、乳鉢にて粉砕して粉末状のものとした。以下、本リチウムバナジウム複合酸化物を、便宜的に比較例のリチウムバナジウム複合酸化物ということにする。
【００４８】
〈負極活物質としての特性調査〉
実施例のリチウムバナジウム複合酸化物を活物質とした電極を作製し、サイクリックボルタモグラムによる単極評価試験を行うことにより負極活物質としての特性を調査した。まず、７０重量部のリチウムバナジウム複合酸化物に、導電材としての２５重量部のカーボンおよび結着剤としての５重量部のポリテトラフルオロエチレンを混合して電極合材を得た。次いで、この電極合材の１０ｍｇを、ステンレス製のメッシュ上に約０．６ｔｏｎ／ｃｍ²の圧力で圧着して電極とした。
【００４９】
そして、参照極を銀塩化銀電極とし、また、対極を白金ワイヤー（φ０．３ｍｍ×５ｍｍ；コイル状）として、３極式のビーカーセルを用い、走査速度を２ｍＶ／ｓとして試験を行った。この試験により、電流と電位との関係を示すＣＶ曲線を得た。得られたＣＶ曲線を図２に示す。
【００５０】
図２から、実施例のリチウムバナジウム複合酸化物は、酸化還元電位を１つしか持たないいわゆる２相共存型のリチウム吸蔵・脱離挙動を示すことがわかる。すなわち、充放電の際、リチウムの吸蔵・脱離に伴い格子定数等が連続に変化し電位が変化していくＬｉＮｉＯ₂やＬｉＣｏＯ₂とは異なり、実施例のリチウムバナジウム複合酸化物は、充放電の際には一定の電位を示す。そして、その充放電の電位は２．４Ｖ付近（vs.Ｌｉ／Ｌｉ⁺）であり、この電位は水の電気分解による水素発生が生じない電位範囲内である。したがって、実施例のリチウムバナジウム複合酸化物は、水の電気分解による水素発生が生じない電位範囲において、可逆的に大量のリチウムイオンの吸蔵・脱離が可能であり、水系リチウム二次電池における負極活物質として好適であることが確認できた。
【００５１】
また、正極活物質の一つとして用いられる組成式ＬｉＦｅＰＯ₄で表されるオリビン構造のリチウム鉄複合リン酸化物についても、同様に単極評価試験を行った。その結果を、実施例のリチウムバナジウム複合酸化物の結果と併せて図３に示す。図３より、リチウム鉄複合リン酸化物における充放電の電位は、３．５Ｖ付近（vs.Ｌｉ／Ｌｉ⁺）であり、この電位は水の電気分解による酸素発生が生じない電位範囲内である。つまり、正極活物質として組成式ＬｉＦｅＰＯ₄で表されるリチウム鉄複合リン酸化物を用い、負極活物質として実施例のリチウムバナジウム複合酸化物を用いることにより、約１Ｖ級の水系リチウム二次電池が構成できることが確認できた。
【００５２】
〈リチウム二次電池の作製〉
（１）負極の作製
上記実施例および比較例のリチウムバナジウム複合酸化物をそれぞれ負極活物質に用いた負極を作製した。まず、７０重量部のリチウムバナジウム複合酸化物に、導電材としての２５重量部のカーボンおよび結着剤としての５重量部のポリテトラフルオロエチレンを混合して負極合材を得た。次いで、各負極合材の１０ｍｇを、予めコインセルの内側に溶接したステンレス製のメッシュ上に約０．６ｔｏｎ／ｃｍ²の圧力で圧着して負極とした。
【００５３】
（２）正極の作製
正極活物質には、組成式ＬｉＭｎＯ₂で表される層状岩塩構造のリチウムマンガン複合酸化物、組成式ＬｉＦｅＰＯ₄で表されるオリビン構造のリチウム鉄複合リン酸化物、組成式ＬｉＮｉ_0.8Ｃｏ_0.15Ａｌ_0.05Ｏ₂で表される層状岩塩構造のリチウムニッケル複合酸化物をそれぞれ用いた。各複合酸化物の合成方法を以下に述べる。
【００５４】
（ａ）リチウムマンガン複合酸化物の合成
６．２９ｇのＬｉＯＨ・Ｈ₂Ｏを８０ｍｌの水に溶解して、ＬｉＯＨ水溶液を調製した。このＬｉＯＨ水溶液に、ＭｎＯ₂を２．６１ｇ添加し（Ｌｉ／Ｍｎはモル比で５となる）、３０分間超音波分散して分散水溶液を調製した。次いで、この分散水溶液をオートクレーブに入れ、２００℃の温度で７日間反応させた。反応後、オートクレーブを冷却し、容器内の沈殿物を濾過、水洗、１２０℃で乾燥して、リチウムマンガン複合酸化物を得た。なお、得られたリチウムマンガン複合酸化物は、乳鉢にて粉砕して粉末状のものとした。
【００５５】
（ｂ）リチウム鉄複合リン酸化物の合成
ＬｉＯＨ・Ｈ₂Ｏと、ＦｅＣ₂Ｏ₄・２Ｈ₂Ｏと、（ＮＨ₄）₂ＨＰＯ₄とをそれぞれＬｉ：Ｆｅ：Ｐがモル比で１：１：１となるように混合した。混合は、自動乳鉢にて３０分間行った。この混合物を、アルゴン気流中で、３５０℃で５時間仮焼した後、自動乳鉢にて３０分間混合した。その後さらに、アルゴン気流中で、６５０℃で６時間焼成し、リチウム鉄複合リン酸化物を得た。なお、得られたリチウム鉄複合リン酸化物は、乳鉢にて粉砕して粉末状のものとした。
【００５６】
（ｃ）リチウムニッケル複合酸化物の合成
２Ｍの硝酸ニッケル、硝酸コバルト、硝酸アルミニウムの各水溶液を、Ｎｉ：Ｃｏ：Ａｌがモル比で８：１．５：０．５となるように混合し、５００ｍＬの水溶液とした。この水溶液を４Ｍの水酸化ナトリウム水溶液に滴下し、ニッケル、コバルト、アルミニウムを含む水酸化物粒子を析出合成した。そして、析出した水酸化物粒子を濾別、洗浄した後、水に投入して水酸化物スラリーを得た。水酸化物スラリー中のＮｉ、Ｃｏ、およびＡｌの総量（Ｎｉ＋Ｃｏ＋Ａｌ）に対して、Ｌｉの原子比がＬｉ／（Ｎｉ＋Ｃｏ＋Ａｌ）＝１となるように、３Ｍの水酸化リチウム水溶液を上記スラリーに添加して懸濁液を調製し、この懸濁液を窒素雰囲気下で噴霧乾燥した。乾燥後、得られた複合酸化物前駆体を、窒素雰囲気下、３５０℃で１時間予備焼成を行い、さらに、酸素雰囲気下、７５０℃で１２時間焼成を行ってリチウムニッケル複合酸化物を得た。
【００５７】
上記（ａ）〜（ｃ）で得られた複合酸化物、リン酸化物をそれぞれ正極活物質として正極を作製した。負極の場合と同様、７０重量部の上記複合酸化物等に、導電材としてのカーボンを２５重量部、結着剤としてのポリテトラフルオロエチレンを５重量部混合して正極合材を得た。次いで、この正極合材の１０ｍｇをあらかじめコインセルの内側に溶接したステンレス製のメッシュ上に、約０．６ｔｏｎ／ｃｍ²で圧着して正極とした。
【００５８】
（３）実施例のリチウム二次電池の作製
（ａ）実施例１のリチウム二次電池
実施例のリチウムバナジウム複合酸化物を負極活物質とした負極、およびリチウム鉄複合リン酸化物を正極活物質とした正極を、その間にセルロース系のセパレータを挟んで対向させ、水系電解液を注入後、コインセルを密封して２０１６型（外径２０ｍｍφ、厚さ１６ｍｍ）リチウム二次電池を完成させた。なお、水系電解液は、リチウム塩であるＬｉＮＯ₃を水に溶解した飽和濃度のＬｉＮＯ₃水溶液であり、そのｐＨ値は７程度である。このリチウム二次電池を、実施例１のリチウム二次電池とした。
【００５９】
（ｂ）実施例２のリチウム二次電池
上記実施例１のリチウム二次電池の作製において、正極を変更し、リチウムマンガン複合酸化物を正極活物質とした正極を用いた以外は、上記実施例１のリチウム二次電池と同様に作製した。このリチウム二次電池を、実施例１のリチウム二次電池とした。
【００６０】
（ｃ）実施例３のリチウム二次電池
上記実施例１のリチウム二次電池の作製において、正極を変更し、リチウムニッケル複合酸化物を正極活物質とした正極を用いた以外は、上記実施例１のリチウム二次電池と同様に作製した。このリチウム二次電池を、比較例３のリチウム二次電池とした。
【００６１】
（４）比較例のリチウム二次電池の作製
（ａ）比較例１のリチウム二次電池
上記実施例１のリチウム二次電池の作製において、負極を変更し、比較例のリチウムバナジウム複合酸化物を負極活物質とした負極を用いた以外は、上記実施例１のリチウム二次電池と同様に作製した。このリチウム二次電池を、比較例１のリチウム二次電池とした。
【００６２】
（ｂ）比較例２のリチウム二次電池
上記実施例２のリチウム二次電池の作製において、負極を変更し、比較例のリチウムバナジウム複合酸化物を負極活物質とした負極を用いた以外は、上記実施例２のリチウム二次電池と同様に作製した。このリチウム二次電池を、比較例２のリチウム二次電池とした。
【００６３】
〈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て充放電サイクル試験を行った。充放電サイクル試験は、６０℃の環境温度下、電流密度０．５ｍＡ／ｃｍ²の定電流で電池電圧１．４Ｖまで充電し、その後、電流密度０．５ｍＡ／ｃｍ²の定電流で電池電圧０．１Ｖまで放電する充放電をを１サイクルとし、このサイクルを５０サイクル繰り返すものとした。なお、充放電サイクルにおける充電休止時間および放電休止時間はそれぞれ１分間とした。
【００６４】
充放電サイクル試験の結果として、測定したそれぞれのリチウム二次電池の正極活物質単位重量あたりの初回放電容量を下記表１に示す。なお、参考例として、負極活物質にＬｉＭｎ₂Ｏ₄を用いて二次電池を構成し、その初回放電容量を測定した結果も併せて示す。
【００６５】
【表１】

【００６６】
表１から明らかなように、負極活物質としてＬｉＶ₂Ｏ₄を用いた実施例１、２の各リチウム二次電池は、負極活物質としてＬｉＶ₃Ｏ₈を用いた比較例１、２の各リチウム二次電池と比較して、初回放電容量が大きいことが判る。また、負極活物質としてＬｉＭｎ₂Ｏ₄を用いた二次電池と比較しても、初回放電容量は極めて大きい。つまり、本発明のリチウムバナジウム複合酸化物を負極活物質に用いた本発明のリチウム二次電池は、大容量の水系リチウム二次電池であることが確認できた。なお、実施例３のリチウム二次電池は、初回放電容量は小さいものとなった。これは、正極活物質にリチウムニッケル複合酸化物を用いたためであり、上記電圧範囲内ではあまり容量を取り出せなかったと考えられる。したがって、本発明のリチウム二次電池に用いる正極活物質としては、組成式ＬｉＭｎＯ₂で表される層状岩塩構造のリチウムマンガン複合酸化物、組成式ＬｉＦｅＰＯ₄で表されるオリビン構造のリチウム鉄複合リン酸化物が好適であるといえる。
【００６７】
また、上記充放電サイクル試験のもう一つの結果として、図４に、実施例および比較例の各二次電池の充放電サイクル試験における各サイクルの正極活物質単位重量あたりの放電容量を示す。図４から明らかなように、負極活物質としてＬｉＶ₂Ｏ₄を用いた実施例１、２の各リチウム二次電池は、負極活物質としてＬｉＶ₃Ｏ₈を用いた比較例１、２の各リチウム二次電池と比較して、サイクルを繰り返した場合であっても大きな容量を維持していることが判る。この結果から、本発明のリチウムバナジウム複合酸化物を負極活物質に用いた本発明のリチウム二次電池は、サイクル特性についても良好な水系リチウム二次電池であることが確認できた。
【００６８】
【発明の効果】
本発明のリチウムバナジウム複合酸化物は、容量が大きく、かつサイクル特性の良好なリチウム二次電池を構成することができる負極活物質として好適である。また、本発明の製造方法によれば、そのリチウムバナジウム複合酸化物を簡便に製造することができる。また、本発明のリチウムバナジウム複合酸化物を負極活物質として水系電解液を用いた水系リチウム二次電池を構成することで、本発明のリチウム二次電池は、水系リチウム二次電池であることによる高安全性、低コストという利点を維持しつつ、大容量かつ出力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】 正極活物質として用いることのできる代表的な各種リチウム遷移金属複合酸化物の容量と電位（vs.Ｌｉ／Ｌｉ⁺）との関係を示す。
【図２】 サイクリックボルタモグラムによる単極評価試験で得られた実施例のリチウムバナジウム複合酸化物のＣＶ曲線を示す。
【図３】 サイクリックボルタモグラムによる単極評価試験で得られた実施例のリチウムバナジウム複合酸化物および組成式ＬｉＦｅＰＯ₄で表されるオリビン構造のリチウム鉄複合リン酸化物のＣＶ曲線を示す。
【図４】 実施例および比較例の各二次電池の充放電サイクル試験における各サイクルの正極活物質単位重量あたりの放電容量を示す。[0001]
BACKGROUND OF THE INVENTION
The present invention utilizes the lithium absorption / desorption phenomenon. Water system Negative electrode active material capable of constituting lithium secondary battery, method for producing the same, and using the same Water system The present invention relates to a lithium secondary battery.
[0002]
[Prior art]
Lithium secondary batteries that use the lithium absorption / desorption phenomenon have a high energy density, and as a result, miniaturization of mobile phones, personal computers, etc. has led to widespread use in the fields of communication equipment and information-related equipment. . In the field of automobiles, the development of electric vehicles has been urgently promoted due to resource issues and environmental issues, and lithium secondary batteries have been studied as power sources for the electric vehicles.
[0003]
Lithium secondary batteries currently in practical use generally have a positive electrode using a lithium transition metal composite oxide as a positive electrode active material, a negative electrode using a carbon material or the like as a negative electrode active material, and a lithium salt dissolved in an organic solvent. A non-aqueous electrolyte solution having a high voltage of 4V class is mainstream. In order to further improve the battery characteristics, various active material materials for the positive electrode and the negative electrode have been studied. For example, as the negative electrode active material, in addition to the above-described carbon material, LiWO ₂ , LiMoO ₂ LiTiS ₂ , LiV _Three O ₈ , VO ₂ Etc. have been tried.
[0004]
On the other hand, since the lithium secondary battery configured to include the non-aqueous electrolyte uses a non-aqueous organic solvent having a low flash point for the electrolyte, ignition and combustion of the organic solvent become a problem. In addition, for example, when it reaches an overcharged state or is exposed to a high temperature environment, the electrolyte may decompose and flammable gas may be generated, and for the purpose of ensuring safety, It is common to equip devices such as PTC elements and safety valves. However, since a flammable solvent is used, it is difficult to secure sufficient safety. In particular, secondary batteries as power sources for automobiles and the like are large and have a large amount of organic solvent to be used, and are expected to be used under severe conditions such as operating temperature. High safety is required.
[0005]
Further, if even a slight amount of moisture is present in the battery, various problems such as generation of gas due to water electrolysis reaction, consumption of lithium due to reaction between water and lithium, and corrosion of battery constituent materials occur. For this reason, in the manufacture of lithium secondary batteries, a thorough dry environment is required, requiring special equipment and a great deal of labor to completely remove moisture, which increases the cost of the battery. It is a cause.
[0006]
Under such circumstances, development of a water-based lithium secondary battery using an aqueous solution as an electrolytic solution has been promoted as a secondary battery that basically does not have the above-described problem. In general, since an aqueous solution has better conductivity than a non-aqueous solution, the reaction resistance of the battery is reduced in an aqueous lithium secondary battery, and the output characteristics and rate characteristics of the battery are improved. However, water-based lithium secondary batteries need to be charged / discharged in a potential range where water electrolysis does not occur, so it is possible to ensure a large discharge capacity compared to non-aqueous lithium secondary batteries. Has the disadvantage of being difficult.
[0007]
Therefore, in particular, in water-based lithium secondary batteries, in addition to being stable in an aqueous solution, a large amount of lithium ions can be reversibly absorbed and desorbed in a potential range where oxygen and hydrogen are not generated by electrolysis of water. Therefore, it is desired to use an active material having a large capacity.
[0008]
[Problems to be solved by the invention]
When the present inventor conducted various tests focusing on the negative electrode active material, LiV, a material that has been studied conventionally, _Three O ₈ , VO ₂ It has been found that it is difficult to take out a sufficient capacity in a potential range where no electrolysis reaction of water occurs, and there is a problem in stability in an aqueous solution. Therefore, when an aqueous lithium secondary battery is actually constructed using these as a negative electrode active material, the capacity and cycle characteristics of the lithium secondary battery are not practically satisfactory.
[0009]
As a result of further research and many experiments on the negative electrode active material, the present inventor has come to find a negative electrode active material having a large capacity particularly in the battery voltage range of a water based lithium secondary battery. An object of the present invention is to provide a negative electrode active material capable of constituting a lithium secondary battery having a large capacity and good cycle characteristics, and a simple manufacturing method thereof. Then, an object of the present invention is to provide a lithium secondary battery that is low in cost, high in safety, large in capacity, and good in cycle characteristics by forming a water-based lithium secondary battery using the negative electrode active material.
[0010]
[Means for Solving the Problems]
Of the present invention Water system The lithium vanadium composite oxide for lithium secondary battery negative electrode active material is: Basic Composition is LiV ₂ O _Four And having a spinel structure. As will be described in detail later, an electrode using the lithium vanadium composite oxide of the example as an active material was prepared, and a unipolar evaluation test by a cyclic voltammogram was performed. From the obtained current-potential curve (CV curve), This lithium vanadium composite oxide was found to exhibit a so-called two-phase coexistence type lithium occlusion / desorption behavior having only one oxidation-reduction potential.
[0011]
That is, during charging and discharging, LiNiO in which the lattice constant and the like continuously change and the potential changes with the insertion and extraction of lithium ₂ And LiCoO ₂ Unlike the above, the present lithium vanadium composite oxide exhibits a substantially constant potential during charging and discharging. The charge / discharge potential is around 2.4 V (vs. Li / Li ⁺ This potential is within a potential range where hydrogen generation due to water electrolysis does not occur. Moreover, since the rise of the peak of the obtained CV curve was steep and the polarization was small, it was also found that the reaction resistance in the oxidation-reduction reaction was small. Therefore, the present lithium vanadium composite oxide can reversibly absorb and desorb a large amount of lithium ions in a potential range where hydrogen generation due to electrolysis of water does not occur, particularly in an aqueous lithium secondary battery. It can be said that it is suitable as a negative electrode active material.
[0012]
Therefore, the lithium secondary battery of the present invention, which is an aqueous lithium secondary battery using the present lithium vanadium composite oxide as a negative electrode active material, maintains the advantages of high safety and low cost due to being an aqueous lithium secondary battery. However, the secondary battery has a large capacity and excellent output characteristics. Furthermore, as will become clear in later experiments, the lithium secondary battery of the present invention has a small capacity drop even after repeated charge and discharge, and has good cycle characteristics, particularly cycle characteristics at high temperatures. Next battery.
[0013]
Moreover, although the manufacturing method in particular does not specifically limit the lithium vanadium complex oxide of this invention, According to the manufacturing method of this invention, it can manufacture more simply. That is, the method for producing a lithium vanadium composite oxide of the present invention includes a raw material mixing step of mixing a lithium compound as a lithium source, a vanadium compound as a vanadium source, and a carbon material to obtain a raw material mixture, and the raw material mixture. Baked From the lithium compound and the vanadium compound in a reducing atmosphere generated by combustion of the carbon material in the raw material mixture The basic composition is LiV ₂ O _Four And a baking step of obtaining a lithium vanadium composite oxide having a spinel structure.
[0014]
Conventionally, a lithium compound and a vanadium compound are mixed, and the mixture is fired at a high temperature to obtain a lithium vanadium composite oxide. However, since it was fired at a high temperature, in order to construct a secondary battery having a practical capacity using the obtained lithium vanadium composite oxide as a negative electrode active material, the lithium vanadium composite oxide was once crushed, Further heat treatment was required. Therefore, it is desired to perform firing at a relatively low temperature from the viewpoint of reduction in the number of manufacturing steps, manufacturing cost, and the like, but in order to perform firing at a low temperature, it has been necessary to perform in a strong reducing atmosphere containing hydrogen.
[0015]
In this manufacturing method, since the carbon material is mixed with the raw material compound, the carbon material becomes a reducing atmosphere during firing. That is, even when firing at a relatively low temperature, the lithium vanadium composite oxide of the present invention can be easily produced without using hydrogen that is difficult to handle. Furthermore, the lithium vanadium composite oxide of the present invention can be produced by an extremely simple method of mixing each raw material compound and a carbon material at a predetermined ratio and firing the raw material mixture.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the lithium vanadium composite oxide for a lithium secondary battery negative electrode active material of the present invention and the production method of the present invention, which is a production method thereof, will be described in order, and then the utilization of the lithium vanadium composite oxide of the present invention will be described. The lithium secondary battery of the present invention in the form will be described.
[0017]
<Lithium vanadium composite oxide>
The lithium vanadium composite oxide for a lithium secondary battery negative electrode active material of the present invention has a basic composition of LiV. ₂ O _Four And having a spinel structure. Here, the basic composition means a typical composition of the lithium vanadium composite oxide. In addition, it is not necessarily limited to a stoichiometric composition, for example, a non-stoichiometric composition in which a cation atom such as lithium inevitably produced in production is deficient or an oxygen atom is deficient. Is also included.
[0018]
<Method for producing lithium vanadium composite oxide>
The production method of the lithium vanadium composite oxide of the present invention is not particularly limited, but can be easily produced by the production method of the present invention. That is, the method for producing a lithium vanadium composite oxide of the present invention includes a raw material mixing step and a firing step. Hereinafter, each step will be described.
[0019]
(1) Raw material mixing process
This step is a step of obtaining a raw material mixture by mixing a lithium compound serving as a lithium source, a vanadium compound serving as a vanadium source, and a carbon material. Lithium compounds include Li ₂ CO _Three , LiOH, LiNO _Three , Li ₂ SO _Four Etc. can be used. Vanadium compounds include V ₂ O _Five , NH _Four VO _Three Etc. can be used. V ₂ O _Five Has the advantage of being a cheaper raw material, so as a vanadium compound V ₂ O _Five It is more desirable to select. In this case, the mixing ratio of the lithium compound and the vanadium compound is set so that the lithium and vanadium contained in the lithium compound and vanadium each have a composition ratio of the target lithium vanadium composite oxide.
[0020]
The type of the carbon material is not particularly limited. For example, graphite materials such as natural graphite, spherical or fibrous artificial graphite, graphitizable carbon such as coke and carbon black, and carbonaceous materials such as non-graphitizable carbon such as a phenol resin fired body are powdered. Can be used. These materials can be used alone or in admixture of two or more. Among these, it is desirable to use carbon black when considering dispersibility in the raw material mixture.
[0021]
The mixing ratio of the carbon material is 100% of the total weight of the mixture of the lithium compound and the vanadium compound. Parts by weight If 0.5 Parts by weight More than 5 Parts by weight The following is desirable. 0.5 Parts by weight This is because the reducing action by the carbon material is small when the ratio is less than the above range. Conversely, 5 Parts by weight This is because the mixed state in the raw material mixture is less likely to be uniform than in the above preferred range, and when a secondary battery is configured, the capacity is reduced. A part of the mixed carbon material disappears as carbon dioxide or the like by later firing the raw material mixture. The carbon material remaining in the lithium vanadium composite oxide is considered to play a role of a conductive material.
[0022]
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.
[0023]
(2) Firing process
This step is a step of firing the raw material mixture obtained in the raw material mixing step to obtain a lithium vanadium composite oxide. The firing temperature is desirably 600 ° C. or higher and 1000 ° C. or lower. This is because if the firing temperature is lower than 600 ° C., the reaction does not proceed sufficiently and a single-phase spinel phase cannot be obtained. On the other hand, when the temperature exceeds 1000 ° C., an excessively reduced state is generated, and a low oxidation number vanadium compound is generated. In particular, when considering the suppression of grain growth by suppressing grain growth, the firing temperature is desirably 800 ° C. or lower.
[0024]
As described above, when firing at a relatively low temperature of 800 ° C. or lower, it is necessary to fire in a strong reducing atmosphere containing hydrogen, but in the production method of the present invention, the firing is performed by the carbon material in the raw material mixture. It is a reducing atmosphere. Therefore, the firing may be performed in an inert gas stream such as argon gas. The firing time may be a time sufficient to complete the firing, and is usually performed for about 4 to 24 hours.
[0025]
<Lithium secondary battery>
An embodiment of the lithium secondary battery of the present invention, which is one application form of the lithium vanadium composite oxide of the present invention, will be described. As described above, the lithium secondary battery moves the lithium ions between the negative electrode and the positive electrode formed including an active material, the separator sandwiched between the negative electrode and the positive electrode, and the negative electrode and the positive electrode. It consists of electrolyte solution. The lithium secondary battery of this embodiment may follow this configuration. The following description will be given for each of these components.
[0026]
(1) Negative electrode active material
In the lithium secondary battery of the present invention, the lithium vanadium composite oxide of the present invention is used as the negative electrode active material. When the lithium vanadium composite oxide is used as a negative electrode active material of a lithium secondary battery, a powdery one is generally used. Therefore, what was obtained by baking as mentioned above should just grind | pulverize and use for manufacture of a battery.
[0027]
(2) Positive electrode active material
In the lithium secondary battery of the present invention, the positive electrode active material uses a material having a higher lithium insertion / extraction potential than the lithium vanadium composite oxide. As this substance, for example, the basic composition is LiCoO. ₂ Layered rock salt structure lithium cobalt composite oxide with basic composition LiNiO ₂ Layered rock salt structure lithium nickel composite oxide with basic composition LiMn ₂ O _Four Spinel structure lithium manganese composite oxide with a basic composition of LiMnO ₂ Layered structure lithium manganese composite oxide with a basic composition of LiFePO _Four And olivine structure lithium iron composite phosphorous oxide. All of these have a high lithium insertion / extraction potential, and can be combined with the lithium vanadium composite oxide to form a 1V class secondary battery.
[0028]
Again, the basic composition means a typical composition of the lithium transition metal composite oxide listed above, and in addition to what is represented by the above composition formula, for example, transition of lithium sites, Co, Ni, Mn, etc. It includes a composition in which the metal site is partially substituted with one or more elements such as other transition metals and Al. In addition, it is not necessarily limited to those having a stoichiometric composition, for example, those having a non-stoichiometric composition in which a cation atom such as lithium inevitably generated in production is deficient or oxygen atoms are deficient. Is also included.
[0029]
Among the listed lithium transition metal composite oxides, the basic composition of the lithium secondary battery of the present invention is LiMnO. ₂ Lithium manganese composite oxide having a layered crystal structure or a basic composition of LiFePO _Four It is desirable that the olivine-structured lithium iron composite phosphorous oxide as described above be included as a positive electrode active material. These lithium manganese composite oxides and lithium iron composite phosphorous oxides can reversibly store and release a large amount of lithium ions in a potential range where oxygen generation due to electrolysis of water does not occur. It is suitable as a positive electrode active material in an aqueous lithium secondary battery. Therefore, the lithium secondary battery of the present invention of this aspect is a larger capacity aqueous lithium secondary battery.
[0030]
FIG. 1 shows the capacity and potential (vs. Li / Li) of various typical lithium transition metal composite oxides that can be used as the positive electrode active material. ⁺ ). As is clear from FIG. 1, LiCoO ₂ , LiMn ₂ O _Four In the potential range where oxygen generation due to electrolysis of water does not occur, the capacity cannot be taken out very much, and Li (Ni, Co) O ₂ However, it is only about half the original capacity. Actually, a minute amount of Li dissolves as LiOH in water, and the electrolytic solution tends to be alkaline, and the oxygen generation potential is lowered. Meanwhile, LiMnO ₂ And LiFePO _Four Can absorb and release a large amount of lithium ions reversibly in a potential range where oxygen generation due to electrolysis of water does not occur, so that a sufficient capacity can be taken out. Therefore, LiMnO ₂ Or LiFePO _Four It can be confirmed that the lithium secondary battery of the present invention in which A is used as the positive electrode active material is a large capacity secondary battery.
[0031]
Here, the lithium manganese composite oxide having a layered structure includes a hexagonal layered structure, a lithium manganese composite oxide having a so-called layered rock salt structure (a space group is represented by the following formula 1), and an orthorhombic system. Lithium-manganese composite oxide (space group C2 / m) having a layered structure of 1 and a lithium-manganese composite oxide (space group Pmnm) having a monoclinic zigzag layered structure, one of these being used alone In addition, two or more kinds can be mixed and used.
[0032]
[Chemical 1]

[0033]
Among these, it is desirable to use a lithium manganese composite oxide having a hexagonal layered rock salt structure. The lithium manganese composite oxide having a hexagonal layered rock salt structure does not cause a transition to a spinel structure with a small capacity that can be taken out even after repeated charge and discharge. The secondary battery is an aqueous lithium secondary battery that can maintain a larger discharge capacity more stably.
[0034]
The method for producing the lithium manganese composite oxide having a hexagonal layered rock salt structure is not particularly limited, and can be fired by a so-called solid phase reaction method. Further, for example, a dispersion aqueous solution preparation in which manganese dioxide as a manganese source and a lithium hydroxide aqueous solution as a lithium source are mixed at a ratio such that Li / Mn is 2 or more and 10 or less in molar ratio is obtained. It can also be produced by a production method comprising a step and a hydrothermal treatment step of heating and holding the dispersed aqueous solution at a temperature of 120 ° C. or higher and 250 ° C. or lower. This production method is a simple and industrially advantageous production method because the target lithium composite oxide can be synthesized by one-stage hydrothermal treatment.
[0035]
Also, the production method of the olivine-structured lithium iron composite phosphorus oxide is not particularly limited. For example, LiOH / H as a lithium source ₂ O and FeC as iron source ₂ O _Four ・ 2H ₂ O and phosphorus source (NH _Four ) ₂ HPO _Four Can be produced by a production method comprising a raw material mixing step of obtaining a mixture by mixing and a baking step of baking the mixture at a temperature of 600 ° C. or higher and 700 ° C. or lower.
[0036]
(3) Negative electrode and positive electrode
For both the negative electrode and the positive electrode, a powdered active material is mixed with a conductive material and a binder, and a mixture of the negative electrode and the positive electrode, respectively, is crimped to the surface of a metal current collector, or applied and dried. Can be formed.
[0037]
The conductive material is for ensuring the electrical conductivity of the electrode, and for example, a material obtained by mixing one or two or more carbon material powders such as carbon black, acetylene black, and graphite can be used. . In addition, the binder plays a role of connecting the active material particles and the conductive material particles. For example, the binder is a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene. Can be used.
[0038]
(4) Aqueous electrolyte
The electrolytic solution used in the lithium secondary battery of the present invention is an aqueous electrolytic solution in which a lithium salt as an electrolyte is dissolved in water. The lithium salt is dissociated by dissolving in water and becomes lithium ions and exists in the electrolytic solution. In general, an oxide-based active material is present more stably in a neutral to alkaline aqueous solution. In consideration of further activating the lithium ion occlusion / desorption reaction, it is desirable that the electrolytic solution used be neutral to alkaline. Here, neutral means pH = 6 to 8 in terms of pH.
[0039]
For example, when a neutral electrolytic solution having pH = 7 is used, the hydrogen generation potential by water electrolysis is 2.62 V, and the oxygen generation potential is 3.85 V (vs. Li / Li ⁺ In the case of using an alkaline electrolyte with pH = 14, the hydrogen generation potential is 2.21 V and the oxygen generation potential is 3.44 V (vs. Li / Li ⁺ ). That is, when a neutral electrolyte is used, since the oxygen generation potential due to the electrolysis of water is high, as described above, the positive electrode active material can occlude / desorb more lithium ions. A large capacity can be taken out. Therefore, when making a secondary battery with a larger capacity, it is desirable to use an electrolyte solution close to neutrality, specifically, an electrolyte solution having a pH of 6 to 10.
[0040]
In general, the aqueous solution has better conductivity than the non-aqueous solution. For example, the neutral aqueous solution has a conductivity 10 times or more that of the non-aqueous solution, and the alkaline aqueous solution has a conductivity 100 times or more that of the non-aqueous solution. Have a rate. Therefore, a secondary battery using an aqueous solution as an electrolyte has a smaller internal resistance, particularly a reaction resistance, compared to a non-aqueous secondary battery. When an alkaline electrolyte is used, the internal resistance is It will be smaller. Therefore, in order to obtain a secondary battery with better output characteristics and rate characteristics, it is desirable to use a strongly alkaline electrolyte, specifically, an electrolyte having a pH of 10 to 12.
[0041]
The lithium salt that can be used as the electrolyte is not particularly limited as long as it is soluble in water. However, in view of the stability of the oxide as the positive electrode active material, the electrolyte solution is neutral to alkaline after dissolution. It is desirable to use a lithium salt such that Specifically, for example, it is desirable to use lithium nitrate, lithium hydroxide, lithium iodide, or the like. These lithium salts may be used alone, or two or more of these may be used in combination. In particular, it is desirable to use lithium nitrate in order to obtain a neutral electrolyte because of its high solubility and therefore good conductivity, and in order to obtain a strong alkaline electrolyte, lithium nitrate and water. It is desirable to use a mixture with lithium oxide. Note that the concentration of the lithium salt in the electrolytic solution is desirably a saturated concentration or a concentration close thereto because the electric conductivity of the electrolytic solution can be increased to reduce the internal resistance of the secondary battery.
[0042]
(5) Other components
In the lithium secondary battery of the present invention, the electrode body is formed by making the positive electrode and the negative electrode face each other. It is desirable to sandwich a separator between the positive electrode and the negative electrode. This separator separates a positive electrode and a negative electrode and holds an electrolytic solution, and a cellulose-based one can be used.
[0043]
The shape of the lithium secondary battery of the present invention is not particularly limited, and various types such as a cylindrical type, a stacked type, and a coin type can be used. Regardless of the shape, the electrode body formed according to the battery shape is housed in a predetermined battery case, and the positive electrode terminal and the negative electrode terminal communicated to the outside from the positive electrode current collector and the negative electrode current collector. A current collector lead or the like is connected, and the electrode body is impregnated with the electrolyte solution and sealed in a battery case to complete a lithium secondary battery.
[0044]
<Acceptance of other embodiments>
As described above, the embodiment of the lithium vanadium composite oxide of the present invention, the manufacturing method thereof, and the lithium secondary battery using the same have been described. However, the embodiment described above is merely an embodiment, and the lithium vanadium composite oxide of the present invention The lithium secondary battery using the product, its manufacturing method and the lithium secondary battery using the same can be implemented in various forms including various modifications and improvements based on the knowledge of those skilled in the art including the above-described embodiments.
[0045]
【Example】
The lithium vanadium composite oxide used as the negative electrode active material in the lithium secondary battery of the present invention was actually synthesized, and the characteristics as the negative electrode active material were investigated. Subsequently, the lithium secondary battery of the present invention using the lithium vanadium composite oxide as a negative electrode active material was produced, and the battery characteristics were evaluated. For comparison, LiV, which has been studied in the past _Three O ₈ An aqueous lithium secondary battery using, for example, a negative electrode active material was prepared, and it was confirmed that the lithium secondary battery of the present invention was excellent.
[0046]
<Synthesis of lithium vanadium composite oxide>
(1) Lithium vanadium composite oxide of Example
Composition formula LiV ₂ O _Four A lithium vanadium composite oxide having a spinel structure represented by the following formula was synthesized. First, lithium carbonate (Li ₂ CO _Three ) And vanadium pentoxide (V ₂ O _Five ) Was mixed at a ratio such that the molar ratio of Li: V was 1: 2. Mixing was performed in an automatic mortar for 20 minutes. Next, 2 parts by weight of ketjen black was mixed with 100 parts by weight of the mixture to obtain a raw material mixture. Mixing was performed in an automatic mortar for 20 minutes. The raw material mixture was calcined at 750 ° C. for 24 hours in an argon stream and quenched to obtain a lithium vanadium composite oxide. In addition, in order to use the obtained lithium vanadium complex oxide as a negative electrode active material, it grind | pulverized in the mortar and it was set as the powdery thing. Hereinafter, this lithium vanadium composite oxide will be referred to as the lithium vanadium composite oxide of the examples for convenience.
[0047]
(2) Lithium vanadium composite oxide of comparative example
Composition formula LiV _Three O ₈ A lithium vanadium composite oxide represented by the following formula was synthesized. Lithium carbonate (Li ₂ CO _Three ) And vanadium pentoxide (V ₂ O _Five ) Was mixed at a ratio such that the molar ratio of Li: V was 1: 3. Mixing was performed in an automatic mortar for 20 minutes. Next, the mixture was heated to 700 ° C. in an argon atmosphere, held at that temperature for 12 hours, and then cooled in a furnace to obtain a lithium vanadium composite oxide. In addition, the obtained lithium vanadium composite oxide was pulverized in a mortar to obtain a powder. Hereinafter, the lithium vanadium composite oxide will be referred to as a lithium vanadium composite oxide of a comparative example for convenience.
[0048]
<Characteristic investigation as negative electrode active material>
An electrode using the lithium vanadium composite oxide of the example as an active material was prepared, and the characteristics as the negative electrode active material were investigated by performing a monopolar evaluation test using a cyclic voltammogram. First, an electrode mixture was obtained by mixing 70 parts by weight of lithium vanadium composite oxide with 25 parts by weight of carbon as a conductive material and 5 parts by weight of polytetrafluoroethylene as a binder. Next, 10 mg of this electrode mixture is placed on a stainless steel mesh at about 0.6 ton / cm. ² The electrode was pressure-bonded at a pressure of
[0049]
The test was conducted using a silver / silver chloride electrode as a reference electrode, a platinum wire (φ0.3 mm × 5 mm; coiled) as a counter electrode, a tripolar beaker cell, and a scanning speed of 2 mV / s. This test yielded a CV curve showing the relationship between current and potential. The obtained CV curve is shown in FIG.
[0050]
From FIG. 2, it can be seen that the lithium vanadium composite oxide of the example shows a so-called two-phase coexistence type lithium occlusion / desorption behavior having only one oxidation-reduction potential. That is, during charging and discharging, LiNiO in which the lattice constant and the like continuously change and the potential changes with the insertion and extraction of lithium ₂ And LiCoO ₂ In contrast, the lithium vanadium composite oxide of the example shows a constant potential during charging and discharging. The charge / discharge potential is around 2.4 V (vs. Li / Li ⁺ This potential is within a potential range where hydrogen generation due to water electrolysis does not occur. Therefore, the lithium vanadium composite oxide of the example can reversibly absorb and desorb a large amount of lithium ions in a potential range where hydrogen generation due to water electrolysis does not occur, and the negative electrode in an aqueous lithium secondary battery It was confirmed that it was suitable as an active material.
[0051]
In addition, the composition formula LiFePO used as one of the positive electrode active materials _Four Similarly, a unipolar evaluation test was performed on the lithium iron composite phosphor oxide having an olivine structure represented by the following formula. The results are shown in FIG. 3 together with the results of the lithium vanadium composite oxide of the example. From FIG. 3, the charge / discharge potential in the lithium iron composite phosphate is around 3.5 V (vs. Li / Li ⁺ ), And this potential is within a potential range where oxygen generation due to electrolysis of water does not occur. That is, the composition formula LiFePO as a positive electrode active material _Four It was confirmed that an about 1 V-class aqueous lithium secondary battery can be constructed by using the lithium iron composite phosphorus oxide represented by the above and using the lithium vanadium composite oxide of the example as the negative electrode active material.
[0052]
<Production of lithium secondary battery>
(1) Production of negative electrode
Negative electrodes were prepared using the lithium vanadium composite oxides of the above examples and comparative examples as negative electrode active materials. First, negative electrode mixture was obtained by mixing 70 parts by weight of lithium vanadium composite oxide with 25 parts by weight of carbon as a conductive material and 5 parts by weight of polytetrafluoroethylene as a binder. Next, 10 mg of each negative electrode mixture is about 0.6 ton / cm on a stainless steel mesh previously welded to the inside of the coin cell. ² A negative electrode was formed by pressure bonding under the pressure of
[0053]
(2) Production of positive electrode
For the positive electrode active material, the composition formula LiMnO ₂ Lithium manganese composite oxide having a layered rock salt structure represented by the formula: LiFePO _Four Lithium iron composite phosphorus oxide having an olivine structure represented by the formula: LiNi _0.8 Co _0.15 Al _0.05 O ₂ A lithium nickel composite oxide having a layered rock salt structure represented by: A method for synthesizing each composite oxide will be described below.
[0054]
(A) Synthesis of lithium manganese composite oxide
6.29 g of LiOH.H ₂ O was dissolved in 80 ml of water to prepare a LiOH aqueous solution. To this LiOH aqueous solution, MnO ₂ Was added (Li / Mn was 5 in molar ratio), and ultrasonic dispersion was performed for 30 minutes to prepare a dispersed aqueous solution. Next, this dispersed aqueous solution was put in an autoclave and reacted at a temperature of 200 ° C. for 7 days. After the reaction, the autoclave was cooled, and the precipitate in the container was filtered, washed with water, and dried at 120 ° C. to obtain a lithium manganese composite oxide. The obtained lithium manganese composite oxide was pulverized in a mortar to form a powder.
[0055]
(B) Synthesis of lithium iron composite phosphorus oxide
LiOH ・ H ₂ O and FeC ₂ O _Four ・ 2H ₂ O and (NH _Four ) ₂ HPO _Four Were mixed such that the molar ratio of Li: Fe: P was 1: 1: 1. Mixing was performed in an automatic mortar for 30 minutes. This mixture was calcined at 350 ° C. for 5 hours in an argon stream, and then mixed in an automatic mortar for 30 minutes. Thereafter, it was further fired in an argon stream at 650 ° C. for 6 hours to obtain a lithium iron composite phosphorus oxide. The obtained lithium iron composite phosphate was pulverized in a mortar to form a powder.
[0056]
(C) Synthesis of lithium nickel composite oxide
Each aqueous solution of 2M nickel nitrate, cobalt nitrate, and aluminum nitrate was mixed so that the molar ratio of Ni: Co: Al was 8: 1.5: 0.5 to obtain a 500 mL aqueous solution. This aqueous solution was dropped into a 4M sodium hydroxide aqueous solution to precipitate and synthesize hydroxide particles containing nickel, cobalt, and aluminum. Then, the precipitated hydroxide particles were filtered and washed, and then poured into water to obtain a hydroxide slurry. 3M lithium hydroxide aqueous solution was added to the slurry so that the atomic ratio of Li was Li / (Ni + Co + Al) = 1 with respect to the total amount of Ni, Co, and Al (Ni + Co + Al) in the hydroxide slurry. A suspension was prepared and the suspension was spray dried under a nitrogen atmosphere. After drying, the obtained composite oxide precursor was pre-fired at 350 ° C. for 1 hour in a nitrogen atmosphere, and further fired at 750 ° C. for 12 hours in an oxygen atmosphere to obtain a lithium nickel composite oxide. .
[0057]
A positive electrode was produced using the composite oxide and the phosphor oxide obtained in (a) to (c) above as the positive electrode active material. As in the case of the negative electrode, 70 parts by weight of the composite oxide and the like were mixed with 25 parts by weight of carbon as a conductive material and 5 parts by weight of polytetrafluoroethylene as a binder to obtain a positive electrode mixture. Next, about 10 ton / cm of the positive electrode composite material on a stainless steel mesh previously welded to the inside of the coin cell. ² A positive electrode was formed by pressure bonding.
[0058]
(3) Production of lithium secondary battery of Example
(A) Lithium secondary battery of Example 1
After injecting the aqueous electrolyte, the negative electrode using the lithium vanadium composite oxide of the example as the negative electrode active material and the positive electrode using the lithium iron composite phosphate as the positive electrode active material are opposed to each other with a cellulose separator interposed therebetween. The coin cell was sealed to complete a 2016 type (outer diameter 20 mmφ, thickness 16 mm) lithium secondary battery. The aqueous electrolyte is LiNO, which is a lithium salt. _Three Of LiNO dissolved in water _Three It is an aqueous solution, and its pH value is about 7. This lithium secondary battery was referred to as the lithium secondary battery of Example 1.
[0059]
(B) Lithium secondary battery of Example 2
The lithium secondary battery of Example 1 was prepared in the same manner as the lithium secondary battery of Example 1 except that the positive electrode was changed and a positive electrode using lithium manganese composite oxide as the positive electrode active material was used. . This lithium secondary battery was referred to as the lithium secondary battery of Example 1.
[0060]
(C) Lithium secondary battery of Example 3
The lithium secondary battery of Example 1 was prepared in the same manner as the lithium secondary battery of Example 1 except that the positive electrode was changed and a positive electrode using a lithium nickel composite oxide as a positive electrode active material was used. . This lithium secondary battery was used as the lithium secondary battery of Comparative Example 3.
[0061]
(4) Preparation of lithium secondary battery of comparative example
(A) Lithium secondary battery of Comparative Example 1
In the production of the lithium secondary battery of Example 1, the negative electrode was changed and the negative electrode using the lithium vanadium composite oxide of the comparative example as a negative electrode active material was used. It was prepared. This lithium secondary battery was used as the lithium secondary battery of Comparative Example 1.
[0062]
(B) Lithium secondary battery of Comparative Example 2
In the production of the lithium secondary battery of Example 2, the negative electrode was changed and the negative electrode using the lithium vanadium composite oxide of Comparative Example as the negative electrode active material was used. It was prepared. This lithium secondary battery was used as the lithium secondary battery of Comparative Example 2.
[0063]
<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 cycle test was conducted at an ambient temperature of 60 ° C. and a current density of 0.5 mA / cm. ² Is charged to a battery voltage of 1.4 V at a constant current of 0.5 mA / cm. ² Charging / discharging to a battery voltage of 0.1 V at a constant current of 1 cycle was defined as one cycle, and this cycle was repeated 50 cycles. Note that the charge pause time and the discharge pause time in the charge / discharge cycle were each 1 minute.
[0064]
Table 1 shows the initial discharge capacity per unit weight of the positive electrode active material of each lithium secondary battery as a result of the charge / discharge cycle test. As a reference example, the negative electrode active material is LiMn. ₂ O _Four The result of having measured the first time discharge capacity is also shown together.
[0065]
[Table 1]

[0066]
As is clear from Table 1, LiV is used as the negative electrode active material. ₂ O _Four In each of the lithium secondary batteries of Examples 1 and 2 using LiFe, LiV was used as the negative electrode active material. _Three O ₈ It can be seen that the initial discharge capacity is large as compared with the lithium secondary batteries of Comparative Examples 1 and 2 using Moreover, LiMn as a negative electrode active material ₂ O _Four Compared to a secondary battery using, the initial discharge capacity is extremely large. That is, it was confirmed that the lithium secondary battery of the present invention using the lithium vanadium composite oxide of the present invention as a negative electrode active material is a large capacity aqueous lithium secondary battery. In addition, the lithium secondary battery of Example 3 had a small initial discharge capacity. This is because lithium nickel composite oxide was used as the positive electrode active material, and it was considered that the capacity could not be taken out within the above voltage range. Therefore, as the positive electrode active material used in the lithium secondary battery of the present invention, the composition formula LiMnO ₂ Lithium manganese composite oxide having a layered rock salt structure represented by the formula: LiFePO _Four It can be said that the lithium iron composite phosphorus oxide having an olivine structure represented by
[0067]
As another result of the charge / discharge cycle test, FIG. 4 shows the discharge capacity per unit weight of the positive electrode active material in each cycle in the charge / discharge cycle test of each secondary battery of the example and the comparative example. As is clear from FIG. 4, LiV is used as the negative electrode active material. ₂ O _Four In each of the lithium secondary batteries of Examples 1 and 2 using LiFe, LiV was used as the negative electrode active material. _Three O ₈ It can be seen that a large capacity is maintained even when the cycle is repeated as compared with each of the lithium secondary batteries of Comparative Examples 1 and 2 using the above. From this result, it was confirmed that the lithium secondary battery of the present invention using the lithium vanadium composite oxide of the present invention as a negative electrode active material was an aqueous lithium secondary battery having good cycle characteristics.
[0068]
【The invention's effect】
The lithium vanadium composite oxide of the present invention is suitable as a negative electrode active material capable of constituting a lithium secondary battery having a large capacity and good cycle characteristics. Moreover, according to the manufacturing method of this invention, the lithium vanadium complex oxide can be manufactured simply. In addition, the lithium secondary battery of the present invention is an aqueous lithium secondary battery by constituting an aqueous lithium secondary battery using an aqueous electrolyte using the lithium vanadium composite oxide of the present invention as a negative electrode active material. While maintaining the advantages of high safety and low cost, the lithium secondary battery has a large capacity and excellent output characteristics.
[Brief description of the drawings]
FIG. 1 shows the capacity and potential (vs. Li / Li) of typical various lithium transition metal composite oxides that can be used as a positive electrode active material. ⁺ ).
FIG. 2 shows a CV curve of a lithium vanadium composite oxide of an example obtained in a monopolar evaluation test using a cyclic voltammogram.
FIG. 3 shows lithium vanadium composite oxide and compositional formula LiFePO obtained in a single pole evaluation test by cyclic voltammogram. _Four The CV curve of the lithium iron complex phosphoric acid oxide of the olivine structure represented by these is shown.
FIG. 4 shows the discharge capacity per unit weight of the positive electrode active material in each cycle in the charge / discharge cycle test of each secondary battery in Examples and Comparative Examples.

Claims (7)

A lithium vanadium composite oxide for a negative electrode active material of an aqueous lithium secondary battery having a basic composition of LiV ₂ O ₄ and a spinel structure. A raw material mixing step of mixing a lithium compound serving as a lithium source, a vanadium compound serving as a vanadium source, and a carbon material to obtain a raw material mixture, firing the raw material mixture, and burning the carbon material in the raw material mixture aqueous lithium which comprises a firing step to obtain a lithium vanadium composite oxide having to spinel structure and LiV ₂ O ₄ the basic composition from said vanadium compound and the lithium compound in the generated reducing atmosphere by The manufacturing method of lithium vanadium complex oxide for secondary battery negative electrode active materials. The aqueous lithium secondary according to claim 2, wherein the mixing ratio of the carbon material is 0.5 parts by weight or more and 5 parts by weight or less when the total weight of the mixture of the lithium compound and the vanadium compound is 100 parts by weight. Method for producing lithium vanadium composite oxide for battery negative electrode active material . The method for producing a lithium vanadium composite oxide for a negative electrode active material for an aqueous lithium secondary battery according to claim 2 or 3 , wherein a temperature for firing the raw material mixture in the firing step is 600 ° C or higher and 800 ° C or lower. A negative electrode using a spinel structure lithium vanadium composite oxide having a basic composition of LiV ₂ O ₄ as a negative electrode active material, and a positive electrode using a material having a higher lithium absorption / desorption potential than the lithium vanadium composite oxide as a positive electrode active material; And a lithium secondary battery comprising an aqueous electrolyte obtained by dissolving a lithium salt in water. The lithium secondary battery according to claim 5 , wherein the substance having a lithium occlusion / desorption potential higher than that of the lithium vanadium composite oxide includes a lithium manganese composite oxide having a basic composition of LiMnO ₂ and a layered rock salt structure. The lithium secondary battery according to claim 6 , wherein the substance having a higher lithium insertion / extraction potential than the lithium vanadium composite oxide includes a lithium iron composite phosphate having a basic composition of LiFePO ₄ and an olivine structure.

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