JP3568499B2 - Lithium-containing electrode material, method for producing the same, and electrochemical element using lithium-containing electrode material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium-containing electrode material with high stability in the atmosphere as an electrode material capable of constituting an electrochemical element with high capacity and high cycle characteristics. SOLUTION: This lithium-containing electrode material contains oxygen and an element, forming an alloy with lithium in at least a surface layer part of particles of a lithium transition metal composite nitride, and the lithium- containing electrode material is prepared in such a way that the lithium transition metal composite nitride and a compound containing an element forming an alloy with at least lithium are mixed, the compound containing the element forming the alloy with at least lithium is attached to the surface of the particles of the lithium transition metal composite nitride, and they are baked.

Description

【００３５】
上記式中、Ｒ^１、Ｒ^２及びＲ^３は、それぞれ独立に水素原子、アルキル基、アルケニル基、シクロアルキル基、アリール基、又はこれらの基以外でケイ素に直結する基が炭素である基、アルキルシリル基、アルキルアミノ基、アルコキシ基を表わす。ここで、前記リチウム遷移金属複合窒化物の粒子の表面に、前記少なくともリチウムと合金を形成することができる元素を含む化合物を緻密に付着させ、確実に前記リチウム遷移金属複合窒化物の粒子の少なくとも表層部に、酸素と、リチウムと合金を形成することができる元素とを含有させるために、焼成により架橋構造を形成するポリシラザンであることが好ましく、よってＲ^１、Ｒ^２及びＲ^３の少なくとも１つは水素原子であることが好ましい。
【００３６】
また、上記ポリシラザンはそれ単独で使用することもできるが、シラザン（モノマー）と他のモノマーとの共重合体や、ポリシラザンと他の化合物との混合物としても使用できる。用いるポリシラザンには、鎖状、環状又は架橋構造を有するもの、あるいは分子内にこれら複数の構造を同時に有するものがあり、これら単独でも、あるいはこれらの混合物でも使用できる。中でも、前記リチウム遷移金属複合窒化物の粒子の表面に緻密に付着させることができるという点からはペルヒドロポリシラザンが最も好ましい。前記一般式（Ｉ）においてＲ^１、Ｒ^２及びＲ^３に水素原子を有するものが、ペルヒドロポリシラザンであり、その製造法は、例えば特公昭６３−１６３２５号公報、Ｄ． Ｓｅｙｆｅｒｔｈ らＣｏｍｍｕｎｉｃａｔｉｏｎ ｏｆ Ａｍ．Ｃｅｒ． Ｓｏｃ．， Ｃ−１３， Ｊａｎｕａｒｙ（１９８３）などに報告されている。ペルヒドロポリシラザンの構造の一例を以下に示す。
【００３７】
【化２】

【００３８】
前記一般式（Ｉ）においてＲ^１及びＲ^２に水素原子、Ｒ^３にメチル基を有するポリシラザンの製造方法は、Ｄ． Ｓｅｙｆｅｒｔｈ らＰｏｌｙｍ． Ｐｒｅｐｒ．， Ａｍ． Ｃｈｅｍ． Ｓｏｃ．， Ｄｉｖ． Ｐｏｌｙｍ． Ｃｈｅｍ．， ２５， １０（１９８４）に報告されている。この方法により得られるポリシラザンは、繰り返し単位が −（ＳｉＨ_２ＮＣＨ_３）− の鎖状ポリマーと環状ポリマーであり、いずれも架橋構造を持たない。
【００３９】
また、前記一般式（Ｉ）でＲ^１及びＲ^３に水素原子、Ｒ^２に有機基を有するポリオルガノ（ヒドロ）シラザンの製造法は、Ｄ． Ｓｅｙｆｅｒｔｈ らＰｏｌｙｍ． Ｐｒｅｐｒ．， Ａｍ． Ｃｈｅｍ． Ｓｏｃ．， Ｄｉｖ． Ｐｏｌｙｍ． Ｃｈｅｍ．， ２５， １０（１９８４）、又は特開昭６１−８９２３０号公報などに報告されている。これらの方法により得られるポリシラザンには、−（Ｒ^２ＳｉＨＮＨ）− を繰り返し単位として、主として重合度が３〜５の環状構造を有するものや、（Ｒ^３ＳｉＨＮＨ）_ｘ〔（Ｒ^２ＳｉＨ）_１．５Ｎ〕_１−ｘ（０．４＜ｘ＜１）の化学式で示される分子内に鎖状構造と環状構造を同時に有するものがある。
【００４０】
前記一般式（Ｉ）でＲ^１に水素原子、Ｒ^２及びＲ^３に有機基を有するポリシラザン、またＲ^１及びＲ^２に有機基、Ｒ^３に水素原子を有するポリシラザンは、−（Ｒ^１Ｒ^２ＳｉＮＲ^３）− を繰り返し単位として、主に重合度が３〜５の環状構造を有している。
【００４１】
本発明に用いるポリシラザンは、前記一般式（Ｉ）で表わされる単位からなる主骨格を有するが、一般式（Ｉ）で表わされる単位は、上記にも明らかなように環状化することがあり、その場合にはその環状部分が末端基となり、このような環状化がされない場合には、主骨格の末端はＲ^１、Ｒ^２、Ｒ^３と同様の基又は水素原子となる。
【００４２】
前記ポリオルガノ（ヒドロ）シラザンの中には、Ｄ． Ｓｅｙｆｅｒｔｈ らＣｏｍｍｕｎｉｃａｔｉｏｎ ｏｆ Ａｍ． Ｃｅｒ． Ｓｏｃ．， Ｃ−１３２， Ｊｕｌｙ（１９８４）で報告されているような分子内に架橋構造を有するものもある。また、特開昭４９−６９７１７号公報に報告されているようなＲ^１ＳｉＸ_３（Ｘ：ハロゲン）のアンモニア分解によって得られる架橋構造を有するポリシラザン（Ｒ^１Ｓｉ（ＮＨ）_ｘ）、あるいはＲ^１ＳｉＸ_３及びＲ^１Ｒ^２ＳｉＸ_２の共アンモニア分解によって得られる下記の構造を有するポリシラザンも出発材料として用いることができる。
【００４３】
【化３】

【００４４】
なお、上記式中、ｍ及びｎは正の整数である。
【００４５】
また、本発明でリチウムと合金を形成することができる元素を含む化合物として用いるＳｉ−Ｎ結合を有する化合物として、ポリシラザンの変性物である金属原子を含むポリメタロシラザンも用いることができる。その他、特開昭６２−１９５０２４号公報に報告されているような繰り返し単位が〔（ＳｉＨ_２）_ｎ（ＮＨ）_ｍ〕及び〔（ＳｉＨ_２）_ｒＯ〕（式中、ｎ、ｍ、ｒはそれぞれ１、２又は３である）で表されるポリシロキサザン、特開平２−８４４３７号公報に報告されているような耐熱性に優れたポリボロシラザン、特開昭６３−８１１２２号、同６３−１９１８３２号、特開平２−７７４２７号公報に報告されているようなポリメタロシラザン、特開平１−１３８１０８号、同１−１３８１０７号、同１−２０３４２９号、同１−２０３４３０号公報に報告されているような分子量を増加させた改質ポリシラザン、特開平４−６３８３３号、同３−３２０１６７号公報に報告されているような耐加水分解性を向上させた改質ポリシラザン、特開平２−１７５７２６号、同５−８６２００号、同５−３３１２９３号、同３−３１３２６号公報に報告されているようなポリシラザンに有機成分を導入した厚膜化に有利な共重合ポリシラザン、特開平５−２３８８２７号、同６−１２２８５２号、同６−２９９１８８号、同６−３０６３２９号、同６−２４０２０８号、同７−１９６９８６号公報に報告されているようなポリシラザンに酸化を促進するための触媒化合物を付加又は添加した低温反応ポリシラザン等も同様に使用できる。
【００４６】
本発明における前記リチウム遷移金属複合窒化物の粒子の表面へのＳｉ−Ｎ結合を有する化合物の付着は、Ｓｉ−Ｎ結合を有する化合物（ポリシラザン等）を溶剤に溶解させてコーティング用組成物を調製し、これと前記リチウム遷移金属複合窒化物とを混合することにより行われる。コーティング用組成物に用いられる溶剤としては、脂肪族炭化水素、脂環式炭化水素、芳香族炭化水素の炭化水素溶剤、ハロゲン化メタン、ハロゲン化エタン、ハロゲン化ベンゼン等のハロゲン化炭化水素、脂肪族エーテル、脂環式エーテル等のエーテル類を使用することができる。好ましい溶剤は、ペンタンヘキサン、イソヘキサン、メチルペンタン、ヘプタン、イソヘプタン、オクタン、イソオクタン、シクロペンタン、メチルシクロペンタン、シクロヘキサン、メチルシクロヘキサン、ベンゼン、トルエン、キシレン、エチルベンゼン、エチルシクロヘキサン、シクロヘキセン、ｐ−メンタン、リモネン、デカリン、テトラリン、フェニルシクロヘキサン、シクロヘキサノン、ノナン、デカン、ｎ−ヘキサン、ペンタン、ジメチルジエトキシシラン、メチルトリエトキシシラン等の炭化水素、塩化メチレン、クロロホルム、四塩化炭素、ブロモホルム、塩化エチレン、塩化エチリデン、トリクロロエタン、テトラクロロエタン等のハロゲン化炭化水素、エチルエーテル、イソプロピルエーテル、エチルブチルエーテル、ブチルエーテル、１，２−ジオキシエタン、ジオキサン、ジメチルジオキサン、テトラヒドロフラン、テトラヒドロピラン等のエーテル類、等である。これらの溶剤を使用する場合、ポリシラザンの溶解度や溶剤の蒸発速度を調節するために、２種類以上の溶剤を混合してもよい。溶剤の使用量（割合）はポリシラザンの平均分子量、分子量分布、その構造によって異なるので、適宜、自由に混合することができる。好ましくは固形分濃度で１〜５０質量％の範囲で混合することができる。
【００４７】
Ｓｉ−Ｎ結合を有する化合物（ポリシラザン等）を付着したリチウム遷移金属複合窒化物の焼成は、窒素ガスに酸素ガスを混入した酸化性ガス雰囲気下で行われる。これにより、ポリシラザン中の窒素及び水素が酸素に置き換わることにより前記リチウム遷移金属複合窒化物の粒子の少なくとも表層部に、酸素、及びリチウムと合金を形成することができる元素とを含有させることができる。
【００４８】
焼成温度は特に限定されないが、一般に、３００〜５００℃の範囲内の温度が好ましい。焼成時間も特に限定されないが、一般に、０．５〜３時間の焼成時間が好ましい。
【００４９】
次に、本発明の電気化学素子として、リチウム二次電池について説明する。このリチウム二次電池は、負極と、正極と、非水電解質を有し、負極と正極はセパレータにより隔離されている。
【００５０】
負極は、前記リチウム含有電極材料に、必要であれば導電助剤、更に結着剤を加えて混合し、得られた合剤を適宜の手段で成形又は金属箔等の基体に塗布することにより形成できる。
【００５１】
前記導電助剤としては、構成されたリチウム二次電池において化学変化を起こさない電子伝導性材料であれば特に限定されない。通常、天然黒鉛（鱗状黒鉛、鱗片状黒鉛、土状黒鉛等）、人造黒鉛、カーボンブラック、アセチレンブラック、ケッチェンブラック、炭素繊維、金属粉（銅粉、ニッケル粉、アルミニウム粉、銀粉等）、金属繊維等、あるいは特開昭５９−２０９７１号公報に記載のポリフェニレン誘導体等の導電性材料を１種又はこれらの混合物として使用することができる。
【００５２】
前記結着剤としては、通常、でんぷん、ポリビニルアルコール、カルボキシメチルセルロース、ヒドロキシプロピルセルロース、再生セルロース、ジアセチルセルロース、ポリビニルクロリド、ポリビニルピロリドン、ポリテトラフルオロエチレン（ＰＴＦＥ）、ポリフッ化ビニリデン、ポリエチレン、ポリプロピレン、エチレン−プロピレン−ジエンターポリマー（ＥＰＤＭ）、スルホン化ＥＰＤＭ、スチレンブタジエンゴム、ブタジエンゴム、ポリブタジエン、フッ素ゴム、ポリエチレンオキシド、等の多糖類、熱可塑性樹脂、ゴム弾性を有するポリマー等やこれらの変成体を用いることができる。
【００５３】
正極材料としては、リチウムを含有する、チタン、モリブデン、タングステン、ニオブ、バナジウム、マンガン、鉄、クロム、ニッケル、コバルト等の遷移金属の複合酸化物、複合硫化物、複合硫酸化物等を用いることができる。例えば、ＬｉＣｏＯ_２のような一般式ＬｉＭＯ_２（Ｍは遷移金属元素）で表されるＲ３ｍを空間群に持つ層状酸化物、あるいはＬｉＭｎ_２Ｏ_４のような一般式ＬｉＭ_２Ｏ_４（Ｍは遷移金属元素）で表されるＦｄ３ｍを空間群に持つスピネル型酸化物等が正極材料としては好適である。
【００５４】
非水電解質としては、例えば、ＬｉＣＦ_３ＳＯ_３、ＬｉＣｌＯ_４、ＬｉＰＦ_６、ＬｉＢＦ_４、ＬｉＣ_４Ｆ_９ＳＯ_３等より選ばれる電解質を、１，２−ジメトキシエタン、１，２−ジエトキシエタン、プロピレンカーボネート、エチレンカーボネート、γ−ブチロラクトン、テトラヒドロフラン、１，３−ジオキソラン、ジエチルカーボネート、メチルエチルカーボネート等の有機溶媒から選ばれる１種又は２種以上の混合溶媒に溶解した非水電解液や、前記非水電解液を基材に含浸させてゲル化させたポリマーゲル電解質等が用いられ、リチウム二次電池に対する要求特性やその用途等に応じて適宜使い分けられる。
【００５５】
セパレータとしては、例えば不織布や微孔性フィルムが用いられる。前記不織布の材質としては、ポリプロピレン、ポリエチレン、ポリエチレンテレフタレート、ポリブチレンテレフタレート等がある。また、前記微孔性フィルムの材質としては、ポリプロピレン、ポリエチレン、エチレン−プロピレン共重合体等がある。
【００５６】
【実施例】
以下、本発明を実施例により説明する。但し、本発明はそれら実施例のみに限定されるものではない。また、下記の記載において「部」及び「％」は、特に断りがない限り、「質量部」及び「質量％」を意味する。
【００５７】
先ず、酸素、及びリチウムと合金を形成することができる元素を含む化合物１〜４を以下のように合成した。
【００５８】
（化合物１の合成）
窒素導入管を備え付けた反応容器にテトラヒドロフラン（ＴＨＦ）２５０部及びジメチルシロキサンと水酸基を有する反応性シリコーン（チッソ（株）製の“ＦＭ−４４１１”）８６．７部を計り込み、窒素ガス雰囲気下で沸点まで昇温した。この反応容器に４，４’−ジフェニルメタンジイソシアネート１３．３部を１時間にわたって滴下し、滴下終了後に還流しながら２．５時間反応させた。反応後の溶液は、不揮発分６４％の少なくとも酸素、及びリチウムと合金を形成することができるケイ素を含むウレタン系樹脂化合物であった。
【００５９】
（化合物２の合成）
窒素導入管を備え付けた反応容器にＴＨＦ３００部及びジメチルシロキサンと水酸基を有する反応性シリコーン（チッソ（株）製の“ＦＭ−４４２１”）９８．０部を計り込み、窒素ガス雰囲気下で沸点まで昇温した。この反応容器に４，４’−ジフェニルメタンジイソシアネート２．０部を１時間にわたって滴下し、滴下終了後に還流しながら３時間反応させた。反応後の溶液は、不揮発分４９％の少なくとも酸素、及びリチウムと合金を形成することができるケイ素を含むウレタン系樹脂化合物であった。
【００６０】
（化合物３の合成）
窒素導入管を備え付けた反応容器にＴＨＦ１５０部、ジメチルシロキサンと水酸基を有する反応性シリコーン（チッソ（株）製の“ＦＭ−４４２５”）９８．８部及び４，４’−ジフェニルメタンジイソシアネート１．２部を計り込み、窒素ガス雰囲気下で沸点まで昇温した後、還流しながら３時間反応させた。反応後の溶液は、不揮発分５８％の少なくとも酸素、及びリチウムと合金を形成することができるケイ素を含むウレタン系樹脂化合物であった。
【００６１】
（化合物４の合成）
メチルアクリレート１９．６部、ｎ−ブチルメタクリレート１５．４部、ラウリルメタクリレート１０．０部、スチレン３５．０部、ジメチルシロキサンと炭素数３のアルキレン基とからなるシリコーン系マクロモノマー（信越化学社製の“Ｘ−２２−１７４ＤＸ”）２０．０部、パーオキシエステル（日本油脂社製の“パーブチルＯ”）８．０部を混合して溶液を調製した。
【００６２】
次に、窒素導入管を備え付けた反応容器にＴＨＦ１００部を計り込み、窒素ガス雰囲気下で沸点まで昇温した。この反応容器に上記溶液全量を２時間にわたって滴下し、滴下終了後に還流しながら８時間反応させた。反応後の溶液は、不揮発分６１％の少なくとも酸素、及びリチウムと合金を形成することができるケイ素を含むアクリル系樹脂化合物であった。
【００６３】
次に、以下のようにリチウム含有電極材料を製造した。
【００６４】
（実施例１）
窒化リチウム（Ｌｉ_３Ｎ）と金属コバルト（Ｃｏ）をモル比で２：１の割合で混合し、その混合物を銅製のるつぼに入れ、窒素ガス雰囲気下７００℃で６時間焼成し、リチウム遷移金属複合窒化物（Ｌｉ_５ＣｏＮ_２）の粉末を得た。ＴＨＦ３ｇに前記化合物１を１ｇ溶解させた溶液中に上記リチウム遷移金属複合窒化物粉末１ｇを分散させ、ＴＨＦ２．５ｇ／ヘキサン２．５ｇの混合溶媒で希釈した。その後、この溶液をヘキサン１０ｇ中に滴下することにより、表面に前記化合物１を析出させたリチウム遷移金属複合窒化物の沈澱を得た。沈殿物を回収し、溶媒を除去した後、その沈殿物を銅製のるつぼに入れ、窒素ガス雰囲気下５００℃で１時間焼成し、本発明のリチウム含有電極材料の粉末を得た。得られた粉末の平均粒径は約１５μｍであった。
【００６５】
（実施例２）
前記化合物１に代えて前記化合物２を用いたこと以外は実施例１と同様の方法で、本発明のリチウム含有電極材料の粉末を得た。得られた粉末の平均粒径は約１５μｍであった。
【００６６】
（実施例３）
前記化合物１に代えて前記化合物３を用いたこと以外は実施例１と同様の方法で、本発明のリチウム含有電極材料の粉末を得た。得られた粉末の平均粒径は約１５μｍであった。
【００６７】
（実施例４）
前記化合物１に代えて前記化合物４を用いたこと以外は実施例１と同様の方法で、本発明のリチウム含有電極材料の粉末を得た。得られた粉末の平均粒径は約２０μｍであった。
【００６８】
（実施例５）
実施例１と同様の方法で、リチウム遷移金属複合窒化物の粉末を得た。次に、下記化学式（化４）で表される化合物５と前記リチウム遷移金属複合窒化物の粉末を各１ｇずつ混合し、この混合物を銅製のるつぼに入れ、窒素ガス雰囲気下５００℃で１時間焼成し、本発明のリチウム含有電極材料の粉末を得た。得られた粉末の平均粒径は約２０μｍであった。
【００６９】
【化４】

【００７０】
（実施例６）
実施例１と同様の方法で、リチウム遷移金属複合窒化物の粉末を得た。次に、得られた粉末をペルヒドロポリシラザン２０％含有キシレン溶液に分散し、しばらく攪拌した後に沈降させることで、表面にペルヒドロポリシラザンを付着させたリチウム遷移金属複合窒化物を得た。これを窒素９５体積％、酸素５体積％の混合ガス雰囲気下５００℃で１時間焼成し、本発明のリチウム含有電極材料の粉末を得た。得られた粉末の平均粒径は約１０μｍであり、ケイ素の含有量は１１％であった。
【００７１】
（実施例７）
実施例１と同様の方法で、リチウム遷移金属複合窒化物の粉末を得た。次に、得られた粉末を、あらかじめ重合させたテトラエトキシシランを溶解させたトルエン中に分散させ、ＳｉＯ_ｘ（０＜ｘ≦２）とＬｉ_５ＣｏＮ_２からなるスラリーを得た。得られたスラリーの溶媒を除去し、真空乾燥を行った後、その混合物を銅製のるつぼに入れ、窒素ガス雰囲気下５００℃で１時間焼成し、本発明のリチウム含有電極材料の粉末を得た。得られた粉末の平均粒径は約２０μｍであった。
【００７２】
（実施例８）
窒化リチウム（Ｌｉ_３Ｎ）と金属銅（Ｃｕ）をモル比で２：１の割合で混合し、その混合物を銅製のるつぼに入れ、窒素ガス雰囲気下６００℃で６時間焼成し、リチウム遷移金属複合窒化物（Ｌｉ_５ＣｕＮ_２）の粉末を得た。ＴＨＦ３ｇに前記化合物１を１ｇ溶解させた溶液中に上記リチウム遷移金属複合窒化物の粉末１ｇを分散させ、ＴＨＦ２．５ｇ／ヘキサン２．５ｇの混合溶媒で希釈した。その後、この溶液をヘキサン１０ｇ中に滴下することにより、表面に前記化合物１を析出させたリチウム遷移金属複合窒化物の沈澱を得た。沈殿物を回収し、溶媒を除去した後、その沈殿物を銅製のるつぼに入れ、窒素ガス雰囲気下５００℃で１時間焼成し、本発明のリチウム含有電極材料の粉末を得た。得られた粉末の平均粒径は約２０μｍであった。
【００７３】
（実施例９）
実施例８と同様の方法で、リチウム遷移金属複合窒化物の粉末を得た。次に、得られた粉末をペルヒドロポリシラザン２０％含有キシレン溶液に分散し、しばらく攪拌した後に沈降させることで、表面にペルヒドロポリシラザンを付着させたリチウム遷移金属複合窒化物を得た。これを窒素９５体積％、酸素５体積％の混合ガス雰囲気下５００℃で１時間焼成し、本発明のリチウム含有電極材料の粉末を得た。得られた粉末の平均粒径は約２０μｍであった。
【００７４】
（実施例１０）
ぺルヒドロポリシラザンを付着させたリチウム遷移金属複合窒化物の焼成条件が、窒素ガス雰囲気下５００℃で１時間焼成したこと以外は実施例６と同様の方法で、本発明のリチウム含有電極材料の粉末を得た。得られた粉末の平均粒径は約１０μｍであり、ケイ素の含有量は１２％であった。
【００７５】
次に、上記実施例１〜１０で製造したリチウム含有電極材料の粒子の表層部の組成分析を行った。分析方法はＸ線光電子分光法（ＸＰＳ）を用い、分析装置はＵＬＶＡＣ ＰＨＩ（アルバック ファイ）製のＥＳＣＡ ５５００ＭＣを用いた。その結果を図１〜図４に示す。図１〜図４は、実施例１で製造したリチウム含有電極材料の粒子の表面から１０００ｎｍまでの酸素、ケイ素、コバルト及びリチウムのそれぞれの含有量を任意単位でプロットした図である。図１〜図４から明らかなように、前記リチウム遷移金属複合窒化物の粒子に含有された酸素及びケイ素の含有量は、粒子の表面近傍から内部に向かって減少し、一方、コバルトの含有量は、粒子の表面近傍から内部に向かって増加し、傾斜材料を形成していることがわかる。更に、リチウムの含有量は、深さ方向にほぼ一様に分布していることもわかる。また、実施例２〜１０で製造したリチウム含有電極材料も全て同様に傾斜材料を形成していた。
【００７６】
（比較例１）
実施例１と同様の方法で、リチウム遷移金属複合窒化物の粉末を得た。得られた粉末に酸素、及びリチウムと合金を形成することができる元素の導入は行わなかった。得られた粉末の平均粒径は約１５μｍであった。
【００７７】
（比較例２）
実施例８と同様の方法で、リチウム遷移金属複合窒化物の粉末を得た。得られた粉末に酸素、及びリチウムと合金を形成することができる元素の導入は行わなかった。得られた粉末の平均粒径は約１５μｍであった。
【００７８】
（比較例３）
実施例１と同様の方法で、リチウム遷移金属複合窒化物の粉末を得た。得られた粉末に一酸化ケイ素（ＳｉＯ）粉末を混合した後、その混合物を銅製のるつぼに入れ、窒素ガス雰囲気下５００℃で１時間焼成し、リチウム含有電極材料の粉末を得た。得られた粉末の平均粒径は約２０μｍであった。
【００７９】
次に、比較例３で製造したリチウム含有電極材料の粒子の表層部の組成分析を行った。分析は前記実施例１〜１０と同様に行った。その結果、比較例３においてはリチウム遷移金属複合窒化物の粒子の表層部には、リチウムと合金を形成することができるケイ素は含有されていなかった。
【００８０】
次に、上記のようにして得られた実施例１〜１０及び比較例１〜３のリチウム含有電極材料の大気下での安定性を評価した。評価は、温度２５℃、湿度６０％の大気中に上記リチウム含有電極材料を放置し、放置開始からＸ線回折プロファイルに変化が生じるまでの経過時間を測定して行った。即ち、製造直後のリチウム含有電極材料のＸ線回折プロファイルと、大気中に放置した後のＸ線回折プロファイルを比較して、図５に示すようなＬｉＯＨやＬｉ_２ＣＯ_３等の分解生成物のピークを確認することにより、大気中での安定性を評価した。なお、図５は、比較例１のリチウム含有電極材料の放置開始前及び放置開始から４時間後のＸ線回折プロファイル、並びに実施例１０のリチウム含有電極材料の放置開始から４時間後のＸ線回折プロファイルをそれぞれ示したものである。また、測定は０．５時間ごとに行った。その結果を表１に示す。
【００８１】
【表１】

【００８２】
表１に示すように、酸素、及びリチウムと合金を形成することができる元素を導入しなかった比較例１及び比較例２のリチウム含有電極材料では大気下での安定性が低いのに対し、酸素、及びリチウムと合金を形成することができる元素を表層部に含む実施例１〜９のリチウム含有電極材料の大気下での安定性は高かった。また、表層部にリチウムと合金を形成することができる元素のみを含有させた実施例１０のリチウム含有電極材料は、同じ条件で酸素をも含有させた実施例６のリチウム含有電極材料に比べて大気下での安定性が多少劣っていた。
【００８３】
また、リチウム含有電極材料の製造方法として、リチウムと合金を形成することができる元素を含む単一の化合物を予め合成した後、その化合物をリチウム遷移金属複合窒化物の粒子の表面に付着させて焼成する手法を採用した実施例１〜１０のリチウム含有電極材料は、酸素、及びリチウムと合金を形成することができる元素を含む化合物と、リチウム遷移金属複合窒化物とを混合したのみで、表面に付着させる処理を行わずに焼成した比較例３のリチウム含有電極材料に比べ、大気下でも高い安定性を示した。
【００８４】
次に、上記実施例１〜１０で製造したリチウム含有電極材料を用いてリチウム二次電池を試作した。なお、リチウム含有電極材料は、温度２５℃、湿度６０％の大気中での放置時間が１時間以内のものを用いた。
【００８５】
先ず、負極材料として前記リチウム含有電極材料１０ｍｇ、導電助剤として人造黒鉛９ｍｇ、結着剤としてＰＴＦＥ１ｍｇを混合して調製した負極合剤を金型内に充填し、９８ＭＰａの圧力で直径１０ｍｍの円盤状に加圧成形して負極を作製した。また、正極材料としてコバルト酸リチウム（ＬｉＣｏＯ_２）６７ｍｇ、導電助剤としてアセチレンブラック３．７ｍｇ、結着剤としてＰＴＦＥ３．７ｍｇを混合して調製した正極合剤を金型内に充填し、９８ＭＰａの圧力で直径１０ｍｍの円盤状に加圧成形して正極を作製した。これらの正極、負極の間にポリエチレン製のセパレータを配置し、非水電解質としてはエチレンカーボネートとメチルエチルカーボネートとの体積比１：２の混合溶媒にＬｉＰＦ_６を１．２ｍｏｌ／ｄｍ^３溶解したものを用いて、直径２０ｍｍ、厚さ１．６ｍｍのコイン型リチウム二次電池を作製した。これらの９種類の電池について、電流値０．５ｍＡ／ｃｍ^２、室温で０．０１〜１．４Ｖの電圧範囲で充放電特性を調べた。その結果を表２に示す。
【００８６】
【表２】

【００８７】
表２から明らかなように、初期放電容量はいずれも８００ｍＡｈ／ｇ以上と高容量であった。また、すべての電池で３０サイクル後の放電容量は、初期放電容量の８０％以上を維持しており、サイクル特性も良好であった。
【００８８】
【発明の効果】
以上のように、本発明によれば、高容量で且つサイクル特性に優れた電気化学素子を構成することのできる電極材料として、大気中での安定性に優れたリチウム含有電極材料を提供することができる。
【図面の簡単な説明】
【図１】実施例１で製造したリチウム含有電極材料の粒子の表面から１０００ｎｍまでの酸素の含有量（任意単位）をプロットした図である。
【図２】実施例１で製造したリチウム含有電極材料の粒子の表面から１０００ｎｍまでのケイ素の含有量（任意単位）をプロットした図である。
【図３】実施例１で製造したリチウム含有電極材料の粒子の表面から１０００ｎｍまでのコバルトの含有量（任意単位）をプロットした図である。
【図４】実施例１で製造したリチウム含有電極材料の粒子の表面から１０００ｎｍまでのリチウムの含有量（任意単位）をプロットした図である。
【図５】実施例１０で製造したリチウム含有電極材料の大気中放置４時間後、及び比較例１で製造したリチウム含有電極材料の大気中放置前及び放置４時間後のＸ線回折プロファイルを示した図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a lithium-containing electrode material that can be used for electrochemical devices such as lithium secondary batteries and capacitors, a method for producing the same, and an electrochemical device using the lithium-containing electrode material for a negative electrode.
[0002]
[Prior art]
Conventionally, as an electrode material used as a negative electrode active material of an electrochemical element having a non-aqueous electrolyte, for example, in a lithium secondary battery, a layered compound such as graphite capable of reversibly inserting and removing lithium. Alternatively, a metal or metal oxide capable of forming an alloy with lithium has been used. In the former graphite-based negative electrode material, only the insertion and extraction of lithium between the layers of the layered compound occur during charging and discharging, and as a result, there is no significant change in the crystal structure of the electrode material itself. Shows good reversibility to cycles. In the latter negative electrode material which is alloyed with lithium, for example, in the case of silicon, three or more lithiums react with one silicon, so that a large discharge capacity exceeding 1000 mAh / g is exhibited.
[0003]
[Problems to be solved by the invention]
However, since the graphite-based negative electrode material utilizes the ingress and egress of lithium between the layers, it can only react with at most one lithium to six carbons. Therefore, a capacity of 372 mAh / g or more is theoretically obtained. It is difficult to achieve a large capacity from the current situation. Further, in the case of the negative electrode material accompanied by the alloying, the ratio of expansion and contraction of the electrode material particles due to the alloying with lithium is large, so that the electrode material particles are finely divided and the cycle characteristics are deteriorated. There is.
[0004]
Then, this invention was made in order to solve the said conventional problem, and it aims at providing the lithium-containing electrode material which can comprise an electrochemical element with high capacity and excellent cycle characteristics. .
[0005]
[Means for Solving the Problems]
In order to achieve the above object, the lithium-containing electrode material of the present invention has at least a surface layer of lithium transition metal composite nitride particles, Silicon Characterized by containing When I do.
[0006]
Further, the lithium-containing electrode material of the present invention, at least the surface layer of lithium transition metal composite nitride particles, oxygen, Silicon Characterized by containing When I do.
[0007]
The skeleton of the particles of the lithium-containing electrode material of the present invention has the same structure as that of the lithium transition metal composite nitride mainly having a layer structure. Thereby, while exhibiting good reversibility with respect to insertion / desorption of lithium due to an electrochemical oxidation-reduction cycle, high capacity can be achieved. Further, the particles of the lithium-containing electrode material include: Silicon Is contained. Therefore, lithium and Silicon Can also occlude a large amount of lithium by forming an alloy with the same. Further, in the surface layer of the particles of the lithium-containing electrode material, The silicon , The contact of the lithium transition metal composite nitride with moisture in the atmosphere can be reliably reduced, and the electrode material becomes stable in the atmosphere and can be easily handled. Further, the surface layer of the particles of the lithium-containing electrode material, oxygen, The silicon The stability in the atmosphere is further improved by containing.
[0008]
Further, the lithium-containing electrode material of the present invention, lithium transition metal composite nitride, Silicon Is formed by attaching the compound to the surface of the particles of the lithium-transition metal composite nitride by, for example, mixing the compound with a compound containing the compound in a solvent, followed by firing.
[0009]
In the lithium-containing electrode material of the present invention, it is preferable that the transition metal element constituting the lithium transition metal composite nitride is at least one element selected from the group consisting of cobalt, copper, iron and nickel. . These elements are Li ₃ This is because it can be used as an electrochemical element while maintaining the structure of N.
[0010]
In addition, The silicon Is chemically stable and inexpensive.
[0011]
Further, the lithium-containing electrode material of the present invention was contained in the surface layer of the particles of the lithium transition metal composite nitride. Silicon Is preferably reduced from near the surface to the inside of the particles of the lithium transition metal composite nitride. As a result, even when stress is applied to the lithium-containing electrode material during the production or charge / discharge of a battery or the like, cracks are generated in the material and finer powdering proceeds, The silicon It is possible to prevent problems such as desorption from occurring, and maintain characteristics for a long period of time. Similarly, it is preferable that the content of oxygen contained in the surface layer of the particles of the lithium transition metal composite nitride also decreases from near the surface of the particles of the lithium transition metal composite nitride to the inside.
[0012]
The distribution of lithium and the transition metal element originally contained in the particles of the lithium transition metal composite nitride is not particularly limited, but lithium is substantially inward from near the surface of the electrode material toward the inside. It is preferable that the transition metal element is uniformly distributed, while the transition metal element increases from the vicinity of the surface of the electrode material toward the inside.
[0013]
That is, in the lithium-containing electrode material of the present invention, in the surface layer, Silicon In a preferred embodiment, the content of oxygen or transition metal element continuously changes in the depth direction and is inclined.
[0014]
The expression “from the vicinity of the surface” as described above means that the composition only needs to change as described above from a portion slightly inside the surface of the particle toward the inside of the particle. In other words, it means that the composition change does not necessarily have to be at the very near portion (the portion having a depth of about 100 nm from the surface) of the particle surface.
[0015]
Next, the method for producing a lithium-containing electrode material of the present invention includes a lithium transition metal composite nitride, Silicon And then, after adhering the compound to the surface of the particles of the lithium-transition metal composite nitride, firing the compound.
[0016]
In the method for producing a lithium-containing electrode material of the present invention, the transition metal element constituting the lithium transition metal composite nitride is at least one element selected from the group consisting of cobalt, copper, iron and nickel. Is preferred.
[0017]
Further, in the method for producing a lithium-containing electrode material of the present invention, Silicon Is to use ,Previous As described above, it is chemically stable and the price is low.
[0018]
Further, the electrochemical device of the present invention is characterized in that the lithium-containing electrode material is used for a negative electrode. Thereby, an electrochemical device having high capacity and excellent cycle characteristics can be easily configured.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described.
[0020]
First, the lithium-containing electrode material of the present invention will be described. The lithium-containing electrode material of the present invention contains at least the surface layer of lithium transition metal composite nitride particles containing oxygen and an element capable of forming an alloy with lithium.
[0021]
As the lithium transition metal composite nitride, for example, Li _j Co _k N _m , Li _j Cu _k N _m , Li _j Fe _k N _m , Li _j Ni _k N _m Etc. can be used. Here, in general, j, k, and m are numbers of j> 0, k> 0, and m> 0, and preferably, are numbers of 0 <j <20, 0 <k <20, and 0 <m <20. Yes, most preferably 0 <j <7, 0 <k <2, 0 <m <5. Examples of lithium transition metal composite nitrides containing two or more transition metals include, for example, Li _j Co _k Ni _m N _n , Li _j Co _k Cu _m N _n , Li _j Cu _k Ni _m N _n Etc. can be used. Here, in general, j, k, m, and n are numbers of j> 0, k> 0, m> 0, and n> 0, and preferably, 0 <j <50, 0 <k <50, and 0 <. m <50, 0 <n <50, and most preferably 0 <j <30, 0 <k <10, 0 <m <5, 0 <n <20.
[0022]
The particle diameter of the particles of the lithium transition metal composite nitride is usually 0.1 to 100 μm, and more preferably 0.1 to 20 μm.
[0023]
The element capable of forming an alloy with oxygen and lithium contained in the particles of the lithium transition metal composite nitride only needs to be present at least in the surface layer of the particles, and the lithium storage / release capability in the surface layer is maintained. What is necessary is just to adjust the content within the range which sags. Specifically, in the mass of the lithium-containing electrode material of the present invention, it is considered that about 0.01 to 15% of oxygen and about 0.01 to 20% of an element capable of forming an alloy with lithium are more preferable. Is considered that oxygen is 0.05 to 10% and an element capable of forming an alloy with lithium is 0.05 to 15%.
[0024]
In addition, as an element capable of forming an alloy with lithium, silicon, tin, aluminum, or the like can be used.
[0025]
In addition, the lithium-containing electrode material of the present invention has a content of an element capable of forming an alloy with oxygen or lithium contained in the particles of the lithium transition metal composite nitride (hereinafter, also referred to as a surface element). It is preferable that the particle size decreases from near the surface of the lithium transition metal composite nitride particles toward the inside. The range in which the content of these surface elements changes is, for example, in the case where the particle size of the lithium transition metal composite nitride is about 5 to 20 μm, the range from the surface to the inside to about 1000 nm. preferable. However, these surface layer elements may be present inside beyond the range of 1000 nm.
[0026]
Next, in the method for producing a lithium-containing electrode material of the present invention, a lithium transition metal composite nitride and a compound containing an element capable of forming an alloy with at least lithium are mixed. After attaching a compound containing an element capable of forming an alloy with at least lithium to the surface of the particles, the compound is fired.
[0027]
Here, the method for producing a lithium-containing electrode material of the present invention will be described more specifically. The lithium transition metal composite nitride is preferably a mixture of transition metals such as lithium and cobalt, oxides, nitrides, halides, carbonates, and the like, preferably in an inert gas atmosphere such as nitrogen gas, or in an inert gas atmosphere. It can be obtained by a method such as firing in a temperature range of about 200 to 1000 ° C. in a reducing gas atmosphere in which a gas is mixed with hydrogen gas. The particle size of the fired lithium-containing electrode material is about the same as or slightly larger than the particle size of the original lithium transition metal composite nitride particles.
[0028]
Next, as a compound containing an element capable of forming an alloy with lithium, a simple substance such as silicon, tin, and aluminum, an oxide, a nitride, a halide, a carbonate, and an organic compound containing these elements are preferable. To synthesize a compound containing at least an element capable of forming an alloy with lithium. The obtained compound containing an element capable of forming an alloy with at least lithium is dissolved in an organic solvent, and the lithium transition metal composite nitride is dispersed in the solution. Subsequently, by mixing the dispersion and an organic solvent that does not dissolve a compound containing an element capable of forming an alloy with at least lithium, the at least lithium and the alloy are formed on the surface of the lithium transition metal composite nitride. By depositing a compound containing an element capable of forming, and then firing, oxygen and an element capable of forming an alloy with lithium are formed on at least the surface layer of the lithium transition metal composite nitride particles. Thus, a lithium-containing electrode material of the present invention can be obtained.
[0029]
The calcination can be performed in an inert gas atmosphere such as a nitrogen gas atmosphere or in a reducing gas atmosphere in which a hydrogen gas is mixed into an inert gas, and includes an element capable of forming an alloy with the lithium. When a compound containing no oxygen is selected as the compound, firing may be performed in an oxidizing gas atmosphere containing oxygen. The firing temperature is not particularly limited, but generally, a temperature in the range of 200 to 1000C is preferable. The baking time is not particularly limited, but generally, a baking time in the range of several tens minutes to several hours is preferable. By baking, an element capable of forming an alloy with oxygen and lithium can be contained in at least the surface layer of the lithium transition metal composite nitride particles. Further, since the surface element is diffused from the surface of the particle to the inside by this firing, in the lithium-containing electrode material produced according to the present invention, the content of the surface element is less than the particle of the lithium transition metal composite nitride. An inclined structure that decreases from near the surface to the inside can be realized.
[0030]
Note that a compound containing no oxygen was attached to the surface of the lithium transition metal composite nitride particles, and this was fired under a gas atmosphere containing no oxygen, such as under an inert gas atmosphere or a reducing gas atmosphere. In this case, a lithium-containing electrode material containing an element capable of forming an alloy with lithium in the surface layer portion but not containing oxygen is obtained. Such an electrode material is somewhat inferior in stability in the air as compared with the case where oxygen is also contained in the surface layer portion, but since the effects intended by the present invention can be obtained, such an electrode material is also used in the present invention. It is included in the lithium-containing electrode material of the invention. Also in the lithium-containing electrode material containing no oxygen in the surface layer, the particle size of the lithium transition metal composite nitride, firing conditions, the inclined structure of the surface layer, and the like are the same as those described above. However, in order to more effectively achieve the object of the present invention, it is desirable to include both an element capable of forming an alloy with lithium and oxygen in at least the surface layer of the lithium transition metal composite nitride particles. In the present invention, such a lithium-containing electrode material is a more preferred embodiment.
[0031]
In addition, a compound having an Si—N (silazane) bond is used as a compound containing an element capable of forming an alloy with lithium, and the compound having the Si—N bond is provided on the surface of the lithium transition metal composite nitride particles. After attaching the compound, the same lithium-containing electrode material as described above can also be obtained by baking the compound. Of course, when oxygen is not contained in the compound having a Si-N bond, firing is performed in an oxidizing gas atmosphere containing oxygen, so that at least the surface layer of the lithium-containing electrode material contains silicon and oxygen. This is the same as described above in that if firing is performed in an inert gas atmosphere or a reducing gas atmosphere, only silicon can be contained.
[0032]
As the compound having an Si—N (silazane) bond used as a compound containing an element capable of forming an alloy with lithium in the present invention, polysilazane such as hexamethyldisilazane and perhydropolysilazane is preferably used.
[0033]
Polysilazane is an inorganic polymer having a number average molecular weight of 100 to 50,000 having a main skeleton composed of a structural unit represented by the following general formula (I).
[0034]
Embedded image

[0035]
In the above formula, R ¹ , R ² And R ³ Each independently represents a hydrogen atom, an alkyl group, an alkenyl group, a cycloalkyl group, an aryl group, or a group other than these groups in which a group directly bonded to silicon is carbon, an alkylsilyl group, an alkylamino group, or an alkoxy group . Here, a compound containing an element capable of forming an alloy with at least lithium is densely attached to the surface of the particles of the lithium transition metal composite nitride, and at least the particles of the lithium transition metal composite nitride are surely adhered. In order for the surface layer to contain oxygen and an element capable of forming an alloy with lithium, it is preferable to use a polysilazane which forms a crosslinked structure by firing. ¹ , R ² And R ³ Is preferably a hydrogen atom.
[0036]
The polysilazane can be used alone, but can also be used as a copolymer of silazane (monomer) and another monomer, or as a mixture of polysilazane and another compound. The polysilazane to be used includes a polysilazane having a chain, cyclic or cross-linked structure, or a polysilazane having a plurality of these structures in the molecule at the same time. Above all, perhydropolysilazane is most preferable because it can be densely attached to the surface of the lithium transition metal composite nitride particles. In the general formula (I), R ¹ , R ² And R ³ Having a hydrogen atom is perhydropolysilazane, and its production method is described, for example, in JP-B-63-16325; Seeyferth et al., Communication of Am. Cer. Soc. , C-13, January (1983). An example of the structure of perhydropolysilazane is shown below.
[0037]
Embedded image

[0038]
In the general formula (I), R ¹ And R ² A hydrogen atom, R ³ The method for producing a polysilazane having a methyl group in Seeyferth et al., Polym. Prepr. , Am. Chem. Soc. , Div. Polym. Chem. , 25 , 10 (1984). The polysilazane obtained by this method has a repeating unit of-(SiH ₂ NCH ₃ )-Is a chain polymer and a cyclic polymer, and neither has a crosslinked structure.
[0039]
In addition, R in the general formula (I) ¹ And R ³ A hydrogen atom, R ² The method for producing a polyorgano (hydro) silazane having an organic group at Seeyferth et al., Polym. Prepr. , Am. Chem. Soc. , Div. Polym. Chem. , 25 , 10 (1984) or JP-A-61-89230. The polysilazanes obtained by these methods include-(R ² SiHNH)-as a repeating unit having a cyclic structure mainly having a degree of polymerization of 3 to 5, or (R ³ SiHNH) _x [(R ² SiH) _1.5 N] _1-x Some of the molecules represented by the chemical formula (0.4 <x <1) have both a chain structure and a cyclic structure in the molecule.
[0040]
In the general formula (I), R ¹ A hydrogen atom, R ² And R ³ A polysilazane having an organic group at ¹ And R ² An organic group, R ³ Is a polysilazane having a hydrogen atom at-(R ¹ R ² SiNR ³ )-Is a repeating unit, and has a cyclic structure mainly having a degree of polymerization of 3 to 5.
[0041]
The polysilazane used in the present invention has a main skeleton composed of the unit represented by the general formula (I). However, the unit represented by the general formula (I) may be cyclized as described above, In that case, the cyclic portion becomes a terminal group, and when such cyclization is not performed, the terminal of the main skeleton is R ¹ , R ² , R ³ And a hydrogen atom.
[0042]
Some of the polyorgano (hydro) silazanes include D.I. Seeyferth et al., Communication of Am. Cer. Soc. , C-132, July (1984) have a crosslinked structure in the molecule. Further, as disclosed in JP-A-49-69717, R ¹ Six ₃ (X: halogen) having a crosslinked structure obtained by ammonia decomposition of polysilazane (R ¹ Si (NH) _x ) Or R ¹ Six ₃ And R ¹ R ² Six ₂ The polysilazane having the following structure obtained by co-ammonium decomposition of
[0043]
Embedded image

[0044]
In the above formula, m and n are positive integers.
[0045]
Further, as the compound having an Si—N bond used as the compound containing an element capable of forming an alloy with lithium in the present invention, polymetallosilazane containing a metal atom which is a modified product of polysilazane can also be used. In addition, a repeating unit as reported in JP-A-62-195024 is [(SiH ₂ ) _n (NH) _m ] And [(SiH ₂ ) _r O] (wherein, n, m and r are 1, 2 and 3 respectively), and polyboroxane excellent in heat resistance as reported in JP-A-2-84437. Silazane, polymetallosilazane reported in JP-A-63-81212, JP-A-63-191832, JP-A-2-77427, JP-A-1-138108, JP-A-138107, JP-A-1-138108 Modified polysilazane having an increased molecular weight as reported in JP-A-203429 and JP-A-203430, and having a hydrolysis resistance as reported in JP-A-4-63833 and JP-A-3-320167. Improved modified polysilazanes, such as those reported in JP-A-2-175726, JP-A-5-86200, JP-A-5-331293 and JP-A-3-31326. Copolymerized polysilazane which is advantageous for thickening by introducing an organic component, JP-A-5-238827, JP-A-6-122852, JP-A-6-299188, JP-A-6-306329, JP-A-6-240208, and JP-A-7-196986 Low-temperature-reacted polysilazane obtained by adding or adding a catalyst compound for accelerating oxidation to polysilazane as reported in Japanese Patent Application Laid-Open No. H10-163, can also be used.
[0046]
In the present invention, the compound having a Si-N bond is attached to the surface of the lithium transition metal composite nitride particles by dissolving the compound having a Si-N bond (such as polysilazane) in a solvent to prepare a coating composition. Then, it is performed by mixing this with the lithium transition metal composite nitride. Examples of the solvent used in the coating composition include aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbon hydrocarbon solvents, halogenated hydrocarbons such as halogenated methane, halogenated ethane, and halogenated benzene; Ethers such as aromatic ethers and alicyclic ethers can be used. Preferred solvents are pentanehexane, isohexane, methylpentane, heptane, isoheptane, octane, isooctane, cyclopentane, methylcyclopentane, cyclohexane, methylcyclohexane, benzene, toluene, xylene, ethylbenzene, ethylcyclohexane, cyclohexene, p-menthane, limonene. , Decalin, tetralin, phenylcyclohexane, cyclohexanone, nonane, decane, n-hexane, pentane, dimethyldiethoxysilane, methyltriethoxysilane and other hydrocarbons, methylene chloride, chloroform, carbon tetrachloride, bromoform, ethylene chloride, ethylidene chloride , Trichloroethane, halogenated hydrocarbons such as tetrachloroethane, ethyl ether, isopropyl ether, ethyl butyl ether Butyl ether, 1,2-dioxyethane, dioxane, dimethyl dioxane, tetrahydrofuran, ethers such as tetrahydropyran, and the like. When these solvents are used, two or more solvents may be mixed in order to adjust the solubility of polysilazane and the evaporation rate of the solvent. The amount (ratio) of the solvent used depends on the average molecular weight, molecular weight distribution, and structure of the polysilazane, and can be freely mixed as appropriate. Preferably, they can be mixed in a solid content range of 1 to 50% by mass.
[0047]
The calcination of the lithium transition metal composite nitride to which the compound having a Si-N bond (such as polysilazane) is attached is performed in an oxidizing gas atmosphere in which oxygen gas is mixed with nitrogen gas. Thereby, by replacing nitrogen and hydrogen in the polysilazane with oxygen, at least the surface layer of the particles of the lithium transition metal composite nitride can contain oxygen and an element capable of forming an alloy with lithium. .
[0048]
The firing temperature is not particularly limited, but generally, a temperature in the range of 300 to 500C is preferable. The firing time is not particularly limited, but generally, a firing time of 0.5 to 3 hours is preferable.
[0049]
Next, a lithium secondary battery will be described as the electrochemical device of the present invention. This lithium secondary battery has a negative electrode, a positive electrode, and a non-aqueous electrolyte, and the negative electrode and the positive electrode are separated by a separator.
[0050]
The negative electrode is prepared by mixing the obtained lithium-containing electrode material with a conductive auxiliary agent, if necessary, further adding a binder, and applying the obtained mixture to a base such as molding or metal foil by an appropriate means. Can be formed.
[0051]
The conductive auxiliary agent is not particularly limited as long as it is an electron conductive material that does not cause a chemical change in the configured lithium secondary battery. Usually, natural graphite (flaky graphite, flaky graphite, earthy graphite, etc.), artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, metal powder (copper powder, nickel powder, aluminum powder, silver powder, etc.), A conductive material such as a metal fiber or a polyphenylene derivative described in JP-A-59-20971 can be used alone or as a mixture thereof.
[0052]
Examples of the binder include starch, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, regenerated cellulose, diacetyl cellulose, polyvinyl chloride, polyvinyl pyrrolidone, polytetrafluoroethylene (PTFE), polyvinylidene fluoride, polyethylene, polypropylene, and ethylene. -Polypropylene such as propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber, butadiene rubber, polybutadiene, fluororubber, polyethylene oxide, thermoplastic resin, rubber elastic polymer and the like, and modified products thereof. Can be used.
[0053]
As the positive electrode material, a composite oxide, a composite sulfide, a composite sulfate of a transition metal such as titanium, molybdenum, tungsten, niobium, vanadium, manganese, iron, chromium, nickel, and cobalt containing lithium may be used. it can. For example, LiCoO ₂ General formula LiMO such as ₂ (M is a transition metal element) a layered oxide having R3m in a space group, or LiMn ₂ O ₄ General formula LiM such as ₂ O ₄ A spinel-type oxide or the like having Fd3m represented by (M is a transition metal element) in a space group is suitable as the positive electrode material.
[0054]
As the non-aqueous electrolyte, for example, LiCF ₃ SO ₃ , LiClO ₄ , LiPF ₆ , LiBF ₄ , LiC ₄ F ₉ SO ₃ An electrolyte selected from the group consisting of organic solvents such as 1,2-dimethoxyethane, 1,2-diethoxyethane, propylene carbonate, ethylene carbonate, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolan, diethyl carbonate, and methyl ethyl carbonate. Non-aqueous electrolyte dissolved in one or two or more kinds of mixed solvents selected from the group consisting of a polymer gel electrolyte and the like, which is obtained by impregnating the substrate with the non-aqueous electrolyte and gelling the same, is used. It can be properly used depending on the required characteristics and its use.
[0055]
As the separator, for example, a nonwoven fabric or a microporous film is used. Examples of the material of the nonwoven fabric include polypropylene, polyethylene, polyethylene terephthalate, and polybutylene terephthalate. Examples of the material of the microporous film include polypropylene, polyethylene, and ethylene-propylene copolymer.
[0056]
【Example】
Hereinafter, the present invention will be described with reference to examples. However, the present invention is not limited to only these examples. In the following description, “parts” and “%” mean “parts by mass” and “% by mass”, respectively, unless otherwise specified.
[0057]
First, compounds 1 to 4 containing elements capable of forming an alloy with oxygen and lithium were synthesized as follows.
[0058]
(Synthesis of Compound 1)
In a reaction vessel equipped with a nitrogen inlet tube, 250 parts of tetrahydrofuran (THF) and 86.7 parts of dimethylsiloxane and a reactive silicone having a hydroxyl group (“FM-4411” manufactured by Chisso Corporation) are measured, and the mixture is placed under a nitrogen gas atmosphere. To the boiling point. To this reaction vessel, 13.3 parts of 4,4′-diphenylmethane diisocyanate was added dropwise over 1 hour, and after the completion of the addition, the mixture was reacted for 2.5 hours while refluxing. The solution after the reaction was a urethane-based resin compound containing at least oxygen having a nonvolatile content of 64% and silicon capable of forming an alloy with lithium.
[0059]
(Synthesis of Compound 2)
In a reaction vessel equipped with a nitrogen inlet tube, 300 parts of THF and 98.0 parts of reactive silicone having dimethylsiloxane and a hydroxyl group (“FM-4421” manufactured by Chisso Corporation) are weighed, and the mixture is heated to the boiling point under a nitrogen gas atmosphere. Warmed up. 2.0 parts of 4,4'-diphenylmethane diisocyanate was added dropwise to this reaction vessel over 1 hour, and after completion of the addition, the mixture was reacted for 3 hours while refluxing. The solution after the reaction was a urethane-based resin compound containing at least oxygen having a nonvolatile content of 49% and silicon capable of forming an alloy with lithium.
[0060]
(Synthesis of Compound 3)
In a reaction vessel equipped with a nitrogen inlet tube, 150 parts of THF, 98.8 parts of dimethylsiloxane and a reactive silicone having a hydroxyl group (“FM-4425” manufactured by Chisso Corporation) and 1.2 parts of 4,4′-diphenylmethane diisocyanate Was measured, the temperature was raised to the boiling point under a nitrogen gas atmosphere, and the mixture was reacted under reflux for 3 hours. The solution after the reaction was a urethane-based resin compound containing at least oxygen having a nonvolatile content of 58% and silicon capable of forming an alloy with lithium.
[0061]
(Synthesis of Compound 4)
19.6 parts of methyl acrylate, 15.4 parts of n-butyl methacrylate, 10.0 parts of lauryl methacrylate, 35.0 parts of styrene, a silicone-based macromonomer composed of dimethylsiloxane and an alkylene group having 3 carbon atoms (manufactured by Shin-Etsu Chemical Co., Ltd.) Of "X-22-174DX") and 8.0 parts of peroxyester ("Perbutyl O" manufactured by NOF CORPORATION) were mixed to prepare a solution.
[0062]
Next, 100 parts of THF was weighed into a reaction vessel equipped with a nitrogen introduction tube, and the temperature was raised to the boiling point under a nitrogen gas atmosphere. The whole amount of the solution was dropped into this reaction vessel over 2 hours, and after the completion of the dropping, the reaction was carried out for 8 hours while refluxing. The solution after the reaction was an acrylic resin compound containing at least oxygen having a nonvolatile content of 61% and silicon capable of forming an alloy with lithium.
[0063]
Next, a lithium-containing electrode material was manufactured as follows.
[0064]
(Example 1)
Lithium nitride (Li ₃ N) and metallic cobalt (Co) at a molar ratio of 2: 1. The mixture was placed in a copper crucible and calcined at 700 ° C. for 6 hours in a nitrogen gas atmosphere to obtain a lithium transition metal composite nitride (Li). ₅ CoN ₂ ) Was obtained. 1 g of the lithium-transition metal composite nitride powder was dispersed in a solution of 1 g of the compound 1 in 3 g of THF, and diluted with a mixed solvent of 2.5 g of THF / 2.5 g of hexane. Thereafter, this solution was dropped into 10 g of hexane to obtain a precipitate of a lithium transition metal composite nitride in which the compound 1 was deposited on the surface. After the precipitate was collected and the solvent was removed, the precipitate was placed in a copper crucible and fired at 500 ° C. for 1 hour in a nitrogen gas atmosphere to obtain a powder of the lithium-containing electrode material of the present invention. The average particle size of the obtained powder was about 15 μm.
[0065]
(Example 2)
A powder of the lithium-containing electrode material of the present invention was obtained in the same manner as in Example 1 except that Compound 2 was used instead of Compound 1. The average particle size of the obtained powder was about 15 μm.
[0066]
(Example 3)
A powder of the lithium-containing electrode material of the present invention was obtained in the same manner as in Example 1, except that Compound 3 was used instead of Compound 1. The average particle size of the obtained powder was about 15 μm.
[0067]
(Example 4)
A powder of the lithium-containing electrode material of the present invention was obtained in the same manner as in Example 1 except that Compound 4 was used instead of Compound 1. The average particle size of the obtained powder was about 20 μm.
[0068]
(Example 5)
In the same manner as in Example 1, a lithium transition metal composite nitride powder was obtained. Next, 1 g of each of the compound 5 represented by the following chemical formula (Chemical Formula 4) and the powder of the lithium transition metal composite nitride are mixed, and the mixture is placed in a copper crucible, and is placed in a nitrogen gas atmosphere at 500 ° C. for 1 hour. By firing, a powder of the lithium-containing electrode material of the present invention was obtained. The average particle size of the obtained powder was about 20 μm.
[0069]
Embedded image

[0070]
(Example 6)
In the same manner as in Example 1, a lithium transition metal composite nitride powder was obtained. Next, the obtained powder was dispersed in a xylene solution containing 20% of perhydropolysilazane, stirred for a while, and then settled to obtain a lithium transition metal composite nitride having perhydropolysilazane adhered to the surface. This was fired at 500 ° C. for 1 hour in a mixed gas atmosphere of 95% by volume of nitrogen and 5% by volume of oxygen to obtain a powder of the lithium-containing electrode material of the present invention. The average particle size of the obtained powder was about 10 μm, and the silicon content was 11%.
[0071]
(Example 7)
In the same manner as in Example 1, a lithium transition metal composite nitride powder was obtained. Next, the obtained powder is dispersed in toluene in which tetraethoxysilane previously polymerized is dissolved, _x (0 <x ≦ 2) and Li ₅ CoN ₂ Was obtained. After removing the solvent of the obtained slurry and performing vacuum drying, the mixture was put into a copper crucible and fired at 500 ° C. for 1 hour under a nitrogen gas atmosphere to obtain a powder of a lithium-containing electrode material of the present invention. . The average particle size of the obtained powder was about 20 μm.
[0072]
(Example 8)
Lithium nitride (Li ₃ N) and metallic copper (Cu) were mixed at a molar ratio of 2: 1. The mixture was placed in a copper crucible and calcined at 600 ° C. for 6 hours in a nitrogen gas atmosphere to obtain a lithium transition metal composite nitride (Li). ₅ CuN ₂ ) Was obtained. 1 g of the lithium transition metal composite nitride powder was dispersed in a solution in which 1 g of the compound 1 was dissolved in 3 g of THF, and diluted with a mixed solvent of 2.5 g of THF / 2.5 g of hexane. Thereafter, this solution was dropped into 10 g of hexane to obtain a precipitate of a lithium transition metal composite nitride in which the compound 1 was deposited on the surface. After the precipitate was collected and the solvent was removed, the precipitate was placed in a copper crucible and fired at 500 ° C. for 1 hour in a nitrogen gas atmosphere to obtain a powder of the lithium-containing electrode material of the present invention. The average particle size of the obtained powder was about 20 μm.
[0073]
(Example 9)
In the same manner as in Example 8, a lithium transition metal composite nitride powder was obtained. Next, the obtained powder was dispersed in a xylene solution containing 20% of perhydropolysilazane, stirred for a while, and then settled to obtain a lithium transition metal composite nitride having perhydropolysilazane adhered to the surface. This was fired at 500 ° C. for 1 hour in a mixed gas atmosphere of 95% by volume of nitrogen and 5% by volume of oxygen to obtain a powder of the lithium-containing electrode material of the present invention. The average particle size of the obtained powder was about 20 μm.
[0074]
(Example 10)
The lithium transition metal composite nitride to which perhydropolysilazane was attached was fired at 500 ° C. for 1 hour in a nitrogen gas atmosphere in the same manner as in Example 6 to obtain the lithium-containing electrode material of the present invention. A powder was obtained. The average particle size of the obtained powder was about 10 μm, and the content of silicon was 12%.
[0075]
Next, the composition analysis of the surface layer portion of the particles of the lithium-containing electrode material produced in Examples 1 to 10 was performed. The analysis method used was X-ray photoelectron spectroscopy (XPS), and the analysis apparatus used was ESCA 5500MC manufactured by ULVAC PHI (ULVAC-PHI). The results are shown in FIGS. 1 to 4 are diagrams in which the respective contents of oxygen, silicon, cobalt, and lithium from the surface of the particles of the lithium-containing electrode material manufactured in Example 1 to 1000 nm are plotted in arbitrary units. As is clear from FIGS. 1 to 4, the content of oxygen and silicon contained in the particles of the lithium transition metal composite nitride decreases from near the surface of the particles toward the inside, while the content of cobalt decreases. It increases from the vicinity of the surface of the particle toward the inside, and it is understood that a gradient material is formed. Further, it can be seen that the lithium content is distributed almost uniformly in the depth direction. In addition, all the lithium-containing electrode materials manufactured in Examples 2 to 10 similarly formed gradient materials.
[0076]
(Comparative Example 1)
In the same manner as in Example 1, a lithium transition metal composite nitride powder was obtained. Oxygen and elements capable of forming an alloy with lithium were not introduced into the obtained powder. The average particle size of the obtained powder was about 15 μm.
[0077]
(Comparative Example 2)
In the same manner as in Example 8, a lithium transition metal composite nitride powder was obtained. Oxygen and elements capable of forming an alloy with lithium were not introduced into the obtained powder. The average particle size of the obtained powder was about 15 μm.
[0078]
(Comparative Example 3)
In the same manner as in Example 1, a lithium transition metal composite nitride powder was obtained. After mixing the obtained powder with silicon monoxide (SiO) powder, the mixture was put into a copper crucible and fired at 500 ° C. for 1 hour in a nitrogen gas atmosphere to obtain a powder of a lithium-containing electrode material. The average particle size of the obtained powder was about 20 μm.
[0079]
Next, the composition of the surface layer of the particles of the lithium-containing electrode material produced in Comparative Example 3 was analyzed. The analysis was performed in the same manner as in Examples 1 to 10. As a result, in Comparative Example 3, silicon capable of forming an alloy with lithium was not contained in the surface portion of the lithium transition metal composite nitride particles.
[0080]
Next, the stability in air of the lithium-containing electrode materials of Examples 1 to 10 and Comparative Examples 1 to 3 obtained as described above was evaluated. The evaluation was performed by allowing the lithium-containing electrode material to stand in the air at a temperature of 25 ° C. and a humidity of 60%, and measuring the elapsed time from the start of the standing until a change occurred in the X-ray diffraction profile. That is, by comparing the X-ray diffraction profile of the lithium-containing electrode material immediately after the production and the X-ray diffraction profile after being left in the air, LiOH or Li as shown in FIG. ₂ CO ₃ The stability in the air was evaluated by confirming the peaks of decomposition products such as. FIG. 5 shows the X-ray diffraction profiles of the lithium-containing electrode material of Comparative Example 1 before and 4 hours after the start of standing, and the X-ray diffraction of the lithium-containing electrode material of Example 10 after 4 hours. It is each showing the diffraction profile. The measurement was performed every 0.5 hour. Table 1 shows the results.
[0081]
[Table 1]

[0082]
As shown in Table 1, the stability under the atmosphere was low in the lithium-containing electrode materials of Comparative Examples 1 and 2 in which oxygen and an element capable of forming an alloy with lithium were not introduced. The stability of the lithium-containing electrode materials of Examples 1 to 9 containing oxygen and an element capable of forming an alloy with lithium in the surface layer portion in the atmosphere was high. In addition, the lithium-containing electrode material of Example 10 in which only the element capable of forming an alloy with lithium was contained in the surface layer portion was compared with the lithium-containing electrode material of Example 6 in which oxygen was also contained under the same conditions. The stability under the atmosphere was somewhat poor.
[0083]
Further, as a method for producing a lithium-containing electrode material, after previously synthesizing a single compound containing an element capable of forming an alloy with lithium, the compound is attached to the surface of lithium transition metal composite nitride particles. The lithium-containing electrode materials of Examples 1 to 10 employing the firing method were prepared by mixing only a compound containing oxygen and an element capable of forming an alloy with lithium, and a lithium transition metal composite nitride. As compared with the lithium-containing electrode material of Comparative Example 3 which was calcined without performing the treatment for attaching to the electrode, it showed higher stability even in the atmosphere.
[0084]
Next, lithium secondary batteries were prototyped using the lithium-containing electrode materials manufactured in Examples 1 to 10 described above. The lithium-containing electrode material used was one that was left in the air at a temperature of 25 ° C. and a humidity of 60% within one hour.
[0085]
First, a negative electrode mixture prepared by mixing 10 mg of the lithium-containing electrode material as a negative electrode material, 9 mg of artificial graphite as a conductive aid, and 1 mg of PTFE as a binder is filled in a mold, and a disc having a diameter of 10 mm is applied under a pressure of 98 MPa. A negative electrode was prepared by press-molding into a shape. Lithium cobaltate (LiCoO) is used as a positive electrode material. ₂ A positive electrode mixture prepared by mixing 67 mg, 3.7 mg of acetylene black as a conductive auxiliary agent, and 3.7 mg of PTFE as a binder is filled in a mold, and pressed into a disk having a diameter of 10 mm at a pressure of 98 MPa. Thus, a positive electrode was produced. A polyethylene separator is disposed between the positive electrode and the negative electrode, and LiPF is used as a nonaqueous electrolyte in a mixed solvent of ethylene carbonate and methyl ethyl carbonate in a volume ratio of 1: 2. ₆ To 1.2 mol / dm ³ A coin-type lithium secondary battery having a diameter of 20 mm and a thickness of 1.6 mm was prepared using the melted material. For these nine types of batteries, a current value of 0.5 mA / cm ² The charge and discharge characteristics were examined at a room temperature and a voltage range of 0.01 to 1.4 V. Table 2 shows the results.
[0086]
[Table 2]

[0087]
As is clear from Table 2, the initial discharge capacities were as high as 800 mAh / g or more. In addition, the discharge capacity after 30 cycles of all batteries maintained 80% or more of the initial discharge capacity, and the cycle characteristics were also good.
[0088]
【The invention's effect】
As described above, according to the present invention, there is provided a lithium-containing electrode material having excellent stability in the atmosphere as an electrode material capable of forming an electrochemical device having high capacity and excellent cycle characteristics. Can be.
[Brief description of the drawings]
FIG. 1 is a diagram plotting the oxygen content (arbitrary unit) from the surface of particles of the lithium-containing electrode material manufactured in Example 1 to 1000 nm.
FIG. 2 is a diagram plotting the content (arbitrary unit) of silicon from the surface of the particles of the lithium-containing electrode material manufactured in Example 1 to 1000 nm.
FIG. 3 is a diagram plotting the cobalt content (arbitrary unit) from the surface of the particles of the lithium-containing electrode material manufactured in Example 1 to 1000 nm.
FIG. 4 is a diagram plotting the lithium content (arbitrary unit) from the surface of the particles of the lithium-containing electrode material manufactured in Example 1 to 1000 nm.
FIG. 5 shows X-ray diffraction profiles of the lithium-containing electrode material manufactured in Example 10 after being left in the air for 4 hours, and the lithium-containing electrode material manufactured in Comparative Example 1 before and after being left in the air for 4 hours. FIG.

Claims (9)

A lithium-containing electrode material characterized in that at least the surface layer of lithium transition metal composite nitride particles contains silicon . A lithium-containing electrode material, characterized in that oxygen and silicon are contained in at least a surface layer of lithium transition metal composite nitride particles. A lithium-containing electrode material formed by depositing a compound containing silicon on the surface of lithium transition metal composite nitride particles and then firing the compound. 4. The lithium-containing electrode material according to claim 1, wherein the transition metal element constituting the lithium transition metal composite nitride is at least one element selected from the group consisting of cobalt, copper, iron and nickel. . The content of silicon contained in the particles of the lithium transition metal composite nitride decreases from near the surface to the inside of the particles of the lithium transition metal composite nitride. Lithium-containing electrode material. The content of oxygen contained in the particles of the lithium transition metal composite nitride decreases from near the surface of the particles of the lithium transition metal composite nitride toward the inside. Lithium-containing electrode material. A lithium-containing electrode comprising mixing a lithium-transition metal composite nitride and a compound containing silicon , adhering the compound to the surface of the particles of the lithium-transition metal composite nitride, and firing the mixture. Material manufacturing method. The method for producing a lithium-containing electrode material according to claim 7, wherein the transition metal element forming the lithium transition metal composite nitride is at least one element selected from the group consisting of cobalt, copper, iron, and nickel. An electrochemical device using the lithium-containing electrode material according to any one of claims 1 to 6 for a negative electrode.

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