JP4456668B2 - Nonaqueous secondary battery and its positive electrode - Google Patents
Nonaqueous secondary battery and its positive electrode Download PDFInfo
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- JP4456668B2 JP4456668B2 JP2002116318A JP2002116318A JP4456668B2 JP 4456668 B2 JP4456668 B2 JP 4456668B2 JP 2002116318 A JP2002116318 A JP 2002116318A JP 2002116318 A JP2002116318 A JP 2002116318A JP 4456668 B2 JP4456668 B2 JP 4456668B2
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Description
【０００１】
【発明の属する技術分野】
本発明は、非水二次電池に関し、さらに詳しくは正極合剤中に用いる導電剤を改良した非水二次電池に関するものである。
【０００２】
【従来の技術】
近年の電子機器の小型化・携帯化に伴い、高エネルギー密度を有する二次電池の要求は、さらに高まっている。現在、この要求に応える二次電池として、正極活物質にリチウムコバルト複合酸化物などのリチウム複合酸化物を用い、負極活物質に主に炭素系材料を用いたリチウムイオン二次電池が商品化され、またさらなる高性能化が続けられている。これらの電池は、平均作動電圧が３．６Ｖと、従来のＮｉ−Ｃｄ電池やニッケル水素電池に比べて、約３倍高く、また負極に炭素系材料を用いていることなどから、軽量化が可能である。
【０００３】
非水二次電池では、反応面積を確保するため、シート状の正極と負極をセパレータや不織布などを介して捲回または積層した構造とされる。これらの電極は、活物質と結着剤を含む合剤層を集電体上に形成してなるものが一般的であるが、正極活物質であるＬｉＣｏＯ2 などのリチウム複合酸化物は、導電性に乏しく、比較的高い電気抵抗を有する。このため、この活物質を用いて正極を作製する際には、結着剤とともに導電剤を添加して合剤層とする必要がある。
【０００４】
ところで、非水二次電池は、前述のように作動電位が高く、また、たとえば、電池が充電されてＬｉＣｏＯ2 がＬｉを放出しＬｉ1-x ＣｏＯ2 となっているような状態では強い酸化作用を示す。このため、高い信頼性を持つ非水二次電池を得るには、導電剤や結着剤にも高い耐酸化性が要求される。
【０００５】
【発明が解決しようとする課題】
しかし、導電剤として広く用いられている黒鉛やカーボンブラックなどは耐酸化性に乏しく、Ｃｏなどの遷移金属の存在下ではより耐酸化性が低下するため、充放電の繰り返しや充電状態での長期貯蔵により内部抵抗が上昇し、サイクル経過や経時変化に伴う容量・負荷特性の低下が問題となる。
【０００６】
本発明は、上記の事情に照らし、耐酸化性にすぐれた導電剤を用いることで、サイクル経過や長期貯蔵によっても内部抵抗を低く保つことができ、もって劣化が少なく信頼性の高い非水二次電池を得ることを目的としている。
【０００７】
【課題を解決するための手段】
本発明者らは、上記の目的を達成するため、鋭意検討した結果、正極合剤中に添加する導電剤としてホウ素含有カーボンブラックを使用すると、このカーボンブラックがすぐれた耐酸化性を示して、サイクル経過や長期貯蔵によっても内部抵抗を低く保つことができ、もって劣化が少なく信頼性の高い非水二次電池が得られることを見い出し、本発明を完成するに至った。
【０００８】
本発明は、正極活物質、導電剤および結着剤を含む正極合剤の成形体からなり、正極合剤中に導電剤として、表面にＢ4 Ｃ層を有するホウ素含有カーボンブラックが０．３〜１０重量％の割合で含有され、かつ上記のホウ素含有カーボンブラック中のホウ素含有量が４〜３０重量％であることを特徴とする非水二次電池用正極に係るものである。
また、本発明は、正極、負極および電解質を有する非水二次電池において、正極は、正極活物質、導電剤および結着剤を含む正極合剤の成形体からなり、正極合剤中に導電剤として、表面にＢ4 Ｃ層を有するホウ素含有カーボンブラックが０．３〜１０重量％の割合で含有され、かつ上記のホウ素含有カーボンブラック中のホウ素含有量が４〜３０重量％であることを特徴とする非水二次電池に係るものである。
【０００９】
【発明の実施の形態】
本発明におけるホウ素含有カーボンブラックは、ホウ素のほとんどが炭素表面にＢ4 Ｃとして共存した、いわば、カーボンブラックとホウ素化合物層の複合化した材料である。ホウ素と炭素とは周期表では隣接しており共有結合半径の値も近いが、最大固溶限界は大きくなく、ホウ素の一部が炭素中に置換固溶する量は僅かであり、上記のように炭素表面にＢ4 Ｃとして共存した形態をとっている。その結果、粒子表面に存在するＢ4 Ｃなどが酸化を受けて表面を被覆する膜を形成し、これがカーボンブラック粒子のさらなる酸化を抑制するものと思われる。また、ホウ素化合物がカーボンブラックの格子欠陥などの活性なサイトを被覆することも耐酸化性に好ましい結果を与えているものと思われる。
【００１０】
このような効果を発現させるため、本発明に用いるホウ素含有カーボンブラックは、ホウ素含有量が４〜３０重量％であることが好ましく、とくに好ましくは８〜２５重量％であるのがよい。ホウ素含有量が４重量％に満たないときは、表面に存在するＢ4 Ｃなどの量が不足し、耐酸化性の改善に好結果が得られない。また、３０重量％を超える量となると、被覆膜の過度な増加により粒子自体の電気抵抗が増加し、導電剤としての効果を損ないやすい。
【００１１】
このようなホウ素含有カーボンブラックは、アセチレンブラック、ケッチェンブラック、サーマルブラック、チャンネルブラック、フアーネスブラック、ランプブラックなどのカーボンブラックと、ホウ素、酸化ホウ素、ホウ酸、炭化ホウ素などのホウ素化合物とを、ホウ素含有量が前記値となるように、適宜の割合で混合し、この混合物を不活性ガス雰囲気中で５００〜３，０００℃、とくに１，２００〜２，４００℃で熱処理することなどにより、得られる。
【００１２】
本発明においては、ホウ素含有カーボンブラックを正極合剤中に導電剤として含有させることを特徴としているが、その含有量は、正極合剤中、０．３〜１０重量％の割合となるようにするのがよい。導電剤としては、ホウ素含有カーボンブラックをこれ単独で使用してもよいし、ホウ素含有カーボンブラックとともに黒鉛などの別の物質を併用してもよい。前者の単独使用の場合、ホウ素含有カーボンブラックのとくに好ましい含有量としては、２〜５重量％である。また後者の併用の場合、ホウ素含有カーボンブラックのとくに好ましい含有量としては、０．５〜３重量％である。
【００１３】
なお、カーボンブラックは、種類や銘柄により、比表面積や沃素吸着量などの物性値がさまざまであり、正極活物質の粉体物性によっても導電剤の必要かつ十分な含有量は異なってくる。正極合剤中の導電剤が少なすぎると十分な導電性を有する正極を得ることができず、逆に多すぎると充填できる活物質量が減少して電池容量の面で損失となる。このことから、導電剤として使用する上記のホウ素含有カーボンブラックは、その原料であるカーボンブラックとさらに正極活物質の種類などに応じて、最適の含有量を決定するのが望ましい。
【００１４】
本発明における非水二次電池用正極は、正極活物質に導電剤として上記割合のホウ素含有カーボンブラックと必要により黒鉛などの別の物質とを混合し、さらに結着剤を加えて、正極合剤を調製し、これをシート状などの成形体としたものである。成形体とする手段は、とくに限定されないが、通常は、正極集電体を使用して、その片面または両面に上記の正極合剤を塗布し乾燥し、必要により、圧延処理して、正極合剤層を形成するという方法が採用される。
【００１５】
正極活物質には、ＬｉＣｏＯ2 、ＬｉＮｉＯ2 、ＬｉＭｎＯ4 などのリチウム含有遷移金属カルコゲナイドが好ましく用いられる。結着剤には、ポリフッ化ビニリデン、ポリテトラフルオロエチレン、ポリアクリル酸、スチレンブタジエンゴム（ＳＢＲ）、フッ素ゴムなどが好ましく用いられる。正極集電体には、アルミニウム、ステンレス鋼、ニッケル、チタンまたはこれらの合金からなる箔、パンチドメタル、エキスバンドメタル、金網などが用いられる。
【００１６】
本発明の非水二次電池は、上記の正極を使用し、この正極とともに負極および電解質を有するものであり、通常は、さらに正極と負極との両極間の電気的短絡を防止するセパレータが用いられる。セパレータは、強度が十分で電解質としての電解液を多く保持できるものが好ましく、厚さが１０〜５０μｍで、開孔率が３０〜７０％のポリプロピレン製、ポリエチレン製またはエチレンとプロピレンのコポリマー製の微孔性フィルムや不織布などが用いられる。
【００１７】
負極を構成する負極活物質には、リチウムまたはリチウム含有化合物が用いられる。リチウム含有化合物としては、代表的には、天然黒鉛や黒鉛化処理を施したコークス、メソフェーズピッチマイクロビーズ、メソフェーズピッチカーボンファイバーなどの黒鉛質材料、乱層構造を有する炭素質材料が用いられる。その他に、錫酸化物、珪素酸化物、ニッケル−珪素系合金、マグネシウム−珪素系合金、タングステン酸化物、リチウム鉄複合酸化物や、リチウム−アルミニウム、リチウム−鉛、リチウム−インジウム、リチウム−ガリウム、リチウム−インジウム−ガリウムなどのリチウム合金が用いられる。
【００１８】
負極は、通常は、上記の負極活物質に結着剤と必要により導電剤を加えて、負極合剤とし、これを負極集電体の片面または両面に塗布し乾燥し、必要により、圧延処理して、負極合剤層を形成することにより、作製される。
結着剤には、ポリフッ化ビニリデン、ポリテトラフルオロエチレン、ポリアクリル酸、スチレンブタジエンゴム（ＳＢＲ）、フッ素ゴムなどが用いられる。負極集電体には、銅、ステンレス鋼、ニッケル、チタンまたはこれらの合金からなる箔、パンチドメタル、エキスバンドメタル、金網などが用いられる。
【００１９】
電解質としては、液状電解質（以下、電解液という）が好ましく用いられる。この電解液には、有機溶媒に溶質を溶解させた非水電解液が用いられる。上記の有機溶媒としては、ジメチルカーボネート、ジエチルカーボネート、エチルメチルカーボネート、メチルプロピルカーボネートなどの鎖状エステル、エチレンカーボネート、プロピレンカーボネート、ブチレンカーボネート、ビニレンカーボネートなどの環状エステル、上記の鎖状エステルと上記の環状エステルとの混合溶媒が用いられる。これらの中でも、鎖状エステルを主溶媒とし、これと環状エステルとの混合溶媒としたものが、とくに好ましく用いられる。また、電解液の溶質としては、ＬｉＰＦ6 、ＬｉＣｌＯ4 、ＬｉＢＦ4 、ＬｉＣＦ3 ＳＯ3 、ＬｉＡｓＦ6 などが単独でまたは２種以上混合して使用できる。
【００２０】
また、本発明において、電解質としては、上記の電解液以外に、固体状またはゲル状の電解質を用いることもできる。このような電解質には、無機固体電解質のほか、ポリエチレンオキサイド、ポリプロピレンオキサイドまたはこれらの誘導体などを主剤にした有機固体電解質などが挙げられる。
【００２１】
【実施例】
以下、本発明の実施例を説明する。ただし、本発明はこれらの例になんら限定されるものではなく、その要旨を変更しない範囲において、適宜変更して実施することが可能である。以下、部とあるのは重量部を意味する。
【００２２】
なお、実施例で用いた、表１に示す▲１▼〜▲５▼の「ホウ素含有カーボンブラック」は、一次粒子径２５ｎｍ、比表面積２００ｍ2 ／ｇの原料カーボンブラックに、ホウ素化合物としてＢ2 Ｏ3 を適量加えて、混合し、アルゴンガス雰囲気中で、２，０００℃で熱処理して、得たものである。
【００２３】
また、表１に示す▲６▼の「ホウ素化合物の混合も熱処理もしていないカーボンブラック」は、上記の原料カーボンブラックそのものを指すものである。さらに、表１に示す▲７▼の「ホウ素化合物なしで熱処理したカーボンブラック」とは、上記の原料カーボンブラックにホウ素化合物を添加しないで上記と同じ条件で熱処理したカーボンブラックを指すものである。
【００２４】
表１
実施例１
正極合剤の導電剤として、▲１▼のホウ素含有カーボンブラックを使用し、円筒型リチウムイオン二次電池を作製した。
正極は、以下のようにして、作製した。
正極活物質としてＬｉＣｏＯ2 （平均粒径５μｍ）９３部に対して、導電剤として▲１▼のホウ素含有カーボンブラック３．５部、結着剤としてポリフッ化ビニリデン３．５部を混合した。この混合は、Ｎ−メチルピロリドンにポリフッ化ビニリデンを溶解した溶液に、正極活物質と導電剤を加え、十分に分散し、正極合剤の塗液とした。これを厚さが２０μｍのＡｌ箔の上に、所定の塗布量で均一に塗布し、乾操した。同様に、Ａｌ箔の裏面にも均一に塗布し、乾燥したのち、圧延処理し、所定の大きさに切断して正極を得た。
【００２６】
負極は、以下のようにして、作製した。
負極活物質として人造黒鉛〔Ｘ線回折法から求められる（００２）面の面間隔（ｄ002 ）が０．３３６２ｎｍであり、平均粒径が２０ｎｍ〕９２部に対して、結着剤としてポリフッ化ビニリデン８部を混合した。この混合は、正極と同様に、Ｎ−メチルピロリドンにポリフッ化ビニリデンを溶解した溶液に、負極活物質を加えて、分散し、負極合剤の塗液とした。これを厚さが１５μｍのＣｕ箔の上に、所定の塗布量で均一に塗布し、乾燥した。同様に、Ｃｕ箔の裏面にも均一に塗布し、乾燥したのち、圧延処理し、所定の大きさに切断して負極を得た。
【００２７】
上記のように作製した帯状正極と帯状負極との間に、厚さが２５μｍの微孔性ポリエチレンフィルムからなるセパレータを配置し、渦巻状に捲回して、渦巻状電極体とした。これを、外径１８mm、高さ６７mmの有底円筒状のステンレス鋼製の電池ケース内に挿入し、負極リード体と缶底の溶接を行った。
【００２８】
その後、この電池ケース内に、電解液として、エチレンカーボネートとジエチルカーボネートとの体積比１：２の混合溶媒にＬｉＰＦ6 を１モル／リットル溶解させてなる非水電解液を注入した。ついで、正極リード体と、正極端子となる封口体とを溶接し、上記電池ケースの開口部を常法にしたがって封口して、外径１８mm、高さ６５mmの筒形非水二次電池を作製した。
【００２９】
この電池構成について、さらに詳しく説明すると、上記電池ケースは負極端子を兼ねていて、その底部には絶縁体が配置され、渦巻状電極体上にも絶縁体が配置されている。電池ケースの開口部には環状の絶縁パッキングを介して封口体が配置され、電池ケースの開口端部の内方への締め付けにより電池内部を密閉構造にしている。上記の封口体には、安全機構として過充電などの異常事態発生時に電池内部に電流を流せなくするための電流遮断機構と、電池内部に発生したガスによって電池内部が異常圧力まで上昇した際に、外部にガスを排出して電池の破裂を防止するための不可逆式ベント機構などが組み込まれている。
【００３０】
実施例２
正極合剤の導電剤として、▲２▼のホウ素含有カーボンブラックを用いた以外は、実施例１と同様にして、円筒型リチウムイオン二次電池を作製した。
【００３１】
実施例３
正極合剤の導電剤として、▲３▼のホウ素含有カーボンブラックを用いた以外は、実施例１と同様にして、円筒型リチウムイオン二次電池を作製した。
【００３２】
実施例４
正極合剤の導電剤として、▲４▼のホウ素含有カーボンブラックを用いた以外は、実施例１と同様にして、円筒型リチウムイオン二次電池を作製した。
【００３３】
実施例５
正極合剤の導電剤として、▲５▼のホウ素含有カーボンブラックを用いた以外は、実施例１と同様にして、円筒型リチウムイオン二次電池を作製した。
【００３４】
実施例６
正極の作製に際し、正極活物質であるＬｉＣｏＯ2 （平均粒径５μｍ）９１．５部に対して、導電剤として▲３▼のホウ素含有カーボンブラック０．５部、人造黒鉛（平均粒径２μｍ）４部、結着剤としてポリフッ化ビニリデン４部を混合した以外は、実施例１と同様にして、円筒型リチウムイオン二次電池を作製した。
【００３５】
実施例７
正極の作製に際し、正極活物質であるＬｉＣｏＯ2 （平均粒径５μｍ）８５部に対して、導電剤として▲３▼のホウ素含有カーボンブラック１０部、結着剤としてポリフッ化ビニリデン５部を混合した以外は、実施例１と同様にして、円筒型リチウムイオン二次電池を作製した。
【００３６】
比較例１
正極合剤の導電剤として、▲６▼のホウ素化合物の混合も熱処理もしていないカーボンブラック（原料カーボンブラック）を用いた以外は、実施例１と同様にして、円筒型リチウムイオン二次電池を作製した。
【００３７】
比較例２
正極合剤の導電剤として、▲７▼のホウ素化合物なしで熱処理したカーボンブラックを用いた以外は、実施例１と同様にして、円筒型リチウムイオン二次電池を作製した。
【００３８】
上記の実施例１〜７および比較例１，２の各円筒型リチウムイオン二次電池について、下記の方法により、サイクル試験、貯蔵試験、負荷特性試験を行った。結果は、表２および表３に示されるとおりであった。
【００３９】
＜サイクル試験＞
１．７Ａの定電流で４．２Ｖまで充電したのち、定電圧方式で、定電流充電と定電圧充電の合計時間が２．５時間となるまで充電した。その後、１．７Ａで終止電圧３．０Ｖまで放電する工程を１サイクルとし、これを繰り返す試験を行った。５００サイクル経過後に維持されている放電容量の比率を求め、比較した。また、サイクル前後でのインピーダンスを測定した。なお、インピーダンスの測定は、いずれも、充電状態にして行った。
【００４０】
＜貯蔵試験＞
１．７Ａの定電流で４．２Ｖまで充電したのち、定電圧方式で、定電流充電と定電圧充電の合計時間が２．５時間となるまで充電した。その後、６０℃の環境下で２０日間貯蔵し、電池のインピーダンスの変化を測定した。なお、インピーダンスの測定は、充電状態にして行った。
【００４１】
＜負荷特性試験＞
１．７Ａの定電流で４．２Ｖまで充電したのち、定電圧方式で、定電流充電と定電圧充電の合計時間が２．５時間となるまで充電した。その後、終止電圧３．０Ｖまで３．４Ａの大電流放電を行い、このときの放電容量と、上記と同様に充電したのちに０．３４Ａで放電した場合の放電容量との比率を求め、比較した。また、その後、上記と同様の方法にて５００サイクルを行ったのち、再度同様の測定を行い、５００サイクル経過後の同比率を求めた。
【００４２】
表２
表３
上記の表２および表３の結果から、本発明の実施例１〜７では、比較例１に比べて、サイクル特性、貯蔵特性、負荷特性特性のいずれの特性についても改善効果がみられている。これに対し、比較例２では、いずれの特性も低下している。これは、カーボンブラックの酸化に起因した導電性の低下とは別に、熱処理によりカーボンブラックの表面官能基が除去され、塗料中の分散安定性が低下して、十分な導電性付与が行われていないためと思われる。本発明のように表面にホウ素化合物が存在するホウ素含有カーボンブラックを使用した実施例１〜７では、このような分散性の低下も生じていないことが明らかに推定される。
【００４５】
【発明の効果】
以上のように、本発明は、正極合剤中に耐酸化性にすぐれた導電剤としてホウ素含有カーボンブラックを特定量含有させる構成としたことにより、サイクル経過や長期貯蔵によっても内部抵抗を低く保つことが可能であり、劣化が少なく、信頼性の高い非水二次電池を提供することができる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous secondary battery, and more particularly to a non-aqueous secondary battery in which a conductive agent used in a positive electrode mixture is improved.
[0002]
[Prior art]
With the recent miniaturization and portability of electronic devices, the demand for secondary batteries having a high energy density has further increased. Currently, lithium ion secondary batteries that use lithium composite oxides such as lithium cobalt composite oxide as the positive electrode active material and mainly carbon-based materials as the negative electrode active material have been commercialized as secondary batteries that meet this demand. Furthermore, further improvement in performance continues. These batteries have an average operating voltage of 3.6 V, which is about three times higher than conventional Ni-Cd batteries and nickel metal hydride batteries, and because they use carbon-based materials for the negative electrode, they can be reduced in weight. Is possible.
[0003]
The non-aqueous secondary battery has a structure in which a sheet-like positive electrode and a negative electrode are wound or laminated via a separator, a nonwoven fabric, or the like in order to secure a reaction area. These electrodes are generally formed by forming a mixture layer containing an active material and a binder on a current collector. However, a lithium composite oxide such as LiCoO 2 that is a positive electrode active material is electrically conductive. It has poor properties and has a relatively high electrical resistance. For this reason, when producing a positive electrode using this active material, it is necessary to add a conductive agent together with a binder to form a mixture layer.
[0004]
By the way, the non-aqueous secondary battery has a high operating potential as described above. For example, in a state where the battery is charged and LiCoO 2 releases Li to become Li 1-x CoO 2 , strong oxidation occurs. Shows the effect. For this reason, in order to obtain a highly reliable non-aqueous secondary battery, high oxidation resistance is also required for the conductive agent and the binder.
[0005]
[Problems to be solved by the invention]
However, graphite and carbon black, which are widely used as conductive agents, have poor oxidation resistance, and in the presence of transition metals such as Co, the oxidation resistance is lowered. The internal resistance increases due to the storage, and the deterioration of capacity / load characteristics with the passage of time and changes with time becomes a problem.
[0006]
In light of the above-described circumstances, the present invention uses a conductive agent having excellent oxidation resistance, so that the internal resistance can be kept low even with cycle progress and long-term storage, and thus there is little deterioration and high reliability. The purpose is to obtain the next battery.
[0007]
[Means for Solving the Problems]
As a result of intensive studies to achieve the above object, the present inventors have shown that, when boron-containing carbon black is used as a conductive agent added to the positive electrode mixture, the carbon black exhibits excellent oxidation resistance, It was found that the internal resistance can be kept low even after cycling and long-term storage, and thus a non-aqueous secondary battery with little deterioration and high reliability can be obtained, and the present invention has been completed.
[0008]
The present invention comprises a molded product of a positive electrode mixture containing a positive electrode active material, a conductive agent and a binder, and boron-containing carbon black having a B 4 C layer on the surface as a conductive agent in the positive electrode mixture is 0.3. it is contained in a proportion of 10 wt%, and Ru der relates to non-aqueous secondary battery positive electrode, wherein the boron content in the boron-containing carbon black described above is 4 to 30 wt%.
Further, the present invention provides a nonaqueous secondary battery having a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode is formed of a positive electrode mixture including a positive electrode active material, a conductive agent, and a binder, and is electrically conductive in the positive electrode mixture. As the agent, boron-containing carbon black having a B 4 C layer on the surface is contained in a proportion of 0.3 to 10% by weight , and the boron content in the boron-containing carbon black is 4 to 30% by weight. the Ru der pertaining to non-aqueous secondary battery comprising.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The boron-containing carbon black in the present invention is a material in which most of boron coexists as B 4 C on the carbon surface, that is, a composite of carbon black and a boron compound layer. Boron and carbon are adjacent to each other in the periodic table and the values of the covalent bond radii are close, but the maximum solid solution limit is not large, and the amount of part of boron substituted and dissolved in carbon is small. In the form of B 4 C coexisting on the carbon surface. As a result, B 4 C existing on the particle surface is oxidized to form a film covering the surface, which is considered to suppress further oxidation of the carbon black particles. Further, it is considered that the boron compound covering active sites such as lattice defects of carbon black also gives a favorable result to the oxidation resistance.
[0010]
In order to express such an effect, the boron-containing carbon black used in the present invention preferably has a boron content of 4 to 30% by weight, particularly preferably 8 to 25% by weight. When the boron content is less than 4% by weight, the amount of B 4 C and the like present on the surface is insufficient, and good results cannot be obtained in improving oxidation resistance. On the other hand, when the amount exceeds 30% by weight, the electrical resistance of the particles themselves increases due to an excessive increase in the coating film, and the effect as a conductive agent tends to be impaired.
[0011]
Such boron-containing carbon blacks include carbon blacks such as acetylene black, ketjen black, thermal black, channel black, furnace black and lamp black, and boron compounds such as boron, boron oxide, boric acid and boron carbide. By mixing at an appropriate ratio so that the boron content becomes the above value, and heat-treating the mixture at 500 to 3,000 ° C., particularly 1,200 to 2,400 ° C. in an inert gas atmosphere. ,can get.
[0012]
The present invention is characterized in that boron-containing carbon black is contained as a conductive agent in the positive electrode mixture, and the content thereof is 0.3 to 10% by weight in the positive electrode mixture. It is good to do. As the conductive agent, boron-containing carbon black may be used alone, or another substance such as graphite may be used in combination with boron-containing carbon black. In the case of the former single use, the particularly preferable content of the boron-containing carbon black is 2 to 5% by weight. In the case of the latter combination, the particularly preferable content of the boron-containing carbon black is 0.5 to 3% by weight.
[0013]
Carbon black has various physical property values such as specific surface area and iodine adsorption amount depending on the type and brand, and the necessary and sufficient content of the conductive agent varies depending on the powder physical properties of the positive electrode active material. When the amount of the conductive agent in the positive electrode mixture is too small, a positive electrode having sufficient conductivity cannot be obtained. On the other hand, when the amount is too large, the amount of the active material that can be filled is reduced and the battery capacity is lost. For this reason, it is desirable to determine the optimum content of the boron-containing carbon black used as the conductive agent according to the carbon black as the raw material and the type of the positive electrode active material.
[0014]
The positive electrode for a non-aqueous secondary battery in the present invention is prepared by mixing the positive electrode active material with boron-containing carbon black in the above ratio as a conductive agent and, if necessary, another substance such as graphite, and further adding a binder. An agent is prepared, and this is formed into a molded body such as a sheet. The means for forming the molded body is not particularly limited. Usually, a positive electrode current collector is used, and the positive electrode mixture is applied to one side or both sides of the positive electrode mixture and dried. If necessary, the positive electrode mixture is rolled. A method of forming an agent layer is employed.
[0015]
As the positive electrode active material, lithium-containing transition metal chalcogenides such as LiCoO 2 , LiNiO 2 , and LiMnO 4 are preferably used. As the binder, polyvinylidene fluoride, polytetrafluoroethylene, polyacrylic acid, styrene butadiene rubber (SBR), fluorine rubber, or the like is preferably used. For the positive electrode current collector, a foil made of aluminum, stainless steel, nickel, titanium, or an alloy thereof, a punched metal, an extended metal, a wire mesh, or the like is used.
[0016]
The non-aqueous secondary battery of the present invention uses the positive electrode described above and has a negative electrode and an electrolyte together with the positive electrode. Usually, a separator that prevents an electrical short circuit between both electrodes of the positive electrode and the negative electrode is used. It is done. The separator preferably has sufficient strength and can hold a large amount of electrolyte as an electrolyte. The separator is made of polypropylene, polyethylene, or a copolymer of ethylene and propylene having a thickness of 10 to 50 μm and a porosity of 30 to 70%. A microporous film or a non-woven fabric is used.
[0017]
Lithium or a lithium-containing compound is used for the negative electrode active material constituting the negative electrode. Typical examples of the lithium-containing compound include natural graphite, graphitized coke, mesophase pitch microbeads, graphite materials such as mesophase pitch carbon fibers, and carbonaceous materials having a layered structure. In addition, tin oxide, silicon oxide, nickel-silicon alloy, magnesium-silicon alloy, tungsten oxide, lithium iron composite oxide, lithium-aluminum, lithium-lead, lithium-indium, lithium-gallium, A lithium alloy such as lithium-indium-gallium is used.
[0018]
The negative electrode is usually a negative electrode mixture obtained by adding a binder and, if necessary, a conductive agent to the negative electrode active material, and applying this to one or both sides of the negative electrode current collector and drying, and if necessary, rolling treatment And it produces by forming a negative mix layer.
As the binder, polyvinylidene fluoride, polytetrafluoroethylene, polyacrylic acid, styrene butadiene rubber (SBR), fluorine rubber, or the like is used. For the negative electrode current collector, a foil made of copper, stainless steel, nickel, titanium, or an alloy thereof, a punched metal, an extended metal, a wire mesh, or the like is used.
[0019]
As the electrolyte, a liquid electrolyte (hereinafter referred to as an electrolytic solution) is preferably used. As this electrolytic solution, a non-aqueous electrolytic solution in which a solute is dissolved in an organic solvent is used. Examples of the organic solvent include chain esters such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and methyl propyl carbonate; cyclic esters such as ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate; A mixed solvent with a cyclic ester is used. Among these, those using a chain ester as a main solvent and a mixed solvent thereof with a cyclic ester are particularly preferably used. Moreover, as a solute of the electrolytic solution, LiPF 6 , LiClO 4 , LiBF 4 , LiCF 3 SO 3 , LiAsF 6 , or the like can be used alone or in combination.
[0020]
In the present invention, as the electrolyte, a solid or gel electrolyte can be used in addition to the above electrolytic solution. Examples of such an electrolyte include inorganic solid electrolytes and organic solid electrolytes mainly composed of polyethylene oxide, polypropylene oxide, or derivatives thereof.
[0021]
【Example】
Examples of the present invention will be described below. However, the present invention is not limited to these examples, and can be appropriately modified and implemented without departing from the scope of the invention. Hereinafter, “parts” means parts by weight.
[0022]
Incidentally, used in Examples are shown in Table 1 ▲ 1 ▼ ~ ▲ 5 ▼ the "boron-containing carbon black" is, B 2 primary particle diameter 25 nm, the raw material of carbon black having a specific surface area of 200 meters 2 / g, as the boron compound An appropriate amount of O 3 was added, mixed, and heat-treated at 2,000 ° C. in an argon gas atmosphere.
[0023]
In addition, “carbon black not mixed with boron compound or heat-treated” in (6) shown in Table 1 refers to the raw material carbon black itself. Furthermore, “carbon black heat-treated without boron compound” in (7) shown in Table 1 refers to carbon black that has been heat-treated under the same conditions as described above without adding a boron compound to the raw material carbon black.
[0024]
Table 1
Example 1
A cylindrical lithium ion secondary battery was produced using the boron-containing carbon black of (1) as the conductive agent of the positive electrode mixture.
The positive electrode was produced as follows.
To 93 parts of LiCoO 2 (average particle size 5 μm) as the positive electrode active material, 3.5 parts of boron-containing carbon black (1) as a conductive agent and 3.5 parts of polyvinylidene fluoride as a binder were mixed. In this mixing, a positive electrode active material and a conductive agent were added to a solution of polyvinylidene fluoride dissolved in N-methylpyrrolidone and dispersed sufficiently to obtain a coating solution for the positive electrode mixture. This was uniformly applied in a predetermined coating amount on an Al foil having a thickness of 20 μm, and dried. Similarly, it was uniformly applied to the back surface of the Al foil, dried, rolled, and cut into a predetermined size to obtain a positive electrode.
[0026]
The negative electrode was produced as follows.
For the negative electrode active material, 92 parts of artificial graphite (the (002) plane spacing (d 002 ) determined by X-ray diffraction method is 0.3362 nm and the average particle size is 20 nm) is 92 parts. 8 parts of vinylidene were mixed. As in the case of the positive electrode, this mixing was performed by adding and dispersing the negative electrode active material to a solution of polyvinylidene fluoride dissolved in N-methylpyrrolidone to obtain a negative electrode mixture coating solution. This was uniformly coated on a Cu foil having a thickness of 15 μm in a predetermined coating amount and dried. Similarly, it was uniformly applied to the back surface of the Cu foil, dried, rolled, and cut into a predetermined size to obtain a negative electrode.
[0027]
A separator made of a microporous polyethylene film having a thickness of 25 μm was disposed between the belt-like positive electrode and the belt-like negative electrode produced as described above, and wound into a spiral shape to obtain a spiral electrode body. This was inserted into a bottomed cylindrical stainless steel battery case having an outer diameter of 18 mm and a height of 67 mm, and the negative electrode lead body and the bottom of the can were welded.
[0028]
Thereafter, a nonaqueous electrolytic solution obtained by dissolving 1 mol / liter of LiPF 6 in a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 2 was injected into the battery case as an electrolytic solution. Next, the positive electrode lead body and the sealing body to be the positive electrode terminal are welded, and the opening of the battery case is sealed in accordance with a conventional method to produce a cylindrical non-aqueous secondary battery having an outer diameter of 18 mm and a height of 65 mm. did.
[0029]
The battery configuration will be described in more detail. The battery case also serves as a negative electrode terminal, an insulator is disposed at the bottom, and an insulator is also disposed on the spiral electrode body. A sealing body is disposed in the opening of the battery case via an annular insulating packing, and the inside of the battery is sealed by tightening the opening end of the battery case inward. The above-mentioned sealing body has a current interruption mechanism for preventing current from flowing inside the battery when an abnormal situation such as overcharge occurs as a safety mechanism, and when the inside of the battery rises to an abnormal pressure due to the gas generated inside the battery. In addition, an irreversible vent mechanism for discharging gas to the outside and preventing the battery from bursting is incorporated.
[0030]
Example 2
A cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that the boron-containing carbon black (2) was used as the conductive agent for the positive electrode mixture.
[0031]
Example 3
A cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that the boron-containing carbon black (3) was used as the conductive agent of the positive electrode mixture.
[0032]
Example 4
A cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that the boron-containing carbon black (4) was used as the conductive agent for the positive electrode mixture.
[0033]
Example 5
A cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that the boron-containing carbon black (5) was used as the conductive agent for the positive electrode mixture.
[0034]
Example 6
In preparation of the positive electrode, 91.5 parts of LiCoO 2 (average particle diameter 5 μm) as the positive electrode active material, 0.5 parts of boron-containing carbon black (3) as a conductive agent, artificial graphite (average particle diameter 2 μm) A cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that 4 parts and 4 parts of polyvinylidene fluoride as a binder were mixed.
[0035]
Example 7
In the production of the positive electrode, 10 parts of boron-containing carbon black (3) as a conductive agent and 5 parts of polyvinylidene fluoride as a binder were mixed with 85 parts of LiCoO 2 (average particle size 5 μm) as a positive electrode active material. Except for the above, a cylindrical lithium ion secondary battery was produced in the same manner as in Example 1.
[0036]
Comparative Example 1
A cylindrical lithium ion secondary battery was prepared in the same manner as in Example 1 except that carbon black (raw carbon black) that was not mixed or heat-treated with the boron compound of (6) was used as the conductive agent of the positive electrode mixture. Produced.
[0037]
Comparative Example 2
A cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that carbon black heat-treated without the boron compound (7) was used as the conductive agent for the positive electrode mixture.
[0038]
About each cylindrical lithium ion secondary battery of said Examples 1-7 and Comparative Examples 1 and 2, the cycle test, the storage test, and the load characteristic test were done with the following method. The results were as shown in Table 2 and Table 3.
[0039]
<Cycle test>
After charging to 4.2 V with a constant current of 1.7 A, charging was performed by a constant voltage method until the total time of constant current charging and constant voltage charging was 2.5 hours. Thereafter, a process of discharging to 1.7 V at a final voltage of 3.0 V was taken as one cycle, and a test was repeated. The ratio of the discharge capacity maintained after the elapse of 500 cycles was determined and compared. Moreover, the impedance before and after the cycle was measured. Note that the impedance was measured in a charged state.
[0040]
<Storage test>
After charging to 4.2 V with a constant current of 1.7 A, charging was performed by a constant voltage method until the total time of constant current charging and constant voltage charging was 2.5 hours. Thereafter, the battery was stored in an environment at 60 ° C. for 20 days, and the change in the impedance of the battery was measured. The impedance was measured in a charged state.
[0041]
<Load characteristic test>
After charging to 4.2 V with a constant current of 1.7 A, charging was performed by a constant voltage method until the total time of constant current charging and constant voltage charging was 2.5 hours. Thereafter, a large current discharge of 3.4 A is performed up to a final voltage of 3.0 V, and the ratio between the discharge capacity at this time and the discharge capacity when discharged at 0.34 A after being charged in the same manner as described above is compared. did. Moreover, after performing 500 cycles by the method similar to the above after that, the same measurement was performed again and the ratio after 500 cycles progressed was calculated | required.
[0042]
Table 2
Table 3
From the results of Table 2 and Table 3 above, in Examples 1 to 7 of the present invention, an improvement effect is seen in any of the cycle characteristics, the storage characteristics, and the load characteristics characteristics as compared with Comparative Example 1. . On the other hand, in the comparative example 2, all the characteristics are deteriorated. This is because, apart from the decrease in conductivity due to the oxidation of carbon black, the surface functional groups of carbon black are removed by heat treatment, the dispersion stability in the paint is decreased, and sufficient conductivity is imparted. It seems that there is not. In Examples 1 to 7 using boron-containing carbon black having a boron compound on the surface as in the present invention, it is clearly estimated that such a decrease in dispersibility does not occur.
[0045]
【The invention's effect】
As described above, the present invention keeps the internal resistance low even after cycling or long-term storage by including a specific amount of boron-containing carbon black as a conductive agent having excellent oxidation resistance in the positive electrode mixture. Therefore, it is possible to provide a highly reliable non-aqueous secondary battery with little deterioration.
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