JP3540080B2 - Battery binder solution and electrode mixture - Google Patents
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- JP3540080B2 JP3540080B2 JP35078295A JP35078295A JP3540080B2 JP 3540080 B2 JP3540080 B2 JP 3540080B2 JP 35078295 A JP35078295 A JP 35078295A JP 35078295 A JP35078295 A JP 35078295A JP 3540080 B2 JP3540080 B2 JP 3540080B2
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- vinylidene fluoride
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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
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Description
【０００１】
【発明の属する技術分野】
本発明は、非水系電池、特にリチウムイオン電池、の粉末電極材料（主として電極活物質および必要に応じて加えられる導電性助剤）を電極に安定的に固着させるために用いられるフッ化ビニリデン系重合体からなるバインダーの溶液、すなわちフッ化ビニリデン系重合体バインダー溶液、およびこれに粉末電極材料を分散させてなる電極合剤に関する。
【０００２】
【従来の技術】
近年電子技術の発展はめざましく、各種の機器が小型軽量化されてきている。この電子機器の小型軽量化と相まって、その電源となる電池の小型軽量化の要望も非常に大きくなってきている。少ない容積及び重量でより大きなエネルギーを得るためには電池一本当たりの電圧が高いことが必要となり、この見地から最近リチウムまたはリチウムイオンを吸蔵可能な炭素質材料を負極活物質とし、正極活物質として例えばリチウムコバルト酸化物を使用した非水系電解液を用いる電池が注目されている。
【０００３】
しかしながら、このような非水系電池においては、水系の電解液のイオン伝導度が通常１０-1Ｓ／ｃｍであるのに対し、非水系電解液のイオン伝導度は１０-2〜１０-4Ｓ／ｃｍ程度と低いため、実用電池としては厚さ数μｍから数百μｍと薄く且つ大面積の電極を用いることが必須となってきている。かかる薄膜で且つ大面積の電極を工業的に安価に得る方法としては、電極活物質のバインダーとなる有機重合体を溶媒に溶解してなるバインダー溶液中に、粉末電極材料を分散させた後、金属電極箔または金属網上に塗布乾燥して電極を成型する方法が知られている。また、このような非水系電池用のバインダー溶液としては、特開平６−９３０２５や特開平６−１７２４５２号各公報に記載されているように、各種のフッ化ビニリデン系重合体を、Ｎ−メチル−２−ピロリドン、ジメチルフォルムアミド、Ｎ，Ｎ−ジメチルアセトアミドなどの極性溶媒に溶解したものが知られている。これは、フッ化ビニリデン系重合体が、耐薬品性、耐候性、耐汚染性等に優れ、上記のような極性溶媒には溶解するが、非水系電池に用いられる非水系電解液に対しては、多少膨潤するものの安定であり、また共重合あるいは変性処理等により金属等の基材に対しても良好な接着性を保持し得るからである。
【０００４】
【発明が解決しようとする課題】
上述したようなフッ化ビニリデン系重合体を、バインダーとして、これを極性有機溶媒に溶解した際に、溶媒のロットによっては溶液粘度が通常より大きく増大する現象が見出された。溶液の粘度が増大すると、電極塗布工程での均一な膜厚が得られ難くなるばかりか、活物質との混練時にゲル化してしまい、製膜そのものが困難になる場合もあった。また製膜が可能であったとしても、塗布乾燥後のバインダーが非水系電解液中で膨潤度が大きくなる現象も見出された。バインダーの非水電解液中での膨潤が激しくなると、粉末電極材料（特に活物質）と金属電極箔や金属網との接触抵抗や活物質間の接触抵抗が大きくなり、電池の内部抵抗が増大する。また繰り返し充放電可能な二次電池の場合には、内部抵抗の増大は充放電サイクル特性の劣化につながり、ひいては電池寿命を短くする恐れがある。
【０００５】
従って、本発明の主要な目的は、それ自体粘度増加を起こすことなく安定であり、また非水系電解液中で過度な膨潤を起すことなく安定な電池電極の形成を可能とする、非水系電池電極形成用のフッ化ビニリデン系重合体バインダー溶液、ならびにこれに粉末電極材料を分散させた安定な特性の電極合剤を提供することにある。
【０００６】
【課題を解決するための手段】
本発明の非水系電池電極形成用のフッ化ビニリデン系重合体バインダー溶液は、上記目的を達成するために開発されたものであり、フッ化ビニリデン系重合体を有機溶媒に溶解してなり、その一部を採取しイオン交換水で１０倍に希釈して得た液のｐＨが９以下になるように酸を添加して調製された非水系電池電極形成用のフッ化ビニリデン系重合体バインダー溶液を特徴とするものである。
【０００７】
本発明の作用・効果について、若干付言する。本発明者等が鋭意研究した結果、上述したフッ化ビニリデン系重合体バインダー溶液における異常な粘度上昇の原因は、系の酸−アルカリ度と関係することが見出された。すなわち、上記したフッ化ビニリデン系重合体の有機溶媒溶液の酸−アルカリ度を直接に判定することは困難であるが、その一部を採取しイオン交換水で１０倍に希釈して得た液のｐＨが一つの目安となる。そして、粘度上昇の生じた系においては、このようにして測定した液のｐＨが９を超えていることが判明した。そして、これはそれ自体は既知の現象であるフッ化ビニリデン系重合体のアルカリ性の媒体中における脱フッ酸反応に関連しており、バインダー性能にも直接関係していること、また、フッ化ビニリデン系重合体に対し良好な溶解性を示す極性溶媒の多くは、Ｎ−メチルピロリドン、ジメチルフォルムアミド等の含窒素有機溶媒であり、溶媒のロット毎による増粘現象の有無、程度の差は、このような含窒素有機溶媒の製造時あるいはその後の保管中におけるアミン等のアルカリ性物質の残留、生成、もしくは遊離による溶媒のアルカリ度の上昇と関連していると解される。なおフッ化ビニリデン系重合体からの脱フッ酸によりバインダーとしての耐溶剤性（耐膨潤性）が低下するのは、脱フッ素部位に酸素が付加しやすく、結果的に非水系電解液への親和性が増大するため、あるいは異種結合の増大が良好な耐溶剤性を与える有力なファクターとしての重合体の結晶性を低下させたためと解される。本発明は、このような解析に基づき、上述したフッ化ビニリデン系重合体バインダー溶液の増粘現象を伴う不都合が、酸を添加して系のｐＨを９以下に低下させることにより有効に抑制されることの知見に基づく。
【０００８】
【発明の実施の形態】
本発明において用いられるフッ化ビニリデン系重合体には、フッ化ビニリデンの単独重合体、共重合体およびこれらの変性物が含まれる。フッ化ビニリデンの単独重合体は、非水系電解液に対する耐久性、特に耐膨潤性、の観点では好ましい。しかし、金属等の電極基体との接着性が若干不足気味であるため、より好ましくは、他のモノマーとの共重合体、特に不飽和二塩基酸のモノエステル、ビニレンカーボネートあるいはエポキシ含有ビニル単量体等との共重合によりカルボキシル基、カーボネート基、エポキシ基等の極性基を導入した共重合体（特開平６−１７２４５２号公報等）が好ましく用いられる。またこれらフッ化ビニリデンの単独または共重合体を溶解または膨潤する溶媒中で、アミノ基またはメルカプト基等のフッ化ビニリデン系重合体と反応性基と加水分解性基を併有するシラン系カップリング剤あるいはチタネート系カップリング剤中で処理してなる変性フッ化ビニリデン系重合体（特開平６−９３０２５号）も好ましく用いられる。但し、全体として非水系電解液に対する耐膨潤性を良好に維持するために、非処理のフッ化ビニリデン単位を９０モル％以上、特に９５モル％以上の範囲で維持することが好ましい。
【０００９】
フッ化ビニリデン系重合体は、固有粘度（樹脂４ｇを１リットルのＮ，Ｎ−ジメチルホルムアミドに溶解させた溶液の３０℃における対数粘度）が、０．５以上、更には０．５〜２．０、特に０．８〜１．５、の範囲内の値を有することが好ましい。
【００１０】
上記フッ化ビニリデン系重合体を溶解して、本発明のバインダー溶液を得るために用いられる有機溶媒は、好ましくは極性のものであり、例えばＮ−メチル−２−ピロリドン、ジメチルフォルムアミド、Ｎ，Ｎ−ジメチルアセトアミド、Ｎ，Ｎ−ジメチルスルフォキシド、ヘキサメチルフォスフォアミド、ジオキサン、テトラヒドロフラン、テトラメチルウレア、トリエチルフォスフェイト、トリメチルフォスフェイト、などが挙げられる。これら有機溶媒は単独でまたは二種以上混合して用いられる。前述したように、フッ化ビニリデン系重合体の良溶媒の多くは含窒素溶媒であり、またバインダー溶液の粘度上昇の原因となるアルカリ性物質を生成し易いものも含窒素有機溶媒である。従って、本発明で好ましく用いられる有機溶媒は含窒素有機溶媒であるが、非窒素系の有機溶媒についても本発明に従い、酸を添加してｐＨを調製しておくことは、何らかの理由によるその後のｐＨ増加に伴う不都合を回避できるので好ましい。このような非窒素系の良溶媒としては、前述したジオキサン、テトラヒドロフラン、トリアルキルホスフェート等があり、単独で又は含窒素有機溶媒と混合して好適に使用される。
【００１１】
本発明のバインダー溶液を得るに当り、これら有機溶媒１００重量部当り、前記フッ化ビニリデン系重合体を０．１〜２０重量部、特に１〜１５重量部、の割合で溶解することが好ましい。０．１重量部未満では、溶液中での重合体の占める割合が小さすぎ、バインダーとしての効果が得られない。また、２０重量部を越えると、溶液の粘度が高くなり過ぎて電極合剤の調整が困難になる。
【００１２】
本発明に従い、酸を添加後のバインダー溶液について、その溶液の一部を採取してイオン交換水による１０倍希釈液のｐＨが９以下になるように調整する。
【００１３】
添加される酸としては、基本的には限定されないが、電極合剤として塗布し、乾燥する工程中に分解や蒸発により飛散して成型電極中に残留しないものが好ましく、また電極活物質と反応し難いものが好ましい。この点で、塩酸や硫酸などの無機酸は電極活物質と反応し易く必ずしも好ましくない。特に黒鉛などの無機物と層間化合物を形成し易い電極活物質を用いる場合には、層間化合物を形成し難い大きな分子直径を有する有機酸を用いることが好ましい。乾燥工程での温度領域（通常ＰＶＤＦの融点である１７５℃以下）で高い蒸気圧を有するか、または分解飛散するものが好ましい。このような観点で好ましく用いられる有機酸の例としては、アクリル酸、ギ酸、クエン酸、酢酸、シュウ酸、乳酸、ピルピン酸、マロン酸、プロピオン酸、マレイン酸、酪酸等が挙げられる。
【００１４】
フッ化ビニリデン系重合体溶液中に酸を添加する方法としては、予め有機溶媒に酸を添加して溶媒のｐＨを調整してから重合体を溶解する方法が好ましいが、有機溶媒に重合体を溶解する際に酸を同時に添加して溶解する方法や、重合体を溶解し終わってから溶液中に酸を添加する方法であっても良い。
【００１５】
酸の添加量は、添加後のバインダー溶液のイオン交換水による１０倍希釈液のｐＨ（好ましい態様に従い予め有機溶媒に酸添加したときの希釈液ｐＨとほぼ一致）が、９以下、好ましくは７．５以下、とする量が添加される。下限は特に限定されないが、一般に３程度までで、それ以上添加しても特に効果の増大は期待できない。但し、上述した好ましい態様である有機酸の場合には、電極の塗布、乾燥による成形工程で分解揮散するので、過剰に加えても有害ではない。予め、酸を添加する前の有機溶媒あるいはバインダー溶液のｐＨが９以下となっている場合でも、その後の溶媒の分解等の何らかの理由によるｐＨ上昇に対する緩衝効果を期待して、一定量の酸を添加することが好ましい。この場合の酸の添加量は、バインダー溶液１リットル中に、１００ｐｐｍ以上、特に３００〜１０，０００ｐｐｍ添加しておくことが好ましい。このような態様のときには、残留しても害の殆どない有機酸を添加することが特に好ましい。
【００１６】
上記のようにして得られた本発明のフッ化ビニリデン系重合体バインダー溶液に、粉末電極材料（活物質および必要に応じて加えられる導電材、その他の助剤）を分散混合することにより電極合剤が得られる。
【００１７】
活物質としては、正極の場合は、一般式ＬｉＭＹ2 （Ｍは、Ｃｏ、Ｎｉ、Ｆｅ、Ｍｎ、Ｃｒ、Ｖ等の遷移金属の少なくとも一種：ＹはＯ、Ｓ等のカルコゲン元素）で表わされる複合金属カルコゲン化合物、特にＬｉＣｏＯ2 をはじめとする複合金属酸化物やＬｉＭｎ2 Ｏ4 などのスピネル構造をとる複合金属酸化物が好ましい。負極の場合は、黒鉛、活性炭、あるいはフェノール樹脂やピッチ等を焼成炭化したもの等の炭素質物質が活物質としては好ましい。
【００１８】
導電材はＬｉＣｏＯ2 等の電子伝導性の小さい活物質を使用する場合に電極合剤層の導電性を向上する目的で添加するもので、カーボンブラック、黒鉛微粉末あるいは繊維等の炭素質物質やニッケル、アルミニウム等の金属微粉末あるいは、繊維が使用される。活物質として導電性の大きい炭素質物質を用いる場合はこれらの導電材は使用する必要がない。
【００１９】
本発明の電極合剤においては、粉末電極材料１００重量部に対し、０．１〜５０重量部、特に１〜２０重量部のフッ化ビニリデン系重合体を混合することが好ましい。
【００２０】
形成された電極合剤を、鉄、ステンレス鋼、鋼、銅、アルミニウム、ニッケル、チタン等の金属箔あるいは金属網等からなり、厚さが５〜１００μｍ、小規模の場合には例えば５〜２０μｍとなるような集電体の少なくとも一面、好ましくは両面に塗布し、例えば５０〜１７０℃で乾燥して、例えば小規模の場合厚さが１０〜１０００μｍの電極合剤層を形成することにより、電極が形成される。
【００２１】
本発明のバインダー溶液および電極合剤を用いて形成される電極は、非水系電池、特にリチウムイオン電池の電極として用いることが好ましく、この際用いられる電解液としては、例えばリチウム塩などの電解質を非水系溶媒（有機溶媒）に溶解したものを用いることができる。
【００２２】
ここで電解質としては、ＬｉＰＦ6 、ＬｉＡｓＦ6 、ＬｉＣｌＯ3 、ＬｉＢＦ4 、ＣＨ3 ＳＯ3 Ｌｉ、ＣＦ3 ＳＯ3 Ｌｉ、ＬｉＣｌ、ＬｉＢｒ等がある。また、電解質の有機溶媒としてはプロピレンカーボネート、エチレンカーボネート、１，２−ジメトキシエタン、１，２−ジエトキシエタン、ジメチルカーボネート、ジエチルカーボネート、メチルエチルカーボネート、γ−プチルラクトン、プロピオン酸メチル、プロピオン酸エチル、およびこれらの混合溶媒などが用いられるが、必ずしもこれらに限定されるものではない。
【００２３】
【実施例】
以下、実施例、比較例により本発明を更に具体的に説明する。
【００２４】
（フッ化ビニリデン系重合体の調製）
特開平６−１７２４５２号公報に記載の実施例１に従い、以下のようにしてカルボキシル基含有フッ化ビニリデン系重合体を得た。
【００２５】
内容量２リットルのオートクレーブに、イオン交換水１０４０ｇ、メチルセルロース０．８ｇ、酢酸エチル２．５ｇ、ジイソプロピルパーオキシジカーボネート４ｇ、フッ化ビニリデン３９６ｇ、マレイン酸モノメチルエステル４ｇを仕込み（フッ化ビニリデン：マレイン酸モノメチルエステル＝１００：１．０１）、２８℃で４７時間懸濁重合を行った。重合完了後、重合体スラリーを脱水、水洗後８０℃で２０時間乾燥して重合体粉末を得た。
【００２６】
重合率は９０重量％で、得られた重合体は、固有粘度１．１、カルボニル基含有量１．２×１０-4モル／ｇであった。
【００２７】
（有機溶媒）
市販のＮ−メチル−２−ピロリドンの２０ロットにわたるものを、約３カ月間室温で保存したものを用いた。以下、「ＮＭＰ」と略記する。イオン交換水による１０倍希釈液のｐＨは、６．５〜１１．０の範囲にわたっていた。
【００２８】
比較例１
イオン交換水による１０倍希釈液のｐＨが９．５を示したロットのＮＭＰに、上記フッ化ビニリデン系重合体を１３重量％の濃度となるように温度５０℃で溶解して、バインダー溶液を得た。
【００２９】
実施例１
上記比較例１で用いたロットのＮＭＰにマレイン酸を１０００ｐｐｍ添加し、再度イオン交換水による１０倍希釈液のｐＨを測定すると３．２であった。次にこのマレイン酸を添加したＮＭＰに、上記フッ化ビニリデン系重合体を比較例１と同じく１３重量％の濃度となるように溶解して、バインダー溶液を得た。
【００３０】
（粘度およびフッ素イオン濃度測定）
上記比較例１および実施例１で得られたバインダー溶液の粘度を、それぞれＪＩＳ Ｋ７１１７に準拠した回転粘度計を用いて測定した。またフッ素イオン（Ｆ- ）の濃度も測定した。
【００３１】
（フィルム形成および膨潤テスト）
上記比較例１および実施例１のバインダー溶液を、それぞれガラス板上にキャストし、１３０℃で２時間乾燥し、厚さ約２００μｍのフィルムを得た。
【００３２】
次いでこれらフィルムを、ＬｉＣｌＯ4 ８．８重量部を、プロピレンカーボネート５３．６重量部とジメトキシエタン３７．６重量部の混合液に溶解して得た電解液に、それぞれ浸漬し、７０℃、７２時間の浸漬を行い、途中２４時間毎にキャストフィルムの重量増加率（増加重量／原フィルム重量×１００）による膨潤度を求めた。
【００３３】
上記測定結果をまとめて、下表１に示す。
【００３４】
【表１】
上表１の結果は、ＮＭＰをそのまま用いた比較例のフッ化ビニリデン系重合体バインダー溶液に比べて、１０００ｐｐｍのマレイン酸を添加して得られたフッ化ビニリデン系重合体バインダー溶液は、粘度上昇、フッ素イオン濃度がともに低く、安定であること、また非水電池用電解液に対する耐膨潤性が顕著に改善されていることがわかる。
【００３６】
比較例２
イオン交換水による１０倍希釈液のｐＨが１０．９を示したロットのＮＭＰに上記カルボキシル基含有フッ化ビニリデン系重合体を１３重量％の濃度となるように溶解してバインダー溶液を形成した。
【００３７】
実施例２
上記比較例２と同じロットのＮＭＰに、１０００ｐｐｍのマロン酸を添加し、その後上記カルボキシル基含有フッ化ビニリデン系重合体を１３重量％の濃度となるように溶解して、バインダー溶液を形成した。
【００３８】
上記比較例２および実施例２のバインダー溶液から、それぞれ上記実施例１と同様にフィルムを形成し、同様に７０℃、７２時間の電解液中浸漬膨潤テストを行った。７２時間後の膨潤度は以下の通りであった。
【００３９】
【表２】
比較例３
イオン交換水による１０倍希釈液のｐＨが、９．５を示したロットのＮＭＰにフッ化ビニリデンホモポリマー（「ＫＦ１１００」、呉羽化学工業（株）製）を１３重量％の濃度となるように温度５０℃で溶解して、バインダー溶液を得た。
【００４１】
実施例３
上記比較例３で用いたロットのＮＭＰにマレイン酸を１０００ｐｐｍ添加し、再度イオン交換水による１０倍希釈液のｐＨを測定すると３．２であった。次にこのマレイン酸を添加したＮＭＰに、上記比較例１で用いたフッ化ビニリデンホモポリマーを同じく１３重量％の濃度となるように溶解してバインダー溶液を得た。
【００４２】
上記比較例３、実施例３で得られたバインダー溶液について実施例１と同様に粘度およびフィルム膨潤度を測定した結果を下表３に示す。
【００４３】
【表３】
ここでも酸添加による、良好なバインダー溶液の粘度上昇防止効果、フィルム耐膨潤性の向上効果が認められる。
【００４５】
実施例４、比較例４
上記実施例１、比較例１で得られたフィルムを、それぞれ、ＬｉＰＦ6 １１．６重量％、エチレンカーボネート５１．０重量％およびジエチルカーボネート３７．４重量％からなる電解液に、７０℃で７２時間浸漬し、放置した。浸漬後のフィルムの膨潤度（膨潤による重量増加率）は、それぞれ１８重量％および２４重量％であった。
【００４６】
【発明の効果】
上述したように、本発明によれば、フッ化ビニリデン系重合体を有機溶媒に溶解して非水系電池電極形成用のバインダー溶液を形成するに際し、酸、好ましくは有機酸を添加することにより、従来はしばしば問題となったバインダー溶液の粘度上昇、ならびに形成される電極中でのバインダーの電解液による膨潤のためのバインダ効果の低減を効果的に防止することが可能となる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a vinylidene fluoride-based material used for stably fixing a powdered electrode material (mainly an electrode active material and an optional conductive additive) to a nonaqueous battery, particularly a lithium ion battery. The present invention relates to a solution of a binder made of a polymer, that is, a vinylidene fluoride-based polymer binder solution, and an electrode mixture obtained by dispersing a powdered electrode material in the binder solution.
[0002]
[Prior art]
In recent years, the development of electronic technology has been remarkable, and various devices have been reduced in size and weight. Along with the reduction in size and weight of the electronic device, the demand for reduction in size and weight of a battery serving as a power source for the electronic device has also become very large. In order to obtain more energy with a small volume and weight, it is necessary to increase the voltage per battery. From this viewpoint, recently, a carbonaceous material capable of storing lithium or lithium ions is used as a negative electrode active material, and a positive electrode active material is used. For example, a battery using a non-aqueous electrolyte using lithium cobalt oxide has attracted attention.
[0003]
However, in such a non-aqueous battery, the ionic conductivity of the aqueous electrolyte is usually 10 -1 S / cm, whereas the ionic conductivity of the non-aqueous electrolyte is 10 -2 to 10 -4 S. / Cm, it is indispensable to use a thin and large-area electrode with a thickness of several μm to several hundred μm as a practical battery. As a method for industrially inexpensively obtaining such a thin-film and large-area electrode, in a binder solution obtained by dissolving an organic polymer serving as a binder for an electrode active material in a solvent, after dispersing the powdered electrode material, 2. Description of the Related Art There is known a method of forming an electrode by coating and drying on a metal electrode foil or a metal net. Further, as described in JP-A-6-93025 and JP-A-6-172452, various types of vinylidene fluoride-based polymers can be used as the binder solution for non-aqueous batteries. Those dissolved in polar solvents such as -2-pyrrolidone, dimethylformamide, and N, N-dimethylacetamide are known. This is because the vinylidene fluoride polymer is excellent in chemical resistance, weather resistance, stain resistance, etc., and dissolves in the polar solvent as described above, but does not react with the non-aqueous electrolyte used in non-aqueous batteries. Is stable because it swells to some extent, and can maintain good adhesion to a base material such as a metal by copolymerization or modification.
[0004]
[Problems to be solved by the invention]
When a vinylidene fluoride polymer as described above was used as a binder and dissolved in a polar organic solvent, a phenomenon was found in which the solution viscosity increased more than usual depending on the lot of the solvent. When the viscosity of the solution increases, not only is it difficult to obtain a uniform film thickness in the electrode coating process, but also gelling occurs during kneading with the active material, which sometimes makes film formation itself difficult. In addition, even if film formation was possible, it was found that the binder after coating and drying had a large degree of swelling in a non-aqueous electrolyte. When the swelling of the binder in the non-aqueous electrolyte becomes severe, the contact resistance between the powdered electrode material (especially the active material) and the metal electrode foil or metal net or the contact resistance between the active materials increases, and the internal resistance of the battery increases. I do. In the case of a secondary battery that can be repeatedly charged and discharged, an increase in the internal resistance leads to deterioration of the charge / discharge cycle characteristics, which may shorten the battery life.
[0005]
Accordingly, a main object of the present invention is to provide a non-aqueous battery which is stable without itself causing an increase in viscosity and which can form a stable battery electrode without excessive swelling in a non-aqueous electrolyte. An object of the present invention is to provide a vinylidene fluoride polymer binder solution for forming an electrode and an electrode mixture having stable characteristics in which a powdered electrode material is dispersed therein.
[0006]
[Means for Solving the Problems]
The vinylidene fluoride polymer binder solution for forming a non-aqueous battery electrode of the present invention has been developed in order to achieve the above object, and is obtained by dissolving a vinylidene fluoride polymer in an organic solvent. A vinylidene fluoride polymer binder solution for forming a non-aqueous battery electrode prepared by adding an acid so that the pH of a solution obtained by collecting a part of the solution and diluting it ten times with ion-exchanged water is 9 or less. It is characterized by the following.
[0007]
The operation and effect of the present invention will be added a little. As a result of diligent studies by the present inventors, it has been found that the cause of the above-mentioned abnormal increase in viscosity in the vinylidene fluoride-based polymer binder solution is related to the acid-alkali degree of the system. That is, although it is difficult to directly determine the acid-alkalinity of the organic solvent solution of the above-mentioned vinylidene fluoride polymer, a part of the solution is obtained by diluting it ten times with ion-exchanged water. PH is one measure. Then, in the system in which the viscosity increased, it was found that the pH of the liquid measured in this manner exceeded 9. This is related to the hydrofluorination reaction of vinylidene fluoride polymer in an alkaline medium, which is a known phenomenon per se, and is directly related to the binder performance. Many polar solvents that show good solubility in a system polymer are nitrogen-containing organic solvents such as N-methylpyrrolidone and dimethylformamide. It is understood that this is related to an increase in the alkalinity of the solvent due to the retention, generation, or release of an alkaline substance such as an amine during the production of such a nitrogen-containing organic solvent or during storage thereafter. The decrease in solvent resistance (swelling resistance) as a binder due to the removal of hydrofluoric acid from the vinylidene fluoride-based polymer is due to the fact that oxygen is easily added to the defluorination site, and as a result, the affinity for the non-aqueous electrolyte solution It is understood that the increase in the heterogeneity bond or the decrease in the crystallinity of the polymer, which is a powerful factor that gives good solvent resistance, is considered. According to the present invention, based on such analysis, the above-described disadvantages associated with the phenomenon of increasing the viscosity of the vinylidene fluoride-based polymer binder solution are effectively suppressed by lowering the pH of the system to 9 or less by adding an acid. Based on the knowledge of
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
The vinylidene fluoride polymer used in the present invention includes vinylidene fluoride homopolymers, copolymers, and modified products thereof. A homopolymer of vinylidene fluoride is preferable from the viewpoint of durability against a non-aqueous electrolyte, particularly swelling resistance. However, since the adhesion to an electrode substrate such as a metal is slightly insufficient, it is more preferable to use a copolymer with another monomer, particularly a monoester of unsaturated dibasic acid, vinylene carbonate or an epoxy-containing vinyl monomer. A copolymer in which a polar group such as a carboxyl group, a carbonate group, an epoxy group or the like is introduced by copolymerization with a polymer or the like (JP-A-6-172452) is preferably used. A silane coupling agent having both a vinylidene fluoride polymer such as an amino group or a mercapto group and a reactive group and a hydrolyzable group in a solvent that dissolves or swells the vinylidene fluoride homopolymer or copolymer. Alternatively, a modified vinylidene fluoride-based polymer treated in a titanate-based coupling agent (JP-A-6-93025) is also preferably used. However, in order to maintain good swelling resistance to the non-aqueous electrolyte as a whole, it is preferable to maintain the untreated vinylidene fluoride unit in a range of 90 mol% or more, particularly 95 mol% or more.
[0009]
The vinylidene fluoride polymer has an intrinsic viscosity (logarithmic viscosity at 30 ° C. of a solution obtained by dissolving 4 g of resin in 1 liter of N, N-dimethylformamide) of 0.5 or more, and more preferably 0.5 to 2. It preferably has a value in the range of 0, especially 0.8 to 1.5.
[0010]
The organic solvent used for dissolving the vinylidene fluoride-based polymer to obtain the binder solution of the present invention is preferably a polar one such as N-methyl-2-pyrrolidone, dimethylformamide, N, N-dimethylacetamide, N, N-dimethylsulfoxide, hexamethylphosphamide, dioxane, tetrahydrofuran, tetramethylurea, triethylphosphate, trimethylphosphate, and the like. These organic solvents are used alone or in combination of two or more. As described above, many of the good solvents of the vinylidene fluoride-based polymer are nitrogen-containing solvents, and those that easily produce an alkaline substance that causes an increase in the viscosity of the binder solution are also nitrogen-containing organic solvents. Accordingly, the organic solvent preferably used in the present invention is a nitrogen-containing organic solvent.However, according to the present invention, the pH of the non-nitrogen-based organic solvent is adjusted by adding an acid, for some reason. This is preferable because inconvenience associated with an increase in pH can be avoided. Such non-nitrogen-based good solvents include the above-mentioned dioxane, tetrahydrofuran, trialkyl phosphate and the like, and are preferably used alone or in combination with a nitrogen-containing organic solvent.
[0011]
In obtaining the binder solution of the present invention, it is preferable to dissolve the vinylidene fluoride-based polymer at a ratio of 0.1 to 20 parts by weight, particularly 1 to 15 parts by weight, based on 100 parts by weight of the organic solvent. If the amount is less than 0.1 part by weight, the proportion of the polymer in the solution is too small, and the effect as a binder cannot be obtained. On the other hand, if the amount exceeds 20 parts by weight, the viscosity of the solution becomes too high, and it becomes difficult to adjust the electrode mixture.
[0012]
According to the present invention, a part of the binder solution to which the acid has been added is sampled and adjusted so that the pH of the 10-fold diluted solution with ion-exchanged water becomes 9 or less.
[0013]
The acid to be added is not fundamentally limited, but is preferably one that is applied as an electrode mixture and scattered by decomposition or evaporation during the drying step and does not remain in the molded electrode, and reacts with the electrode active material. Those that are difficult to perform are preferred. In this respect, inorganic acids such as hydrochloric acid and sulfuric acid are not always preferable because they easily react with the electrode active material. In particular, when an electrode active material that easily forms an interlayer compound with an inorganic substance such as graphite is used, it is preferable to use an organic acid having a large molecular diameter that does not easily form an interlayer compound. Those having a high vapor pressure in the temperature range (usually 175 ° C. or lower, which is the melting point of PVDF) in the drying step, or those which decompose and scatter are preferred. Examples of the organic acid preferably used from such a viewpoint include acrylic acid, formic acid, citric acid, acetic acid, oxalic acid, lactic acid, pyrupic acid, malonic acid, propionic acid, maleic acid, and butyric acid.
[0014]
As a method of adding an acid to a vinylidene fluoride-based polymer solution, a method of adding an acid to an organic solvent in advance to adjust the pH of the solvent and then dissolving the polymer is preferable. A method of simultaneously adding and dissolving an acid at the time of dissolution, or a method of adding an acid to a solution after the polymer has been dissolved may be used.
[0015]
The amount of the acid added is such that the pH of the 10-fold diluted solution of the binder solution with ion-exchanged water after the addition (substantially coincides with the pH of the diluted solution when the acid is added to the organic solvent in advance according to a preferred embodiment) is 9 or less, preferably 7 or less. 0.5 or less. Although the lower limit is not particularly limited, it is generally up to about 3, and even if it is added more, the effect cannot be expected to be particularly increased. However, in the case of the organic acid in the preferred embodiment described above, it is not harmful even if it is added in excess because it is decomposed and volatilized in the forming step by coating and drying the electrode. Even if the pH of the organic solvent or the binder solution before adding the acid is 9 or less in advance, a certain amount of acid is added in anticipation of a buffering effect on the pH increase due to some reason such as decomposition of the solvent thereafter. It is preferred to add. In this case, the amount of the acid added is preferably 100 ppm or more, particularly preferably 300 to 10,000 ppm, per liter of the binder solution. In such an embodiment, it is particularly preferable to add an organic acid which causes little harm even if it remains.
[0016]
The powdered electrode material (active material, conductive material added as necessary, and other auxiliaries) is dispersed and mixed into the vinylidene fluoride polymer binder solution of the present invention obtained as described above, thereby forming an electrode. Agent is obtained.
[0017]
In the case of a positive electrode, the active material is represented by a general formula LiMY 2 (M is at least one of transition metals such as Co, Ni, Fe, Mn, Cr, and V: Y is a chalcogen element such as O and S). A composite metal chalcogen compound, particularly a composite metal oxide such as LiCoO 2 or a composite metal oxide having a spinel structure such as LiMn 2 O 4 is preferable. In the case of the negative electrode, a carbonaceous material such as graphite, activated carbon, or a product obtained by calcining and carbonizing a phenol resin or pitch is preferable as the active material.
[0018]
The conductive material is added for the purpose of improving the conductivity of the electrode mixture layer when using an active material having a small electron conductivity such as LiCoO 2 and the like. Fine metal powder such as nickel or aluminum, or fibers are used. When a conductive carbonaceous material is used as the active material, it is not necessary to use these conductive materials.
[0019]
In the electrode mixture of the present invention, it is preferable that 0.1 to 50 parts by weight, particularly 1 to 20 parts by weight, of a vinylidene fluoride polymer is mixed with 100 parts by weight of the powdered electrode material .
[0020]
The formed electrode mixture is made of a metal foil or a metal net of iron, stainless steel, steel, copper, aluminum, nickel, titanium or the like, and has a thickness of 5 to 100 μm, and for a small scale, for example, 5 to 20 μm. By coating on at least one side of the current collector, preferably both sides, and drying at, for example, 50 to 170 ° C., for example, forming an electrode mixture layer having a thickness of 10 to 1000 μm in a small scale, An electrode is formed.
[0021]
The electrode formed using the binder solution and the electrode mixture of the present invention is preferably used as an electrode of a non-aqueous battery, particularly a lithium ion battery, and the electrolyte used at this time is, for example, an electrolyte such as a lithium salt. A solution dissolved in a non-aqueous solvent (organic solvent) can be used.
[0022]
Here, examples of the electrolyte include LiPF 6 , LiAsF 6 , LiClO 3 , LiBF 4 , CH 3 SO 3 Li, CF 3 SO 3 Li, LiCl, and LiBr. Examples of the organic solvent for the electrolyte include propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyl lactone, methyl propionate, and propionic acid. Ethyl, a mixed solvent thereof and the like are used, but not limited thereto.
[0023]
【Example】
Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples.
[0024]
(Preparation of vinylidene fluoride polymer)
According to Example 1 described in JP-A-6-172452, a carboxyl group-containing vinylidene fluoride polymer was obtained as follows.
[0025]
An autoclave having a content of 2 liters was charged with 1040 g of ion-exchanged water, 0.8 g of methylcellulose, 2.5 g of ethyl acetate, 4 g of diisopropyl peroxydicarbonate, 396 g of vinylidene fluoride, and 4 g of monomethyl maleate (vinylidene fluoride: maleic acid). (Monomethyl ester = 100: 1.01) and suspension polymerization was carried out at 28 ° C. for 47 hours. After completion of the polymerization, the polymer slurry was dehydrated, washed with water and dried at 80 ° C. for 20 hours to obtain a polymer powder.
[0026]
The polymerization rate was 90% by weight, and the obtained polymer had an intrinsic viscosity of 1.1 and a carbonyl group content of 1.2 × 10 -4 mol / g.
[0027]
(Organic solvent)
Twenty lots of commercially available N-methyl-2-pyrrolidone stored at room temperature for about three months were used. Hereinafter, it is abbreviated as “NMP”. The pH of the 10-fold dilution with ion-exchanged water ranged from 6.5 to 11.0.
[0028]
Comparative Example 1
The above vinylidene fluoride polymer was dissolved at a temperature of 50 ° C. so as to have a concentration of 13% by weight in NMP of a lot in which the pH of the 10-fold dilution with ion-exchanged water showed 9.5, and the binder solution was dissolved. Obtained.
[0029]
Example 1
When 1000 ppm of maleic acid was added to the NMP of the lot used in Comparative Example 1 and the pH of the 10-fold diluted solution with ion-exchanged water was measured again, it was 3.2. Next, the vinylidene fluoride-based polymer was dissolved in NMP to which maleic acid was added so as to have a concentration of 13% by weight, similarly to Comparative Example 1, to obtain a binder solution.
[0030]
(Measurement of viscosity and fluorine ion concentration)
The viscosities of the binder solutions obtained in Comparative Example 1 and Example 1 were measured using a rotational viscometer according to JIS K7117. The fluorine ions (F -) concentration were also measured.
[0031]
(Film formation and swelling test)
Each of the binder solutions of Comparative Example 1 and Example 1 was cast on a glass plate and dried at 130 ° C. for 2 hours to obtain a film having a thickness of about 200 μm.
[0032]
Next, these films were immersed in an electrolytic solution obtained by dissolving 8.8 parts by weight of LiClO 4 in a mixture of 53.6 parts by weight of propylene carbonate and 37.6 parts by weight of dimethoxyethane. The film was immersed for a period of time, and the swelling degree was determined every 24 hours on the basis of the weight increase rate of the cast film (increased weight / raw film weight × 100).
[0033]
The above measurement results are summarized in Table 1 below.
[0034]
[Table 1]
The results in Table 1 above show that, compared to the vinylidene fluoride polymer binder solution of the comparative example using NMP as it is, the vinylidene fluoride polymer binder solution obtained by adding 1000 ppm of maleic acid had an increased viscosity. It can be seen that the fluorine ion concentration was low and stable, and that the swelling resistance to the electrolyte for nonaqueous batteries was significantly improved.
[0036]
Comparative Example 2
The above carboxyl group-containing vinylidene fluoride polymer was dissolved in NMP of a lot in which the pH of the 10-fold diluted solution with ion-exchanged water showed 10.9 to a concentration of 13% by weight to form a binder solution.
[0037]
Example 2
1000 ppm of malonic acid was added to NMP of the same lot as in Comparative Example 2, and then the carboxyl group-containing vinylidene fluoride polymer was dissolved to a concentration of 13% by weight to form a binder solution.
[0038]
Films were formed from the binder solutions of Comparative Example 2 and Example 2, respectively, in the same manner as in Example 1, and immersion swelling tests were performed at 70 ° C. for 72 hours in an electrolytic solution. The degree of swelling after 72 hours was as follows.
[0039]
[Table 2]
Comparative Example 3
The concentration of vinylidene fluoride homopolymer (“KF1100”, manufactured by Kureha Chemical Industry Co., Ltd.) was adjusted to 13% by weight to the NMP of the lot showing the pH of the 10-fold dilution with ion-exchanged water of 9.5. The binder was dissolved at a temperature of 50 ° C. to obtain a binder solution.
[0041]
Example 3
When 1000 ppm of maleic acid was added to NMP of the lot used in Comparative Example 3 and the pH of the 10-fold diluted solution with ion-exchanged water was measured again, it was 3.2. Next, the vinylidene fluoride homopolymer used in Comparative Example 1 was dissolved in NMP to which maleic acid was added so as to have a concentration of 13% by weight to obtain a binder solution.
[0042]
The results of measuring the viscosity and the degree of film swelling of the binder solutions obtained in Comparative Example 3 and Example 3 in the same manner as in Example 1 are shown in Table 3 below.
[0043]
[Table 3]
Also in this case, the effect of preventing the increase in viscosity of the binder solution and the effect of improving the swelling resistance of the film by adding the acid are recognized.
[0045]
Example 4, Comparative Example 4
Each of the films obtained in Example 1 and Comparative Example 1 was added to an electrolytic solution comprising 11.6% by weight of LiPF 6 , 51.0% by weight of ethylene carbonate and 37.4% by weight of diethyl carbonate at 70 ° C. for 72 hours. It was immersed for a time and allowed to stand. The degree of swelling of the film after immersion (weight increase by swelling) was 18% by weight and 24% by weight, respectively.
[0046]
【The invention's effect】
As described above, according to the present invention, when forming a binder solution for forming a non-aqueous battery electrode by dissolving a vinylidene fluoride polymer in an organic solvent, by adding an acid, preferably an organic acid, It is possible to effectively prevent the increase in the viscosity of the binder solution, which has often been a problem in the past, and the reduction of the binder effect due to the swelling of the binder in the formed electrode by the electrolytic solution.
Claims (7)
Priority Applications (6)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP35078295A JP3540080B2 (en) | 1995-12-26 | 1995-12-26 | Battery binder solution and electrode mixture |
| US08/779,955 US6200703B1 (en) | 1995-12-26 | 1996-12-23 | Binder solution and electrode-forming composition for non-aqueous-type battery |
| EP96309481A EP0782208B1 (en) | 1995-12-26 | 1996-12-24 | Binder solution and electrode-forming composition for non-aqueous-type battery |
| CA002193935A CA2193935C (en) | 1995-12-26 | 1996-12-24 | Binder solution and electrode-forming composition for non-aqueous-type battery |
| DE69635888T DE69635888T8 (en) | 1995-12-26 | 1996-12-24 | Binder solution and electrode forming composition for non-aqueous battery |
| KR1019960072262A KR100263735B1 (en) | 1995-12-26 | 1996-12-26 | Binder solution and electrode-forming composition for non-aqueous-type battery |
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|---|---|---|---|
| JP35078295A JP3540080B2 (en) | 1995-12-26 | 1995-12-26 | Battery binder solution and electrode mixture |
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| Publication Number | Publication Date |
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| JPH09180725A JPH09180725A (en) | 1997-07-11 |
| JP3540080B2 true JP3540080B2 (en) | 2004-07-07 |
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| JP35078295A Expired - Lifetime JP3540080B2 (en) | 1995-12-26 | 1995-12-26 | Battery binder solution and electrode mixture |
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| Country | Link |
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| JP (1) | JP3540080B2 (en) |
Families Citing this family (20)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| KR100454308B1 (en) | 1996-12-16 | 2004-10-26 | 다이낑 고오교 가부시키가이샤 | Binder for rechargeable battery with nonaqueous electrolyte and battery electrode depolarizing mix prepared using the same |
| JP4253051B2 (en) * | 1997-12-26 | 2009-04-08 | 株式会社クレハ | Nonaqueous battery electrode mixture and nonaqueous battery |
| KR100496276B1 (en) * | 1998-09-29 | 2005-09-09 | 삼성에스디아이 주식회사 | Electrode binder of secondary battery and method of making active material slurry with the same |
| KR100446663B1 (en) * | 2001-12-28 | 2004-09-04 | 주식회사 엘지화학 | Electrode having enhanced adhesive strength and battery comprising said electrode |
| JP4656366B2 (en) * | 2003-06-17 | 2011-03-23 | ソニー株式会社 | Electrode mixture, electrode and secondary battery |
| JP2005310747A (en) * | 2004-03-23 | 2005-11-04 | Kureha Chem Ind Co Ltd | Binder for forming nonaqueous electrochemical element electrode, electrode mix, electrode structure, and electrochemical element |
| KR100657951B1 (en) | 2005-02-18 | 2006-12-14 | 삼성에스디아이 주식회사 | Cathode active material, method of preparing the same, and cathode and lithium battery containing the material |
| US8642210B2 (en) | 2008-09-26 | 2014-02-04 | Mitsuyasu Sakuma | Negative electrode mixture for nonaqueous electrolyte secondary batteries, negative electrode for nonaqueous electrolyte secondary batteries, and nonaqueous electrolyte secondary battery |
| KR101221208B1 (en) * | 2008-11-13 | 2013-01-10 | 가부시끼가이샤 구레하 | Anode mixture for nonaqueous electrolyte secondary cell, anode for nonaqueous electrolyte secondary cell, and nonaqueous electrolyte secondary cell |
| JP5812817B2 (en) * | 2011-11-15 | 2015-11-17 | 電気化学工業株式会社 | Vinylidene fluoride resin composition, resin film, solar cell backsheet, and solar cell module |
| WO2015133424A1 (en) * | 2014-03-03 | 2015-09-11 | 日本ゼオン株式会社 | Secondary cell binder composition |
| JP6593196B2 (en) * | 2016-01-27 | 2019-10-23 | 日本ゼオン株式会社 | Storage method of composition for non-aqueous secondary battery |
| JP6245625B1 (en) | 2016-02-08 | 2017-12-13 | 国立研究開発法人産業技術総合研究所 | Method for producing slurry for positive electrode of non-aqueous electrolyte secondary battery and slurry for positive electrode of non-aqueous electrolyte secondary battery |
| JP6951177B2 (en) | 2017-02-09 | 2021-10-20 | 日本スピンドル製造株式会社 | Slurry production equipment and slurry production method |
| WO2020026486A1 (en) * | 2018-07-31 | 2020-02-06 | パナソニックＩｐマネジメント株式会社 | Positive electrode material and secondary battery |
| KR102621697B1 (en) * | 2018-08-16 | 2024-01-04 | 현대자동차주식회사 | A binder solution for all solid state battery, electrode slurry comprising the same and a method of preparing all solid state battery using the same |
| KR102152982B1 (en) * | 2018-10-15 | 2020-09-08 | 한국과학기술연구원 | Aqueous binder for lithium-sulfur secondary battery, preparation method thereof and lithium-sulfur secondary battery comprising the same |
| US11811065B2 (en) * | 2019-05-31 | 2023-11-07 | Zeon Corporation | Method of producing slurry composition for secondary battery positive electrode, method of producing positive electrode for secondary battery, and method of producing secondary battery |
| WO2023011914A1 (en) * | 2021-08-04 | 2023-02-09 | Varta Microbattery Gmbh | Cathode paste, method for producing same, and use thereof |
| EP4131501A1 (en) * | 2021-08-04 | 2023-02-08 | VARTA Microbattery GmbH | Cathode paste, method for manufacturing same and use of same |
-
1995
- 1995-12-26 JP JP35078295A patent/JP3540080B2/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| JPH09180725A (en) | 1997-07-11 |
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