JP2004200011A

JP2004200011A - Cathode for lithium-ion secondary battery and lithium-ion secondary battery manufactured using the cathode

Info

Publication number: JP2004200011A
Application number: JP2002367408A
Authority: JP
Inventors: Yusuke Watarai; 祐介渡会; Kanji Hisayoshi; 完治久芳; Akio Mizuguchi; 暁夫水口; Shuhin Cho; 守斌張
Original assignee: Mitsubishi Materials Corp
Current assignee: Mitsubishi Materials Corp
Priority date: 2002-12-19
Filing date: 2002-12-19
Publication date: 2004-07-15

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cathode for a lithium-ion secondary battery which is superior in adhesive property and conductivity between a cathode collector and cathode active substance layer, cycle capacity maintaining characteristic can be improved and the cycle characteristic can be improved by controlling the stress arising in insertion and release reaction of charged and discharged lithium ion. <P>SOLUTION: In the cathode material for the lithium-ion secondary battery, a cathode substance layer containing both of a first binding agent and non-organic particles containing at least 1 type of an element selected from a group consisting of Si, Ge, Mg, Sn, Pb, Ag, Al, Zn, Cd, Sb, Bi and In is formed on the surface of the cathode collector. An adhering layer containing both of a second binding agent and a conductive substance in between the cathode collector and the cathode active substance layer is provided. The first binding agent and the second binding agent each contain polymeric compounds containing modified fluoride which is obtained by modifying resin containing fluoride by a modifying substance. The structures of the polymeric compounds containing modified fluoride contained in the first binding agent and the second binding agent are the same or are different from each other. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、リチウムイオン二次電池用負極及び該負極を用いて作製したリチウムイオン二次電池に関する。
【０００２】
【従来の技術】
近年のビデオカメラやノート型パソコン等のポータブル機器の普及により薄型の電池に対する需要が高まっている。この薄型の電池として正極と負極を積層して形成されたリチウムイオン二次電池が知られている。この正極は、シート状の正極集電体の表面に正極活物質層を形成することにより作られ、負極は、シート状の負極集電体の表面に負極活物質層を形成することにより作られる。正極の活物質層と負極の活物質層の間には電解質層が介装される。この電池では、それぞれの活物質における電位差を電流として取出すための正極端子及び負極端子が正極集電体及び負極集電体に設けられ、このように積層されたものをパッケージで密閉することによりリチウムイオン二次電池が形成されている。このリチウムイオン二次電池ではパッケージから引出された正極端子及び負極端子を電池の端子として使用することにより所望の電気が得られるようになっている。
【０００３】
このような構造を有するリチウムイオン二次電池は電池電圧が高く、エネルギー密度も大きいため、非常に注目されている。このリチウムイオン二次電池の放電容量を更に増大させるためにはシート状の正極又は負極の面積を拡大させる必要がある。この正極又は負極の面積を単純に拡大するだけでは広い面積のために、その取扱いが困難になる不具合がある。この点を解消するために、拡大したシート状の正極又は負極を所望の大きさに折畳んだり、捲回したりすることも考えられる。しかし、シート状の正極又は負極を積層した状態で折畳みや捲回を行うと、折目部分における正極又は負極に撓みが生じ、その部分におけるシートが電解質層から剥離して電極と電解質界面の有効表面積が減少して放電容量が減少するとともに、電池内部に抵抗を生じさせて放電容量のサイクル特性を悪化させる不具合がある。また同様に、折目部分に撓みが生じることにより正極又は負極をそれぞれ形成している活物質層が集電体より剥離する問題もあった。更に、この電池は充電及び放電過程において、正極及び負極活物質中へのリチウムイオンの吸蔵、放出によって正極及び負極活物質層の膨張、収縮が起こり、これによって発生する応力により、活物質層が集電体から剥離する問題もあった。
【０００４】
そこで上記諸問題を解決する技術として下記に示すように、活物質層の集電体からの剥離や密着性の低下を防止する技術がそれぞれ提案されている。
平均重合度２０以上のポリビニルアルコール単位を有し、水酸基の一部又は全部が平均モル置換度０．３以上のオキシアルキレン含有基で置換された高分子化合物、該高分子化合物からなるバインダ樹脂、及び高いイオン導電性と高い粘着性を備えているイオン導電性高分子電解質用組成物並びに二次電池が提案されている（例えば、特許文献１参照。）。この発明では、固体高分子電解質の材料やバインダ樹脂の材料としてポリオキシアルキレン部分の導入割合を上げた高分子化合物を用いることで、イオン移動し易くして高分子電解質用ポリマーのイオン導電性を高めている。
【０００５】
イソシアネート化合物とポリオール化合物とを反応させて得られたポリウレタン化合物中のイソシアネート基の一部又は全部に双極子モーメントの大きな置換基を有するアルコール化合物の水酸基を反応させた高分子化合物、バインダ樹脂、イオン導電性高分子電解質用組成物並びに二次電池が提案されている（例えば、特許文献２参照。）。この発明では、双極子モーメントの大きな置換基をポリウレタン中に導入することにより、高い誘電率とイオン導電性塩を高濃度に溶解する能力を保持しながら、電極と電解質間の密着性を良好にして、電解質溶液並みの界面インピーダンスを得ている。
イソシアネート化合物とポリオール化合物とを反応させて得られたポリウレタン化合物中のイソシアネート基の一部又は全部に双極子モーメントの大きな置換基を有するアルコール化合物の水酸基を反応させた高分子化合物と、イオン導電性塩と、架橋可能な官能基を有する化合物とを主成分としたイオン導電性固体高分子用組成物、イオン導電性固体高分子電解質、バインダ樹脂及び二次電池が提案されている（例えば、特許文献３参照。）。
【０００６】
リチウム二次電池の負極用電極材、該電極材を用いた電極構造体、該電極構造体を用いたリチウム二次電池、及び該電極構造体及び該リチウム二次電池の製造方法が提案されている（例えば、特許文献４参照。）。この発明では、電極材に非化学量論比組成の非晶質Ｍ・Ａ・Ｘ合金（ＭはＳｉ、Ｇｅ及びＭｇからなる群より選ばれた少なくとも一種の元素を示し、Ａは遷移金属元素の中から選ばれる少なくとも一種の元素を示し、Ｘは、Ｏ、Ｆ、Ｎ、Ｂａ、Ｓｒ、Ｃａ、Ｌａ、Ｃｅ、Ｃ、Ｐ、Ｓ、Ｓｅ、Ｔｅ、Ｂ、Ｂｉ、Ｓｂ、Ａｌ、Ｉｎ及びＺｎからなる群より選ばれる少なくとも一種の元素を示す。但し、Ｘは含有されていなくてもよく、合金の構成要素Ｍの含量はＭ／(Ｍ＋Ａ＋Ｘ)＝２０〜８０原子％である。）を含有する。このような合金を含有する電極材は優れた特性を有し、リチウム二次電池の負極活性物質として好適である。
【０００７】
リチウムを含有する正極活物質を備えた正極と、リチウムのドープ・脱ドープが可能なケイ素化合物と炭素材料との混合物が結着剤中に分散されてなる負極活物質層を備えた負極と、正極と負極との間に介在される非水電解質とを備え、上記結着剤は、ガラス転位温度が−４０℃以下である非水電解質電池が提案されている（例えば、特許文献５参照。）。この発明では、負極活物質中の結着剤として、ガラス転位温度が−４０℃以下のものを用いているので、リチウムのドープ・脱ドープ時のケイ素化合物負極の体積変化を結着剤が吸収し、負極活物質層全体としての体積変化が抑制されて、サイクル劣化が抑えられる。
更に、リチウムイオンを吸蔵・放出可能な正極及び負極と、リチウムイオン導電性の非水電解質から構成され、正極又は負極の電極合剤中に水溶性ポリマーを含有した非水電解質二次電池が提案されている（例えば、特許文献６参照。）。この発明では、電極合剤中に水溶性ポリマーを含有することにより、集電体への結着性が良好であり、かつ溶媒が水であることから製造コストを削減でき、製造工程における人体への影響等も抑制できる。また比較的低温で乾燥可能であり、合剤やシート電極を構成する材料への熱的ダメージを最小限に留めることができる。
【０００８】
【特許文献１】
ＰＣＴ国際公開番号ＷＯ００／５６７８０号公報
【特許文献２】
ＰＣＴ国際公開番号ＷＯ００／５６７９７号公報
【特許文献３】
ＰＣＴ国際公開番号ＷＯ００／５６８１５号公報
【特許文献４】
ＰＣＴ国際公開番号ＷＯ００／１７９４９号公報
【特許文献５】
特開２０００−２９９１０８号公報
【特許文献６】
特開平９−２８９０２２号公報
【０００９】
【発明が解決しようとする課題】
しかしながら、一般的にリチウムイオン二次電池において、負極集電体と負極活物質層の接着は困難であり、上記特許文献４に示される負極用電極材では、例えば、Ｓｉ、Ｇｅ、Ａｌ、Ｓｎ等の元素を含む負極活物質の場合には、高い充放電容量が得られるものの、充放電に伴う活物質の体積変化が大きく、活物質層と集電体の剥離が生じる問題点があった。また特許文献５に示される結着剤では、柔軟性が特徴であり、活物質の体積変化をある程度吸収することができるが、この種の結着剤では集電体との接着強度が十分とはいえなかった。また特許文献１〜４及び６等は活物質と集電体の接着を向上することを目指して、結着剤の改良を行っているが、これらの結着剤は化学安定性が一般的に電池に使われるポリエチレン、ポリフッ化ビニリデン（以下、ＰＶｄＦという。）等の樹脂より劣ったり、電解質がある環境において接着性が足りなくなるというような問題が残っている。
【００１０】
本発明の第１の目的は、高い化学安定性を備え、かつ無機質粒子への密着性に優れる、リチウムイオン二次電池用負極を提供することにある。
本発明の第２の目的は、負極集電体と負極活物質層との密着性及び導電性に優れ、かつサイクル容量維持特性を向上し得る、リチウムイオン二次電池用負極及び該負極を用いて作製したリチウムイオン二次電池を提供することにある。
本発明の第３の目的は、高い機械強度を保持し得る、リチウムイオン二次電池用負極及び該負極を用いて作製したリチウムイオン二次電池を提供することにある。
本発明の第４の目的は、密着層が電解液中の有機溶媒に対して安定で長期保存性に優れる、リチウムイオン二次電池用負極及び該負極を用いて作製したリチウムイオン二次電池を提供することにある。
本発明の第５の目的は、電池内に発生するフッ酸等の強酸による集電体の腐食を抑制し得る、リチウムイオン二次電池を提供することにある。
本発明の第６の目的は、充放電のリチウムイオンの挿入、脱離反応に伴うストレスを制御し、サイクル特性を向上できる、リチウムイオン二次電池を提供することにある。
【００１１】
【課題を解決するための手段】
請求項１に係る発明は、図１に示すように、負極集電体１４の表面に第１結着剤と、Ｓｉ、Ｇｅ、Ｍｇ、Ｓｎ、Ｐｂ、Ａｇ、Ａｌ、Ｚｎ、Ｃｄ、Ｓｂ、Ｂｉ及びＩｎからなる群より選ばれた少なくとも１種の元素を含む無機質粒子の双方をそれぞれ含む負極活物質層１６が形成され、負極集電体１４と負極活物質層１６との間に第２結着剤と導電性物質の双方をそれぞれ含む密着層１９を有するリチウムイオン二次電池用負極の改良である。その特徴ある構成は、第１結着剤及び第２結着剤がフッ素含有樹脂を変性物質により変性して得られた変性フッ素含有高分子化合物をそれぞれ含み、第１結着剤及び第２結着剤にそれぞれ含まれる変性フッ素含有高分子化合物の構造が互いに同一又は異なるところにある。
請求項１に係る発明では、負極活物質層１６を形成する第１結着剤に含まれるフッ素含有樹脂を変性物質により変性して得られた変性フッ素含有高分子化合物は、フッ素含有高分子化合物が有する高い化学安定性と同様の安定性を備えるだけでなく、無機質粒子への密着性に優れるため、充放電によるリチウムの吸蔵、脱離に伴う無機質粒子の体積変化に起因する無機質粒子の脱落や、電極の剥離等を防止し、サイクル特性を向上することができる。また、負極集電体１４と負極活物質層１６との間にフッ素含有樹脂を変性物質により変性して得られた変性フッ素含有高分子化合物を含む第２結着剤と導電性物質の双方をそれぞれ含む密着層１９を有することで、密着力を向上させているため、無機質粒子を含む負極活物質層の体積変化による剥離を防止する。
【００１２】
請求項２に係る発明は、請求項１に係る発明であって、図２（ｃ）に示すように、第１結着剤又は第２結着剤のどちらか一方又はその双方に含まれる変性フッ素含有高分子化合物２４がフッ素含有樹脂を幹重合体２２とし、幹重合体２２を変性物質２３によりグラフト変性させて得られる高分子化合物である負極である。
請求項３に係る発明は、請求項２に係る発明であって、変性フッ素含有高分子化合物２４を構成する幹重合体２２が、ＰＶｄＦ、ポリフッ化ビニル（以下、ＰＶＦという。）、４フッ化エチレンポリマー、３フッ化エチレンポリマー、２フッ化エチレンポリマー、フッ化ビニリデン-ヘキサフルオロプロピレン共重合体（以下、ＶｄＦ-ＨＦＰ共重合体という。）、エチレン-４フッ化エチレン共重合体、４フッ化エチレン-６フッ化プロピレン共重合体、３フッ化塩化エチレンポリマー及びポリテトラフルオロエチレンからなる群より選ばれた少なくとも１種のフッ素含有樹脂を含む負極である。
請求項３に係る発明では、ＶｄＦ-ＨＦＰ共重合体、ＰＶｄＦが電解液への耐久性が高いため好ましい。
【００１３】
請求項４に係る発明は、請求項２に係る発明であって、変性フッ素含有高分子化合物２４を構成する変性物質２３が、アクリル酸、アクリル酸メチル、メタクリル酸及びメタクリル酸メチルの少なくとも１種である負極である。
請求項４に係る発明では、上記変性物質を用いることにより密着層は負極集電体と良好な密着性を得ることができる。
請求項５に係る発明は、請求項２ないし４いずれか１項に係る発明であって、変性フッ素含有高分子化合物２４を構成する幹重合体２２がＰＶｄＦ又はＶｄＦ-ＨＦＰ共重合体のいずれか一方又は双方を含む混合物であって、変性物質２３がアクリル酸である負極である。
【００１４】
請求項６に係る発明は、請求項２ないし５いずれか１項に係る発明であって、変性フッ素含有高分子化合物２４はフッ素含有樹脂２２と変性物質２３が放射線照射処理によってグラフト化される負極である。
請求項７に係る発明は、請求項６に係る発明であって、フッ素含有樹脂２２への放射線照射処理はγ線照射によって行われ、フッ素含有樹脂のγ線の吸収線量が１０〜９０ｋＧｙである負極である。
請求項８に係る発明は、請求項１ないし７いずれか１項に係る発明であって、第１結着剤に含まれる変性フッ素含有高分子化合物が、第２結着剤に含まれる変性フッ素含有高分子化合物を構成する反復単位をその反復単位として含む負極である。
【００１５】
請求項８に係る発明では、第１結着剤に含まれる変性フッ素含有高分子化合物が、第２結着剤に含まれる変性フッ素含有高分子化合物を構成する反復単位をその反復単位として含むことで、負極活物質層と密着層との密着力がより強固になる。
請求項９に係る発明は、請求項１ないし８いずれか１項に係る発明であって、第１結着剤に変性フッ素含有高分子化合物とは構造の異なるフッ素含有高分子化合物を更に含む負極である。
請求項９に係る発明では、変性フッ素含有高分子化合物だけでなく、フッ素含有高分子化合物を更に含むことで、化学安定性が更に高まり、集電体への密着性も向上する。また、変性フッ素含有高分子化合物よりも柔軟性に富むフッ素含有高分子化合物を含ませることで、充放電によるリチウムの吸蔵、脱離に伴う無機質粒子の体積変化を起因とするストレスを緩和する効果が得られる。
【００１６】
請求項１０に係る発明は、請求項９に係る発明であって、フッ素含有高分子化合物が、変性フッ素含有高分子化合物を構成するフッ素含有樹脂に含まれる反復単位をその反復単位として含む負極である。
請求項１０に係る発明では、フッ素含有高分子化合物の反復単位が、変性フッ素含有高分子化合物を構成するフッ素含有樹脂に含まれる反復単位を含むことで、樹脂結着剤の化学安定性が増すとともに、密着力がより強固になる。
【００１７】
請求項１１に係る発明は、請求項１ないし１０いずれか１項に係る発明であって、第１結着剤に含まれる変性フッ素含有高分子化合物の含有割合が１０重量％〜１００重量％である負極である。
請求項１２に係る発明は、請求項１ないし１０いずれか１項に係る発明であって、第２結着剤に含まれる変性フッ素含有高分子化合物の含有割合が１０重量％〜１００重量％である負極である。
【００１８】
請求項１３に係る発明は、請求項１１又は１２に係る発明であって、第１結着剤に含まれる変性フッ素含有高分子化合物の含有割合が、第２結着剤に含まれる変性フッ素含有高分子化合物の含有割合と同一の割合、又はそれよりも低い割合である負極である。
請求項１４に係る発明は、請求項１ないし１３いずれか１項に係る発明であって、第１結着剤に含まれる変性フッ素含有高分子化合物のグラフト重合割合（変性物質の重量／フッ素含有樹脂の重量）が、第２結着剤に含まれる変性フッ素含有高分子化合物のグラフト重合割合と同一の割合、又はそれよりも低い割合である負極である。
【００１９】
請求項１５に係る発明は、請求項１４に係る発明であって、第１結着剤又は第２結着剤のどちらか一方又はその双方に含まれる変性フッ素含有高分子化合物のグラフト重合割合が、２重量％〜５０重量％である負極である。
請求項１６に係る発明は、請求項１ないし１５いずれか１項に係る発明であって、負極活物質層１６に含まれる第１結着剤の割合が活物質層固形物全体の１重量％〜５０重量％である負極である。
請求項１７に係る発明は、請求項１ないし１６いずれか１項に係る発明であって、密着層１９に含まれる第２結着剤の割合が密着層固形物全体の５〜４０重量％である負極である。
【００２０】
請求項１８に係る発明は、請求項１に係る発明であって、無機質粒子はＳｉ、Ｇｅ、Ｍｇ、Ｓｎ、Ｐｂ、Ａｇ、Ａｌ、Ｚｎ、Ｃｄ、Ｓｂ、Ｂｉ及びＩｎからなる群より選ばれた少なくとも１種の元素が単体、酸化物又は他の金属との合金、単体とリチウムとの合金、及びこれらの金属、リチウムを含む多元合金で構成される負極である。
請求項１９に係る発明は、請求項１又は１８に係る発明であって、無機質粒子の平均粒径が０．０１〜５０μｍである負極である。
請求項２０に係る発明は、請求項１，１８又は１９いずれか１項に係る発明であって、負極活物質層１６に含まれる無機質粒子の割合が活物質層固形物全体の５〜９５重量％である負極である。
【００２１】
請求項２１に係る発明は、図１に示すように、請求項１ないし２０いずれか１項に記載の負極１７を用いて作製したリチウムイオン二次電池である。
請求項２１に係る発明では、本発明の負極を用いて作製したリチウムイオン二次電池は、集電体と活物質との密着性に優れ、高い機械強度を保持できる。また、充放電のリチウムイオンの挿入、脱離反応に伴うストレスを制御し、サイクル特性を向上できる。
【００２２】
【発明の実施の形態】
次に本発明の実施の形態を図面に基づいて説明する。
本発明のリチウムイオン二次電池用負極は、図１に示すように、負極集電体１４の表面に第１結着剤と、Ｓｉ、Ｇｅ、Ｍｇ、Ｓｎ、Ｐｂ、Ａｇ、Ａｌ、Ｚｎ、Ｃｄ、Ｓｂ、Ｂｉ及びＩｎからなる群より選ばれた少なくとも１種の元素を含む無機質粒子の双方をそれぞれ含む負極活物質層１６が形成され、負極集電体１４と負極活物質層１６との間に第２結着剤と導電性物質の双方をそれぞれ含む密着層１９を有する。その特徴ある構成は、第１結着剤及び第２結着剤がフッ素含有樹脂を変性物質により変性して得られた変性フッ素含有高分子化合物をそれぞれ含み、第１結着剤及び第２結着剤にそれぞれ含まれる変性フッ素含有高分子化合物の構造が互いに同一又は異なるところにある。負極活物質層１６を形成する第１結着剤に含まれるフッ素含有樹脂を変性物質により変性して得られた変性フッ素含有高分子化合物は、フッ素含有高分子化合物が有する高い化学安定性と同様の安定性を備えるだけでなく、無機質粒子への密着性に優れるため、充放電によるリチウムの吸蔵、脱離に伴う無機質粒子の体積変化に起因する無機質粒子の脱落や、電極の剥離等を防止し、サイクル特性を向上することができる。また、負極集電体１４と負極活物質層１６との間にフッ素含有樹脂を変性物質により変性して得られた変性フッ素含有高分子化合物を含む第２結着剤と導電性物質の双方をそれぞれ含む密着層１９を有することで、密着力を向上させているため、無機質粒子を含む負極活物質層の体積変化による剥離を防止する。第１及び第２結着剤に含まれる変性フッ素含有高分子化合物は、互いに同一の構造でも良いし、又は異なった構造を有していても良い。この変性フッ素含有高分子化合物２４はフッ素含有樹脂を幹重合体２２とし、幹重合体２２を変性物質２３によりグラフト変性させて得られる高分子化合物である。
【００２３】
ここで「変性」とは、性質が変わることを意味し、本明細書では、幹重合体である樹脂のところどころに変性物質が配列することにより、幹重合体である樹脂自身が持つ性質だけでなく、変性物質が持つ性質も併せ持ったり、両者にない性質を新たに持たせることを意味する。
負極活物質層及び密着層を構成する第１及び第２結着剤に含まれる高分子化合物に、分子内にフッ素を含む化合物を用いるのは、負極活物質層が電池内において化学的、電気化学的、熱的に安定性が要求されるためである。
【００２４】
変性フッ素含有高分子化合物は、変性させたことで化学的に安定性が向上するため、フッ素含有高分子化合物が有する高い化学安定性と同様の安定性を備えるだけでなく、無機質粒子への密着性に優れるため、充放電によるリチウムの挿入、脱離に伴う無機質粒子の体積変化に起因する無機質粒子の脱落や、電極の剥離等を防止し、サイクル特性を向上することができる。また、電解液に対しても溶解されることなく活物質層の集電体からの剥がれが抑制される。また同様の理由から密着層中に分散される導電性物質が崩落することなく保持されるため良好な電子伝導を維持し、長期保存性やサイクル特性に優れる。このように化学的に安定な層に集電体が被覆されるため、電池内部でフッ酸等が発生した場合でも密着層が保護層となり集電体の腐食を抑制できる。
また、変性させたことで熱的にも安定性が向上するため、電池が高温下におかれても電池内溶媒に溶解することがなく電池の劣化を抑制できる。
更に、変性させたことで電気化学的にも安定性が向上するため、劣化することがなく安定した密着力と導電性を保つ。更に電解液が変性高分子化合物中に浸透するのが困難となるため、集電体への電解液の付着がなくなる。
【００２５】
幹重合体のところどころに変性物質を枝重合体として配列して変性させるには、グラフト重合を用いる。幹重合体２２には、ＰＶｄＦ、ＰＶＦ、４フッ化エチレンポリマー、３フッ化エチレンポリマー、２フッ化エチレンポリマー、ＶｄＦ-ＨＦＰ共重合体、エチレン-４フッ化エチレン共重合体、４フッ化エチレン-６フッ化プロピレン共重合体、３フッ化塩化エチレンポリマー及びポリテトラフルオロエチレンからなる群より選ばれた少なくとも１種のフッ素含有樹脂が含まれる。このうち、ＶｄＦ-ＨＦＰ共重合体、ＰＶｄＦが電解液への耐久性が高いため好ましい。また変性物質２３には、アクリル酸、アクリル酸メチル、メタクリル酸及びメタクリル酸メチルの少なくとも１種が挙げられる。これらの変性物質を用いることで、無機質粒子や負極集電体と良好な密着性を得ることができる。
特に、幹重合体２２をＰＶｄＦ又はＶｄＦ-ＨＦＰ共重合体のいずれか一方又は双方を含む混合物とし、変性物質２３をアクリル酸とした変性フッ素含有高分子化合物２４は、ＰＶｄＦ又はＶｄＦ-ＨＦＰ共重合体のいずれか一方又は双方を含む混合物が、負極活物質層中の無機質粒子や負極活物質層と高い密着性を有し、アクリル酸が負極集電体材料と高い密着性を有するため、第１結着剤に含まれる変性フッ素含有高分子化合物、密着層中の第２結着剤に含まれる変性フッ素含有高分子化合物としてそれぞれ優れた性質を有する。
【００２６】
グラフト重合させる方法としては放射線法がある。放射線法では、幹重合体２２となるフッ素含有樹脂と枝重合体となる変性物質とを混合して、混合物に放射線を連続的又は間欠的に放射することにより重合でき、変性物質とフッ素含有樹脂とを接触させる前にフッ素含有樹脂を予備放射することが好ましい。具体的には、フッ素含有樹脂に放射線を照射した後で、被照射物に変性物質を混合することにより、フッ素含有樹脂を幹重合体とし変性物質を枝重合体とした変性フッ素含有高分子化合物を得ることができる。グラフト重合に用いる放射線にはγ線を使用する。フッ素含有樹脂への吸収線量が１〜９０ｋＧｙになるようにγ線を照射する。幹重合体となるフッ素含有樹脂に放射線を照射することにより片末端にラジカルが形成され、変性物質が重合し易くなる。
【００２７】
図２（ａ）に示すように、変性フッ素含有高分子化合物を構成する幹重合体２２は、繰返し単位Ａを基本単位とする。幹重合体の基本単位は、図２（ａ）に示すように１種類でもよいし、図示しないが２種類以上の基本単位を用いてもよい。この基本単位は幹重合体中に５０分子量％以上の割合で含まれることが好ましい。６０〜１００分子量％の割合で含まれるのがより好ましい。図２（ｂ）に示すように、この幹重合体２２に放射線を照射して基本単位Ａの片末端のところどころにラジカル２２ａを形成する。図２（ｃ）に示すように、ラジカルを形成した幹重合体２２に変性物質２３（図２中のＣ）を接触させることにより、ラジカル部分２２ａに変性物質２３が配列して変性フッ素含有高分子化合物２４が形成される。変性物質２３は変性フッ素含有高分子化合物中に２〜５０重量％の割合で含まれることが好ましい。変性フッ素含有高分子化合物中に５〜３０重量％の割合で含まれるのがより好ましい。
グラフト重合は活性化した幹重合体となる樹脂が枝重合体となる変性物質と接触している時間の長さ、放射線による予備活性の程度、枝重合体となる変性物質が幹重合体となる樹脂を透過できるまでの程度、樹脂及び変性物質が接触しているときの温度等によりそれぞれ重合生成が異なる。変性物質が酸である場合、変性物質である化合物を含有する溶液をサンプリングして、アルカリにより滴定し、残留する酸化合物濃度を測定することにより、グラフト化の程度を観測することができる。得られた組成物中のグラフト化の割合は、最終重量の２〜５０分子量％が望ましい。
【００２８】
第１結着剤に含まれる変性フッ素含有高分子化合物は、第２結着剤に含まれる変性フッ素含有高分子化合物を構成する反復単位をその反復単位として含むことが好ましい。これにより負極活物質層と密着層との密着力がより強固になる。
第１結着剤には、変性フッ素含有高分子化合物とは構造の異なるフッ素含有高分子化合物を更に含むことが好ましい。第１結着剤に変性フッ素含有高分子化合物だけでなく、フッ素含有高分子化合物を更に含むことで、化学安定性が更に高まり、集電体への密着性も向上する。また、変性フッ素含有高分子化合物よりも柔軟性に富むフッ素含有高分子化合物を含むことで、充放電によるリチウムの吸蔵、脱離に伴う無機質粒子の体積変化を起因とするストレスを緩和する効果が得られる。
フッ素含有高分子化合物は、変性フッ素含有高分子化合物を構成するフッ素含有樹脂に含まれる反復単位をその反復単位として含むことが好ましい。これにより第１結着剤の化学安定性が更に増すとともに、密着力がより強固になる。
【００２９】
第１結着剤中に含まれる変性フッ素含有高分子化合物の含有割合は１０重量％〜１００重量％に規定される。含有割合が１０重量％未満では活物質層の機械的強度が低くなる不具合を生じる。好ましい含有割合は、１０重量％〜５０重量％である。
また、第２結着剤中に含まれる変性フッ素含有高分子化合物の含有割合は１０重量％〜１００重量％に規定される。含有割合が１０重量％未満では密着層と集電体の接着力が低くなる不具合を生じる。好ましい含有割合は、５０重量％〜１００重量％である。
【００３０】
第１結着剤に含まれる変性フッ素含有高分子化合物の含有割合は、第２結着剤に含まれる変性フッ素含有高分子化合物の含有割合と同一の割合、又はそれよりも低い割合に規定される。
第１結着剤に含まれる変性フッ素含有高分子化合物のグラフト重合割合（変性物質の重量／フッ素含有樹脂の重量）は、第２結着剤に含まれる変性フッ素含有高分子化合物のグラフト重合割合と同一の割合又はそれよりも低い割合に規定される。
【００３１】
第１結着剤又は第２結着剤のどちらか一方又はその双方に含まれる変性フッ素含有高分子化合物のグラフト重合割合は、２重量％〜５０重量％に規定される。２重量％未満であると、変性させた効果が得られず、５０重量％を越えると、高分子の柔軟性が失われる不具合を生じる。好ましいグラフト重合割合は５重量％〜４５重量％である。
負極活物質層１６に含まれる第１結着剤の割合は、活物質層固形物全体の１重量％〜５０重量％である。下限値未満では無機質粒子との密着性が十分に得られず、上限値を越えると充放電容量が低下する。好ましくは活物質層固形物全体の２重量％〜２０重量％である。
【００３２】
第２結着剤は、密着層１９固形物全体に対して５〜４０重量％の割合で含まれる。好ましくは密着層１９固形物全体に対して１０〜３０重量％の割合である。下限値未満であると密着性が得られず、上限値を越えると導電性が悪くなる。
負極活物質層１６に含まれる無機質粒子はＳｉ、Ｇｅ、Ｍｇ、Ｓｎ、Ｐｂ、Ａｇ、Ａｌ、Ｚｎ、Ｃｄ、Ｓｂ、Ｂｉ及びＩｎからなる群より選ばれた少なくとも１種の元素が単体、酸化物又は他の金属との合金、単体とリチウムとの合金、及びこれらの金属、リチウムを含む多元合金で構成される。この無機物粒子はリチウムを吸蔵・放出可能な機能を有する。Ｓｉをベースにした無機質粒子としては、Ｓｉ単結晶、ポリシリコン、アモルファスシリコン、Ｃａ、Ｆｅ、Ｎｉ、Ｃｏ、Ｍｎ、Ｍｇ等と形成するＳｉシリサイド、シリコンオキシカーバイド、シリコンカーバイド等が挙げられる。この無機質粒子の平均粒径は０．０１〜５０μｍである。好ましい平均粒径は０．１〜２０μｍである。負極活物質層１６に含まれる無機質粒子の割合は活物質層固形物全体の５〜９５重量％である。好ましくは活物質層固形物全体の１０〜９０重量％である。
【００３３】
次に、本発明のリチウムイオンポリマー二次電池の製造手順を説明する。
先ず、前述した変性フッ素含有高分子化合物を密着層に用いる第２結着剤に含ませ、この第２結着剤を溶媒に溶解してポリマー溶液を作製し、ポリマー溶液中に導電性物質を分散させて密着層スラリーを調製する。導電性物質には粒径０．５〜３０μｍ、黒鉛化度５０％以上の炭素材が用いられる。第２結着剤と導電性物質との重量比（第３結着剤／導電性物質）が１３／８７〜５０／５０になるように混合して密着層のスラリーを調製する。溶媒にはジメチルアセトアミド、アセトン、ジメチルフォルムアミド、Ｎ-メチルピロリドン（以下、ＮＭＰという。）が用いられる。
次いで、シート状の正極集電体及び負極集電体をそれぞれ用意する。シート状の正極集電体としてはＡｌ箔が、負極集電体としてはＣｕ箔がそれぞれ挙げられる。負極集電体に調製した密着層スラリーをドクターブレード法により塗工及び乾燥し、乾燥後の密着層厚さが０．５〜３０μｍの密着層を有する負極集電体を形成する。ここでドクターブレード法とは、キャリアフィルムやエンドレスベルト等のキャリア上に載せて運ばれるスリップの厚さをドクターブレードと呼ばれるナイフエッジとキャリアとの間隔を調整することによってシートの厚さを精密に制御する方法である。乾燥後の負極の密着層厚さは１〜１５μｍが好ましい。
【００３４】
次に、負極活物質層に必要な成分として、Ｓｉ粒子等の無機質粒子、変性フッ素含有高分子化合物を含む第１結着剤、アセチレンブラック（以下、ＡＢという。）等の導電助剤及びＮＭＰ等の溶媒等をそれぞれ混合して、負極活物質層塗工用スラリーを調製する。また、正極活物質層及び電解質層に必要な成分をそれぞれ混合して正極活物質層塗工用スラリー及び電解質層塗工用スラリーをそれぞれ調製する。
得られた負極活物質層塗工用スラリーを負極集電体上にドクターブレード法により塗布して乾燥し、圧延することにより負極を形成する。
【００３５】
また正極も同様にして、得られた正極活物質層塗工用スラリーを正極集電体上にドクターブレード法により塗布して乾燥し、圧延することにより正極を形成する。正極又は負極活物質層は乾燥後の厚さが、２０〜２５０μｍとなるように形成する。電解質層は得られた電解質層塗工用スラリーを剥離紙上に電解質層の乾燥厚さが１０〜１５０μｍとなるようにドクターブレード法により塗工及び乾燥し、剥離紙より剥がして形成する。また、電解質層塗工用スラリーを正極表面や負極表面に塗工及び乾燥して電解質層を形成してもよい。図１に示すように、それぞれ形成した
正極集電体１１の上に正極活物質層１２が形成されてなる正極１３と、電解質層１８と、負極集電体１４の上に密着層１９を介して負極活物質層１６が形成されてなる負極１７を順に積層し、積層物を熱圧着することにより、シート状の電極体が形成される。
【００３６】
最後に、この電極体にＮｉからなる正極リード及び負極リードをそれぞれ正極集電体及び負極集電体に溶接し、開口部を有する袋状に加工したラミネートパッケージ材に収納し、減圧条件下で熱圧着により開口部を封止して、シート状のリチウムイオンポリマー二次電池が作製できる。
このようにして得られた本発明のリチウムイオン二次電池は、集電体と活物質との密着性に優れ、高い機械強度を保持できる。また、充放電のリチウムイオンの挿入、脱離反応に伴うストレスを制御し、サイクル特性を向上できる。
【００３７】
【実施例】
次に本発明の実施例を比較例とともに詳しく説明する。
＜変性フッ素含有高分子化合物Ａの合成＞
先ず、幹重合体を構成する材料としてＰＶｄＦ粉末５０ｇを、変性物質を構成する材料として３０重量％アクリル酸水溶液４００ｇをそれぞれ用意した。ＰＶｄＦ粉末をポリエチレン製パックに入れて真空パックし、ＰＶｄＦへの吸収線量が３０ｋＧｙとなるように⁶⁰Ｃｏをγ線源としてγ線を照射した。γ線照射したＰＶｄＦ粉末をポリエチレン製パックより取出して窒素雰囲気に移し、３０重量％アクリル酸水溶液４００ｇ中にＰＶｄＦ粉末を供給してアクリル酸水溶液と反応させた。反応中は反応溶液のサンプルを逐次採取し、この採取液に水酸化ナトリウムを用いて滴定を行い、ＰＶｄＦと反応したアクリル酸量を算出した。アクリル酸の消耗量がＰＶｄＦ重量の２０．４８％になった時点で反応を止めて、得られた固体状生成物を純水で洗浄して乾燥させ、変性フッ素含有高分子化合物Ａ（以下、変性化合物Ａという。）を得た。この変性化合物Ａ中の変性物質割合は１７重量％であった。
【００３８】
＜変性フッ素含有高分子化合物Ｂの合成＞
変性物質を構成する材料としてメタクリル酸を用いた以外は変性化合物Ａの合成と同様にして合成を行い、変性フッ素含有高分子化合物Ｂ（以下、変性化合物Ｂという。）を得た。
＜変性フッ素含有高分子化合物Ｃの合成＞
幹重合体を構成する材料としてＶｄＦ-ＨＦＰ共重合体粉末を用いた以外は変性化合物Ａの合成と同様にして合成を行い、変性フッ素含有高分子化合物Ｃ（以下、変性化合物Ｃという。）を得た。
＜変性フッ素含有高分子化合物Ｄの合成＞
ＰＶｄＦへの吸収線量が５０ｋＧｙとなるようにγ線を照射し、変性フッ素含有高分子化合物中のＰＶｄＦ割合を８０重量％、アクリル酸割合を２０重量％となるように合成した以外は変性化合物Ａの合成と同様にして合成を行い、変性フッ素含有高分子化合物Ｄ（以下、変性化合物Ｄという。）を得た。
【００３９】
＜変性フッ素含有高分子化合物Ｅの合成＞
ＰＶｄＦへの吸収線量が５０ｋＧｙとなるようにγ線を照射し、変性フッ素含有高分子化合物中のＰＶｄＦ割合を７５重量％、アクリル酸割合を２５重量％となるように合成した以外は変性化合物Ａの合成と同様にして合成を行い、変性フッ素含有高分子化合物Ｅ（以下、変性化合物Ｅという。）を得た。
＜変性フッ素含有高分子化合物Ｆの合成＞
ＰＶｄＦへの吸収線量が５０ｋＧｙとなるようにγ線を照射し、変性フッ素含有高分子化合物中のＰＶｄＦ割合を９５重量％、アクリル酸割合を５重量％となるように合成した以外は変性化合物Ａの合成と同様にして合成を行い、変性フッ素含有高分子化合物Ｆ（以下、変性化合物Ｆという。）を得た。
【００４０】
＜実施例１＞
先ず、得られた変性化合物Ａを２ｇ採取し、このサンプルにＮＭＰ２８ｇを添加し、攪拌してポリマー溶液とした。この溶液に比表面積１５０ｍ²／ｇの黒鉛粉末８ｇ及びこの黒鉛粉末を分散させるための分散剤１．２ｇをそれぞれ添加し、更にＮＭＰ１５０ｇを加え、密着層スラリーを調製した。
次いで、正極集電体として厚さ２０μｍ、幅２５０μｍのＡｌ箔を、負極集電体として厚さ１４μｍ、幅２５０μｍのＣｕ箔をそれぞれ用意した。負極集電体表面に調製した密着層スラリーをドクターブレード法により塗工及び乾燥し、乾燥後の密着層厚さを１０±１μｍの範囲内に制御した。
【００４１】
次に、無機質粒子として平均粒径０．４μｍのＳｉ粒子を、樹脂結着剤の変性フッ素含有高分子化合物として変性化合物Ａを、導電助剤としてＡＢを、溶媒としてＮＭＰをそれぞれ用意した。無機質粒子が５０重量部、樹脂結着剤が１０重量部、導電助剤が４０重量部及び溶媒が５０重量部となるようにそれぞれ混合してボールミルで２時間混合することにより、負極活物質層塗工用スラリーを調製した。
次に、表１に示される各成分をボールミルで２時間混合することにより、正極活物質層塗工用スラリー及び電解質層塗工用スラリーをそれぞれ調製した。
【００４２】
【表１】

【００４３】
得られた負極活物質層塗工用スラリーを密着層を有する負極集電体表面に負極活物質層の乾燥厚さが８０μｍとなるようにドクターブレード法により塗工及び乾燥し、圧延することにより負極を形成した。同様に、正極活物質層塗工用スラリーを正極集電体表面に塗工及び乾燥し、圧延することにより正極を形成した。更に電解質層塗工用スラリーを乾燥厚さが５０μｍになるように上記正極にドクターブレード法により塗工し、電解質層を有する正極と負極を積層して熱圧着することにより、シート状の電極体を作製した。作製した電極体にＮｉからなる正極リード及び負極リードをそれぞれ正極集電体、負極集電体に溶接し、開口部を有する袋状に加工したラミネートパッケージ材に収納し、減圧条件下で開口部を熱圧着して封止し、シート状電池を作製した。
【００４４】
＜実施例２＞
無機質粒子として平均粒径１μｍのＳｉ粒子を、第１結着剤には変性フッ素含有高分子化合物として変性化合物Ａを、フッ素含有高分子化合物としてＶｄＦ-ＨＦＰ共重合体をそれぞれ用意し、無機質粒子が５０重量部、変性化合物Ａが１０重量部、ＶｄＦ-ＨＦＰ共重合体が５重量部、導電助剤が３５重量部及び溶媒が６０重量部となるようにそれぞれ混合して負極活物質層塗工用スラリーを調製した以外は実施例１と同様にしてシート状電池を作製した。
＜実施例３＞
第２結着剤には変性フッ素含有高分子化合物として変性化合物Ｂを、無機質粒子として平均粒径１μｍのＳｉ粒子を、第１結着剤には変性フッ素含有高分子化合物として変性化合物Ｂをそれぞれ用意し、変性化合物Ｂが１００重量部となるように密着層スラリーを調製し、無機質粒子が４０重量部、変性化合物Ｂが２０重量部、導電助剤が４０重量部及び溶媒が８０重量部となるようにそれぞれ混合して負極活物質層塗工用スラリーを調製した以外は実施例１と同様にしてシート状電池を作製した。
【００４５】
＜実施例４＞
第２結着剤には変性フッ素含有高分子化合物として変性化合物Ｃを、無機質粒子として平均粒径１μｍのＳｉ粒子を、第１結着剤には変性フッ素含有高分子化合物として変性化合物Ｃをそれぞれ用意し、変性化合物Ｃが１００重量部となるように密着層スラリーを調製し、無機質粒子が６０重量部、変性化合物Ｃが１０重量部、導電助剤が３０重量部及び溶媒が５０重量部となるようにそれぞれ混合して負極活物質層塗工用スラリーを調製した以外は実施例１と同様にしてシート状電池を作製した。
＜実施例５＞
第２結着剤には変性フッ素含有高分子化合物として変性化合物Ｄを、無機質粒子として平均粒径１０μｍのＳｉ粒子を、第１結着剤には変性フッ素含有高分子化合物として変性化合物Ｆをそれぞれ用意し、変性化合物Ｄが１００重量部となるように密着層スラリーを調製し、無機質粒子が５０重量部、変性化合物Ｆが１０重量部、導電助剤が４０重量部及び溶媒が５０重量部となるようにそれぞれ混合して負極活物質層塗工用スラリーを調製した以外は実施例１と同様にしてシート状電池を作製した。
【００４６】
＜実施例６＞
第２結着剤には変性フッ素含有高分子化合物として変性化合物Ｅを、フッ素含有高分子化合物としてＰＶｄＦを、無機質粒子として平均粒径１５μｍのＳｉ粒子を、第１結着剤には変性フッ素含有高分子化合物として変性化合物Ｆをそれぞれ用意し、変性化合物Ｅが８０重量部、ＰＶｄＦが２０重量部となるようにそれぞれ混合して密着層スラリーを調製し、無機質粒子を５０重量部、変性化合物Ｆを２０重量部、導電助剤を３０重量部及び溶媒を５０重量部となるようにそれぞれ混合して負極活物質層塗工用スラリーを調製した以外は実施例１と同様にしてシート状電池を作製した。
＜実施例７＞
第２結着剤には変性フッ素含有高分子化合物として変性化合物Ｅを、無機質粒子として平均粒径１μｍのＳｉ粒子を、第１結着剤には変性フッ素含有高分子化合物として変性化合物Ｆを、フッ素含有高分子化合物としてＶｄＦ-ＨＦＰ共重合体をそれぞれ用意し、変性化合物Ｅが１００重量部となるように密着層スラリーを調製し、無機質粒子が４０重量部、変性化合物Ｆが１０重量部、ＶｄＦ-ＨＦＰ共重合体が５重量部、導電助剤が４５重量部及び溶媒が５０重量部となるようにそれぞれ混合して負極活物質層塗工用スラリーを調製した以外は実施例１と同様にしてシート状電池を作製した。
【００４７】
＜比較例１＞
第２結着剤にはフッ素含有高分子化合物としてＰＶｄＦを、無機質粒子として平均粒径１０μｍのＳｉ粒子を、第１結着剤にはＰＶｄＦをそれぞれ用意し、ＰＶｄＦが１００重量部となるように密着層スラリーを調製し、無機質粒子が５０重量部、ＰＶｄＦが１０重量部、導電助剤が４０重量部及び溶媒が５０重量部となるようにそれぞれ混合して負極活物質層塗工用スラリーを調製した以外は実施例１と同様にしてシート状電池を作製した。
＜比較例２＞
無機質粒子として平均粒径５μｍのＳｉ粒子を、樹脂結着剤にはＰＴＦＥをそれぞれ用意し、密着層を形成せず、無機質粒子が５０重量部、ＰＴＦＥが１０重量部、導電助剤が４０重量部及び溶媒が５０重量部となるようにそれぞれ混合して負極活物質層塗工用スラリーを調製した以外は実施例１と同様にしてシート状電池を作製した。
＜比較例３＞
無機質粒子として平均粒径１μｍのＳｉ粒子を、樹脂結着剤にはＶｄＦ-ＨＦＰ共重合体をそれぞれ用意し、密着層を形成せず、無機質粒子が４０重量部、ＶｄＦ-ＨＦＰ共重合体が１５重量部、導電助剤が４５重量部及び溶媒が５０重量部となるようにそれぞれ混合して負極活物質層塗工用スラリーを調製した以外は実施例１と同様にしてシート状電池を作製した。
【００４８】
＜比較試験及び評価＞
実施例１〜７及び比較例１〜３でそれぞれ得られたシート状電池を用いて以下の評価試験を行った。
(1) 電池のサイクル容量維持特性試験
得られたシート状の電池を充放電サイクル試験にかけ、最大充電電圧４Ｖ、負極活物質の充電電流が４００ｍＡｈ／ｇとなるような条件で１０時間の充電を行う充電工程と、充電工程と同様の電流の条件で放電電圧が最低放電電圧となる２．０Ｖまで放電を行う放電工程とを１サイクルとして充放電サイクルを繰返し、充放電試験を行う際に、各サイクルの充放電容量をそれぞれ測定して、１０サイクル目の放電容量保持率（(１０サイクル目の放電容量／初期放電容量)×１００）を求めた。
【００４９】
(2) 密着層の集電体及び活物質層に対する密着性試験
得られたシート状の電池に対して上記評価試験(1)と同様の条件での充放電サイクルを１０サイクル行った。その後、１０サイクルの充放電を終えたシート状電池の収納パッケージを除去し、電池の正極と負極を引き剥がしてそれぞれを分離した。実施例１〜７の電池は、分離した負極の密着層をピンセットでつまんで引っ張ったときに、密着層が集電体から剥離するか否か、密着層が活物質層から剥離するか否かをそれぞれ確認した。また、比較例１〜３の電池は、分離した負極をピンセットでつまんで引っ張ったときに、負極活物質層と負極集電体とが剥離するか否かを確認した。
【００５０】
実施例１〜７及び比較例１〜３のシート状電池でそれぞれ用いた負極活物質層及び密着層を構成する組成を次の表２にそれぞれ示す。また、上記評価試験(1)及び(2)における評価結果を表３にそれぞれ示す。
なお、表３中の密着性評価における記号は、次の意味である。◎：密着が大変良好である。○：部分的な剥離がある。×：完全に剥離されている。
【００５１】
【表２】

【００５２】
【表３】

【００５３】
表３より明らかなように、評価試験(1)のサイクル容量維持特性試験では、比較例１〜３のシート状電池による放電容量保持率は大幅に低下していた。放電容量保持率の低下の原因としては、活物質である無機質粒子の充放電による膨張収縮が、活物質と集電体の界面で発生する剥離を引起こしたと考えられる。この剥離が電池の活物質と集電体の間の電子の流れを止めてしまったので、１０サイクル目における容量保持率を大幅に低下させたと考えられる。これに対して実施例１〜７のシート状電池による放電容量保持率はそれぞれ高い値を示していた。これは、実施例１〜７の樹脂結着剤に含まれる変性フッ素含有高分子化合物が無機質粒子と高い密着性を有しているため、充放電に伴う無機質粒子の体積膨張収縮による無機質粒子の剥離を防止し、サイクル容量を維持したのではないかと考えられる。
【００５４】
また比較例１〜３のシート状電池における評価試験(2)の１０サイクル後における密着性試験では、負極集電体と負極活物質層とが完全に剥離してしまっていた。これは負極活物質層内部において、無機質粒子と樹脂結着剤との接着面が充放電サイクルによる膨張収縮により剥がれてしまい、活物質層自体の凝集強度が失われたと考えられる。また数回の充放電によって本来はシート状であった活物質層が、破片状或いは砂状の形状に変化したのではないかと推察される。これに対して、実施例１〜７のシート状電池では、負極活物質層は負極集電体から剥離し難く、負極活物質層自体の凝集強度も維持していた。このように負極活物質層を形成する負極材料に変性フッ素含有高分子化合物を含む樹脂結着剤を含ませることによって、負極活物質層と負極集電体の接着を向上させ、無機質粒子を含む負極活物質層の体積変化に起因する剥離を防止することができる。
【００５５】
【発明の効果】
以上述べたように、本発明のリチウムイオン二次電池用負極は、負極集電体の表面に第１結着剤と、Ｓｉ、Ｇｅ、Ｍｇ、Ｓｎ、Ｐｂ、Ａｇ、Ａｌ、Ｚｎ、Ｃｄ、Ｓｂ、Ｂｉ及びＩｎからなる群より選ばれた少なくとも１種の元素を含む無機質粒子の双方をそれぞれ含む負極活物質層が形成され、負極集電体と負極活物質層との間に第２結着剤と導電性物質の双方をそれぞれ含む密着層を有する負極の改良である。その特徴ある構成は、第１結着剤及び第２結着剤がフッ素含有樹脂を変性物質により変性して得られた変性フッ素含有高分子化合物をそれぞれ含み、第１結着剤及び第２結着剤にそれぞれ含まれる変性フッ素含有高分子化合物の構造が互いに同一又は異なるところにある。このような構成とすることで次の優れた効果を有する。
(1) 高い化学安定性を備え、かつ無機質粒子への密着性に優れる。
(2) 負極集電体と負極活物質層との密着性及び導電性に優れ、かつサイクル容量維持特性を向上し得る。
(3) 高い機械強度を保持し得る。
(4) 密着層が電解液中の有機溶媒に対して安定で長期保存性に優れる。
(5) 電池内に発生するフッ酸等の強酸による集電体の腐食を抑制し得る。
(6) 充放電のリチウムイオンの挿入、脱離反応に伴うストレスを制御し、サイクル特性を向上できる。
【図面の簡単な説明】
【図１】本発明のリチウムイオン二次電池の電極体を示す部分断面構成図。
【図２】変性フッ素含有高分子化合物の合成を示す模式図。
【符号の説明】
１１正極集電体
１２正極活物質層
１３正極
１４負極集電体
１６負極活物質層
１７負極
１８電解質層[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery manufactured using the negative electrode.
[0002]
[Prior art]
With the spread of portable devices such as video cameras and notebook computers in recent years, demand for thin batteries has been increasing. As this thin battery, a lithium ion secondary battery formed by laminating a positive electrode and a negative electrode is known. The positive electrode is formed by forming a positive electrode active material layer on the surface of a sheet-shaped positive electrode current collector, and the negative electrode is formed by forming a negative electrode active material layer on the surface of a sheet-shaped negative electrode current collector. . An electrolyte layer is interposed between the positive electrode active material layer and the negative electrode active material layer. In this battery, a positive electrode terminal and a negative electrode terminal for taking out a potential difference in each active material as a current are provided on a positive electrode current collector and a negative electrode current collector, and the thus stacked ones are sealed in a package to make lithium. An ion secondary battery is formed. In this lithium ion secondary battery, desired electricity can be obtained by using the positive electrode terminal and the negative electrode terminal drawn out of the package as terminals of the battery.
[0003]
Lithium ion secondary batteries having such a structure have attracted much attention because of their high battery voltage and high energy density. In order to further increase the discharge capacity of this lithium ion secondary battery, it is necessary to increase the area of the sheet-like positive electrode or negative electrode. If the area of the positive electrode or the negative electrode is simply enlarged, the handling becomes difficult due to the large area. In order to solve this problem, it is conceivable to fold or wind the enlarged sheet-like positive electrode or negative electrode to a desired size. However, when folding or winding is performed in a state where the sheet-like positive electrode or negative electrode is laminated, the positive electrode or the negative electrode at the fold portion is bent, and the sheet at that portion is peeled off from the electrolyte layer and the effective interface between the electrode and the electrolyte is formed. There is a problem that the surface area is reduced and the discharge capacity is reduced, and resistance is generated inside the battery to deteriorate the cycle characteristics of the discharge capacity. Similarly, there is also a problem that the active material layer forming the positive electrode or the negative electrode is separated from the current collector due to the bending at the fold portion. Further, in this battery, during the charging and discharging processes, the positive and negative electrode active materials absorb and release lithium ions, causing expansion and contraction of the positive and negative electrode active material layers. There was also a problem of peeling from the current collector.
[0004]
Therefore, as a technique for solving the above-mentioned problems, as described below, techniques for preventing the active material layer from peeling off from the current collector and reducing the adhesion have been proposed.
A polymer compound having a polyvinyl alcohol unit having an average degree of polymerization of 20 or more, and a part or all of hydroxyl groups substituted with an oxyalkylene-containing group having an average degree of molar substitution of 0.3 or more, a binder resin comprising the polymer compound, Also, a composition for an ion-conductive polymer electrolyte having high ionic conductivity and high adhesiveness and a secondary battery have been proposed (for example, see Patent Document 1). In the present invention, by using a polymer compound in which the introduction ratio of a polyoxyalkylene moiety is increased as a material of a solid polymer electrolyte or a material of a binder resin, ion transfer is facilitated and the ionic conductivity of the polymer for a polymer electrolyte is improved. Is increasing.
[0005]
A polymer compound obtained by reacting a hydroxyl group of an alcohol compound having a substituent having a large dipole moment with a part or all of an isocyanate group in a polyurethane compound obtained by reacting an isocyanate compound and a polyol compound, a binder resin, and an ion. A composition for a conductive polymer electrolyte and a secondary battery have been proposed (for example, see Patent Document 2). In the present invention, by introducing a substituent having a large dipole moment into the polyurethane, the adhesion between the electrode and the electrolyte is improved while maintaining a high dielectric constant and the ability to dissolve the ionic conductive salt at a high concentration. As a result, an interface impedance comparable to that of the electrolyte solution is obtained.
A polymer compound obtained by reacting a hydroxyl group of an alcohol compound having a large dipole moment with a part or all of the isocyanate groups in a polyurethane compound obtained by reacting an isocyanate compound with a polyol compound; A composition for an ionic conductive solid polymer, a ionic conductive solid polymer electrolyte, a binder resin, and a secondary battery mainly comprising a salt and a compound having a crosslinkable functional group have been proposed (eg, patents). Reference 3).
[0006]
An electrode material for a negative electrode of a lithium secondary battery, an electrode structure using the electrode material, a lithium secondary battery using the electrode structure, and a method for manufacturing the electrode structure and the lithium secondary battery have been proposed. (For example, see Patent Document 4). In the present invention, an amorphous M.A.X alloy having a non-stoichiometric composition (M is at least one element selected from the group consisting of Si, Ge and Mg, and A is a transition metal element) X represents O, F, N, Ba, Sr, Ca, La, Ce, C, P, S, Se, Te, B, Bi, Sb, Al, In And at least one element selected from the group consisting of Zn and Zn, provided that X does not need to be contained, and the content of the constituent element M of the alloy is M / (M + A + X) = 20 to 80 atomic%.) It contains. An electrode material containing such an alloy has excellent characteristics and is suitable as a negative electrode active material of a lithium secondary battery.
[0007]
A positive electrode provided with a positive electrode active material containing lithium, a negative electrode provided with a negative electrode active material layer in which a mixture of a silicon compound and a carbon material capable of doping / dedoping lithium are dispersed in a binder, A non-aqueous electrolyte battery including a non-aqueous electrolyte interposed between a positive electrode and a negative electrode and having a glass transition temperature of −40 ° C. or lower as the binder has been proposed (for example, see Patent Document 5). ). In the present invention, the binder in the negative electrode active material has a glass transition temperature of −40 ° C. or less, so that the binder absorbs the volume change of the silicon compound negative electrode during lithium doping and undoping. However, a change in volume of the entire negative electrode active material layer is suppressed, and cycle deterioration is suppressed.
Furthermore, a non-aqueous electrolyte secondary battery comprising a positive electrode and a negative electrode capable of inserting and extracting lithium ions, and a non-aqueous electrolyte having lithium ion conductivity, and containing a water-soluble polymer in the electrode mixture of the positive electrode or the negative electrode is proposed. (See, for example, Patent Document 6). In the present invention, the water-soluble polymer is contained in the electrode mixture, so that the binding property to the current collector is good, and since the solvent is water, the production cost can be reduced. Can be suppressed. Further, drying can be performed at a relatively low temperature, and thermal damage to the mixture and the material constituting the sheet electrode can be minimized.
[0008]
[Patent Document 1]
PCT International Publication No. WO00 / 56780
[Patent Document 2]
PCT International Publication No. WO 00/56797
[Patent Document 3]
PCT International Publication No. WO00 / 56815
[Patent Document 4]
PCT International Publication No. WO00 / 17949
[Patent Document 5]
JP-A-2000-299108
[Patent Document 6]
JP-A-9-289022
[0009]
[Problems to be solved by the invention]
However, in general, in a lithium ion secondary battery, it is difficult to bond the negative electrode current collector and the negative electrode active material layer. In the negative electrode material disclosed in Patent Document 4, for example, Si, Ge, Al, Sn In the case of a negative electrode active material containing such elements as above, although a high charge / discharge capacity is obtained, there is a problem that the volume change of the active material due to charge / discharge is large, and the active material layer and the current collector are separated. . The binder disclosed in Patent Document 5 is characterized by flexibility and can absorb a change in volume of the active material to some extent. However, this type of binder has a sufficient adhesive strength with the current collector. I could not say. Patent Documents 1 to 4 and 6 and the like aim at improving the adhesion between the active material and the current collector, and improve the binder. However, these binders generally have chemical stability. There remain problems such as inferiority to resins such as polyethylene and polyvinylidene fluoride (hereinafter referred to as PVdF) used for batteries, and insufficient adhesiveness in an environment in which an electrolyte exists.
[0010]
A first object of the present invention is to provide a negative electrode for a lithium ion secondary battery having high chemical stability and excellent adhesion to inorganic particles.
A second object of the present invention is to provide a negative electrode for a lithium ion secondary battery, which has excellent adhesion and conductivity between a negative electrode current collector and a negative electrode active material layer, and can improve cycle capacity retention characteristics, and uses the negative electrode. To provide a lithium ion secondary battery manufactured by the above method.
A third object of the present invention is to provide a negative electrode for a lithium ion secondary battery, which can maintain high mechanical strength, and a lithium ion secondary battery manufactured using the negative electrode.
A fourth object of the present invention is to provide a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery manufactured using the negative electrode, in which the adhesion layer is stable with respect to the organic solvent in the electrolytic solution and has excellent long-term storage properties. To provide.
A fifth object of the present invention is to provide a lithium ion secondary battery capable of suppressing corrosion of a current collector by a strong acid such as hydrofluoric acid generated in the battery.
A sixth object of the present invention is to provide a lithium ion secondary battery capable of controlling stress accompanying insertion / desorption of lithium ions during charge / discharge and improving cycle characteristics.
[0011]
[Means for Solving the Problems]
As shown in FIG. 1, the invention according to claim 1 includes a first binder, Si, Ge, Mg, Sn, Pb, Ag, Al, Zn, Cd, Sb, A negative electrode active material layer 16 containing both inorganic particles containing at least one element selected from the group consisting of Bi and In is formed, and the second negative electrode active material layer 16 is provided between the negative electrode current collector 14 and the negative electrode active material layer 16. This is an improvement of a negative electrode for a lithium ion secondary battery having an adhesion layer 19 containing both a binder and a conductive substance. The characteristic configuration is such that the first binder and the second binder each include a modified fluorine-containing polymer compound obtained by modifying a fluorine-containing resin with a modifying substance, and the first binder and the second binder are different from each other. The structure of the modified fluorine-containing polymer compound contained in each of the adhesives is the same or different from each other.
In the invention according to claim 1, the modified fluorine-containing polymer obtained by modifying the fluorine-containing resin contained in the first binder forming the negative electrode active material layer 16 with a modifying substance is a fluorine-containing polymer compound. In addition to having high chemical stability similar to that possessed by inorganic particles, it also has excellent adhesion to inorganic particles, so that the inorganic particles fall off due to the volume change of the inorganic particles due to the occlusion and desorption of lithium due to charge and discharge. In addition, it is possible to prevent electrode peeling and the like, and to improve cycle characteristics. Further, both the second binder containing the modified fluorine-containing polymer compound obtained by modifying the fluorine-containing resin with the modifying material between the negative electrode current collector 14 and the negative electrode active material layer 16 and the conductive material are used. Since the adhesive force is improved by having the adhesive layers 19 including the respective layers, peeling due to a volume change of the negative electrode active material layer including the inorganic particles is prevented.
[0012]
The invention according to claim 2 is the invention according to claim 1, wherein, as shown in FIG. 2 (c), a modification contained in one or both of the first binder and the second binder. The fluorine-containing polymer compound 24 is a negative electrode that is a polymer compound obtained by graft-modifying the fluorine-containing resin into the trunk polymer 22 and graft-modifying the trunk polymer 22 with the modifying substance 23.
The invention according to claim 3 is the invention according to claim 2, wherein the trunk polymer 22 constituting the modified fluorine-containing polymer compound 24 is PVdF, polyvinyl fluoride (hereinafter, referred to as PVF), and tetrafluoride. Ethylene polymer, trifluorinated ethylene polymer, difluoroethylene polymer, vinylidene fluoride-hexafluoropropylene copolymer (hereinafter referred to as VdF-HFP copolymer), ethylene-tetrafluoroethylene copolymer, 4-fluoroethylene The negative electrode contains at least one fluorine-containing resin selected from the group consisting of a fluorinated ethylene-6-fluoropropylene copolymer, a trifluorinated ethylene polymer, and polytetrafluoroethylene.
In the invention according to claim 3, the VdF-HFP copolymer and PVdF are preferable because of their high durability to an electrolytic solution.
[0013]
The invention according to claim 4 is the invention according to claim 2, wherein the modified substance 23 constituting the modified fluorine-containing polymer compound 24 is at least one of acrylic acid, methyl acrylate, methacrylic acid, and methyl methacrylate. Is the negative electrode.
In the invention according to claim 4, the use of the modified substance allows the adhesion layer to obtain good adhesion to the negative electrode current collector.
The invention according to claim 5 is the invention according to any one of claims 2 to 4, wherein the trunk polymer 22 constituting the modified fluorine-containing polymer compound 24 is any one of PVdF and VdF-HFP copolymer A negative electrode, which is a mixture containing one or both of them and in which the modifying substance 23 is acrylic acid.
[0014]
The invention according to claim 6 is the invention according to any one of claims 2 to 5, wherein the modified fluorine-containing polymer compound 24 is a negative electrode in which the fluorine-containing resin 22 and the modified substance 23 are grafted by radiation irradiation treatment. It is.
The invention according to claim 7 is the invention according to claim 6, wherein the radiation irradiation treatment on the fluorine-containing resin 22 is performed by γ-ray irradiation, and the absorbed dose of γ-ray of the fluorine-containing resin is 10 to 90 kGy. It is a negative electrode.
The invention according to claim 8 is the invention according to any one of claims 1 to 7, wherein the modified fluorine-containing polymer compound contained in the first binder is modified fluorine contained in the second binder. The negative electrode contains a repeating unit constituting the polymer compound as a repeating unit.
[0015]
In the invention according to claim 8, the modified fluorine-containing polymer compound contained in the first binder contains a repeating unit constituting the modified fluorine-containing polymer compound contained in the second binder as a repeating unit thereof. As a result, the adhesion between the negative electrode active material layer and the adhesion layer becomes stronger.
The invention according to claim 9 is the invention according to any one of claims 1 to 8, wherein the first binder further comprises a fluorine-containing polymer compound having a structure different from that of the modified fluorine-containing polymer compound. It is.
According to the ninth aspect of the present invention, by further containing not only the modified fluorine-containing polymer compound but also the fluorine-containing polymer compound, the chemical stability is further improved, and the adhesion to the current collector is also improved. In addition, by including a fluorine-containing polymer compound that is more flexible than the modified fluorine-containing polymer compound, the effect of alleviating the stress caused by the volume change of the inorganic particles due to the occlusion and desorption of lithium due to charge and discharge. Is obtained.
[0016]
The invention according to claim 10 is the invention according to claim 9, wherein the fluorine-containing polymer compound is a negative electrode containing, as its repeating unit, a repeating unit contained in a fluorine-containing resin constituting the modified fluorine-containing polymer compound. is there.
In the invention according to claim 10, the chemical stability of the resin binder is increased by the fact that the repeating unit of the fluorine-containing polymer compound contains a repeating unit contained in the fluorine-containing resin constituting the modified fluorine-containing polymer compound. At the same time, the adhesion becomes stronger.
[0017]
The invention according to claim 11 is the invention according to any one of claims 1 to 10, wherein the content of the modified fluorine-containing polymer compound contained in the first binder is 10% by weight to 100% by weight. It is a certain negative electrode.
The invention according to claim 12 is the invention according to any one of claims 1 to 10, wherein the content of the modified fluorine-containing polymer compound contained in the second binder is 10% by weight to 100% by weight. It is a certain negative electrode.
[0018]
The invention according to claim 13 is the invention according to claim 11 or 12, wherein the content ratio of the modified fluorine-containing polymer compound contained in the first binder is less than the modified fluorine-containing polymer compound contained in the second binder. The negative electrode has the same ratio as the content ratio of the polymer compound or a lower ratio.
The invention according to claim 14 is the invention according to any one of claims 1 to 13, wherein the graft polymerization ratio of the modified fluorine-containing polymer compound contained in the first binder (weight of modified substance / fluorine-containing substance) (Weight of resin) is the same as or lower than the graft polymerization ratio of the modified fluorine-containing polymer compound contained in the second binder.
[0019]
The invention according to claim 15 is the invention according to claim 14, wherein the graft polymerization ratio of the modified fluorine-containing polymer compound contained in one or both of the first binder and the second binder is And a negative electrode of 2% to 50% by weight.
The invention according to claim 16 is the invention according to any one of claims 1 to 15, wherein the ratio of the first binder contained in the negative electrode active material layer 16 is 1% by weight of the entire solid material of the active material layer. It is a negative electrode of up to 50% by weight.
The invention according to claim 17 is the invention according to any one of claims 1 to 16, wherein the ratio of the second binder contained in the adhesion layer 19 is 5 to 40% by weight of the entire solid of the adhesion layer. It is a certain negative electrode.
[0020]
The invention according to claim 18 is the invention according to claim 1, wherein the inorganic particles are selected from the group consisting of Si, Ge, Mg, Sn, Pb, Ag, Al, Zn, Cd, Sb, Bi, and In. In addition, the negative electrode is composed of a simple substance, an alloy of an oxide or another metal, an alloy of a simple substance and lithium, and a multi-element alloy containing these metals and lithium.
The invention according to claim 19 is the invention according to claim 1 or 18, wherein the inorganic particles have an average particle diameter of 0.01 to 50 μm.
The invention according to claim 20 is the invention according to any one of

claims

1, 18 and 19, wherein the ratio of the inorganic particles contained in the negative electrode active material layer 16 is 5 to 95% by weight of the entire solid material of the active material layer. % Of the negative electrode.
[0021]
The invention according to claim 21 is, as shown in FIG. 1, a lithium ion secondary battery produced using the negative electrode 17 according to any one of claims 1 to 20.
In the invention according to claim 21, a lithium ion secondary battery manufactured using the negative electrode of the present invention has excellent adhesion between the current collector and the active material, and can maintain high mechanical strength. In addition, the stress associated with the insertion and desorption reactions of lithium ions during charge and discharge can be controlled, and cycle characteristics can be improved.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, an embodiment of the present invention will be described with reference to the drawings.
As shown in FIG. 1, the negative electrode for a lithium ion secondary battery of the present invention has a first binder, Si, Ge, Mg, Sn, Pb, Ag, Al, Zn, A negative electrode active material layer 16 including both inorganic particles including at least one element selected from the group consisting of Cd, Sb, Bi, and In is formed, and the negative electrode current collector 14 and the negative electrode active material layer 16 An adhesive layer 19 containing both the second binder and the conductive material is provided between them. The characteristic configuration is such that the first binder and the second binder each include a modified fluorine-containing polymer compound obtained by modifying a fluorine-containing resin with a modifying substance, and the first binder and the second binder are different from each other. The structure of the modified fluorine-containing polymer compound contained in each of the adhesives is the same or different from each other. The modified fluorine-containing polymer obtained by modifying the fluorine-containing resin contained in the first binder forming the negative electrode active material layer 16 with a modifying substance is similar to the high chemical stability of the fluorine-containing polymer. As well as excellent adhesion to inorganic particles, preventing loss of inorganic particles due to volume change of the inorganic particles due to occlusion and desorption of lithium due to charge and discharge, and separation of electrodes, etc. Thus, the cycle characteristics can be improved. Further, both the second binder containing the modified fluorine-containing polymer compound obtained by modifying the fluorine-containing resin with the modifying material between the negative electrode current collector 14 and the negative electrode active material layer 16 and the conductive material are used. Since the adhesive force is improved by having the adhesive layers 19 including the respective layers, peeling due to a volume change of the negative electrode active material layer including the inorganic particles is prevented. The modified fluorine-containing polymer compounds contained in the first and second binders may have the same structure or may have different structures. The modified fluorine-containing polymer compound 24 is a polymer compound obtained by using the fluorine-containing resin as the trunk polymer 22 and graft-modifying the trunk polymer 22 with the modifying substance 23.
[0023]
Here, `` modified '' means that the property changes, and in this specification, the modified substance is arranged everywhere in the resin that is the trunk polymer, so that only the property of the resin itself that is the trunk polymer is obtained. In other words, it means that the properties of the denatured substance are combined, or new properties that are not present in both are added.
The use of a compound containing fluorine in the molecule as the polymer compound contained in the first and second binders constituting the negative electrode active material layer and the adhesion layer is because the negative electrode active material layer is chemically and electrically conductive in the battery. This is because stability is required chemically and thermally.
[0024]
Since the modified fluorine-containing polymer compound chemically improves stability by being modified, it not only has the same stability as the high chemical stability of the fluorine-containing polymer compound, but also adheres to inorganic particles. Because of the excellent properties, it is possible to prevent the inorganic particles from dropping off due to the volume change of the inorganic particles due to the insertion and desorption of lithium due to charge and discharge, the separation of the electrodes, and the like, and to improve the cycle characteristics. Further, peeling of the active material layer from the current collector is suppressed without being dissolved in the electrolytic solution. Further, for the same reason, the conductive substance dispersed in the adhesive layer is maintained without collapsing, so that good electron conduction is maintained, and long-term storage properties and cycle characteristics are excellent. Since the current collector is coated on the chemically stable layer in this way, even when hydrofluoric acid or the like is generated inside the battery, the adhesion layer serves as a protective layer, thereby suppressing corrosion of the current collector.
Further, since the denaturation improves the thermal stability, the battery is not dissolved in the solvent in the battery even when the battery is kept at a high temperature, so that the battery can be prevented from being deteriorated.
Furthermore, since the stability is improved electrochemically by the modification, stable adhesion and conductivity are maintained without deterioration. Further, it becomes difficult for the electrolyte to penetrate into the modified polymer compound, so that the electrolyte does not adhere to the current collector.
[0025]
Graft polymerization is used for arranging and modifying a modified substance as a branch polymer in some places of the backbone polymer. The trunk polymer 22 includes PVdF, PVF, tetrafluoroethylene polymer, trifluoride ethylene polymer, difluoroethylene polymer, VdF-HFP copolymer, ethylene-tetrafluoroethylene copolymer, and tetrafluoroethylene. At least one fluorine-containing resin selected from the group consisting of -6-fluoropropylene copolymer, trifluorinated ethylene polymer and polytetrafluoroethylene is included. Among them, VdF-HFP copolymer and PVdF are preferable because of their high durability to an electrolyte. The modified substance 23 includes at least one of acrylic acid, methyl acrylate, methacrylic acid, and methyl methacrylate. By using these modified substances, good adhesion to the inorganic particles and the negative electrode current collector can be obtained.
In particular, the modified fluorine-containing polymer compound 24 in which the trunk polymer 22 is a mixture containing one or both of PVdF and VdF-HFP copolymer, and the modified substance 23 is acrylic acid, is a PVdF or VdF-HFP copolymer. A mixture containing either one or both of the coalesced particles has high adhesion to the inorganic particles and the negative electrode active material layer in the negative electrode active material layer, and acrylic acid has high adhesion to the negative electrode current collector material. The modified fluorine-containing polymer compound contained in the first binder and the modified fluorine-containing polymer compound contained in the second binder in the adhesive layer have excellent properties.
[0026]
As a method for graft polymerization, there is a radiation method. In the radiation method, the fluorine-containing resin to be the backbone polymer 22 and the modifying substance to be the branching polymer are mixed, and the mixture can be polymerized by continuously or intermittently emitting radiation to the mixture. It is preferable to pre-emit the fluorine-containing resin before contacting Specifically, after irradiating the fluorine-containing resin with radiation, a modified substance is mixed into the irradiation target, so that the fluorine-containing resin is a trunk polymer, and the modified substance is a branched polymer. Can be obtained. Γ-rays are used for radiation used for graft polymerization. Irradiation with γ-rays is performed so that the absorbed dose to the fluorine-containing resin is 1 to 90 kGy. By irradiating the fluorine-containing resin serving as the backbone polymer with radiation, a radical is formed at one end, and the modified substance is easily polymerized.
[0027]
As shown in FIG. 2A, the trunk polymer 22 constituting the modified fluorine-containing polymer compound has a repeating unit A as a basic unit. The basic unit of the backbone polymer may be of one type as shown in FIG. 2A, or two or more types of basic units (not shown). This basic unit is preferably contained in the backbone polymer in a proportion of at least 50 molecular weight%. More preferably, it is contained at a ratio of 60 to 100 molecular weight%. As shown in FIG. 2B, the backbone polymer 22 is irradiated with radiation to form a radical 22a at one end of the basic unit A. As shown in FIG. 2 (c), by bringing the modified substance 23 (C in FIG. 2) into contact with the radical polymer-formed backbone polymer 22, the modified substance 23 is arranged in the radical portion 22a and the modified fluorine content A molecular compound 24 is formed. The modified substance 23 is preferably contained in the modified fluorine-containing polymer at a ratio of 2 to 50% by weight. More preferably, it is contained in the modified fluorine-containing polymer compound at a ratio of 5 to 30% by weight.
In graft polymerization, the length of time during which the activated stem polymer resin is in contact with the modified material that becomes a branch polymer, the degree of preliminary activity by radiation, and the modified material that becomes a branched polymer becomes the stem polymer Polymerization differs depending on the degree to which the resin can pass through, the temperature when the resin and the modified substance are in contact, and the like. When the denaturing substance is an acid, the degree of grafting can be observed by sampling a solution containing the compound as a denaturing substance, titrating the solution with an alkali, and measuring the concentration of the remaining acid compound. The proportion of grafting in the obtained composition is desirably 2 to 50 molecular weight% of the final weight.
[0028]
The modified fluorine-containing polymer compound contained in the first binder preferably contains, as a repeating unit, a repeating unit constituting the modified fluorine-containing polymer compound contained in the second binder. Thereby, the adhesion between the negative electrode active material layer and the adhesion layer becomes stronger.
It is preferable that the first binder further includes a fluorine-containing polymer compound having a structure different from that of the modified fluorine-containing polymer compound. When the first binder further contains not only the modified fluorine-containing polymer compound but also a fluorine-containing polymer compound, the chemical stability is further increased, and the adhesion to the current collector is also improved. In addition, by including a fluorine-containing polymer compound that is more flexible than the modified fluorine-containing polymer compound, the effect of relieving stress caused by a change in volume of the inorganic particles due to occlusion and desorption of lithium due to charge and discharge is obtained. can get.
The fluorine-containing polymer compound preferably contains a repeating unit contained in the fluorine-containing resin constituting the modified fluorine-containing polymer compound as the repeating unit. Thereby, the chemical stability of the first binder is further increased, and the adhesion is further strengthened.
[0029]
The content ratio of the modified fluorine-containing polymer compound contained in the first binder is specified to be 10% by weight to 100% by weight. If the content is less than 10% by weight, a problem occurs in that the mechanical strength of the active material layer is reduced. A preferable content ratio is 10% by weight to 50% by weight.
Further, the content ratio of the modified fluorine-containing polymer compound contained in the second binder is specified to be 10% by weight to 100% by weight. If the content is less than 10% by weight, a problem occurs in that the adhesion between the adhesive layer and the current collector is reduced. A preferable content ratio is 50% by weight to 100% by weight.
[0030]
The content ratio of the modified fluorine-containing polymer compound contained in the first binder is specified to be the same as or lower than the content ratio of the modified fluorine-containing polymer compound contained in the second binder. You.
The graft polymerization ratio of the modified fluorine-containing polymer compound contained in the first binder (the weight of the modified substance / the weight of the fluorine-containing resin) is the graft polymerization ratio of the modified fluorine-containing polymer compound contained in the second binder. Is defined to be the same ratio or a lower ratio.
[0031]
The graft polymerization ratio of the modified fluorine-containing polymer compound contained in one or both of the first binder and the second binder is specified to be 2% by weight to 50% by weight. If the amount is less than 2% by weight, the modified effect cannot be obtained, and if it exceeds 50% by weight, a problem occurs in that the flexibility of the polymer is lost. The preferred graft polymerization ratio is 5% by weight to 45% by weight.
The ratio of the first binder contained in the negative electrode active material layer 16 is 1% by weight to 50% by weight of the entire solid material of the active material layer. Below the lower limit, sufficient adhesion to the inorganic particles cannot be obtained, and above the upper limit, the charge / discharge capacity decreases. Preferably, it is 2% by weight to 20% by weight of the whole solid of the active material layer.
[0032]
The second binder is contained at a ratio of 5 to 40% by weight based on the whole solid matter of the adhesion layer 19. Preferably, the ratio is 10 to 30% by weight based on the whole solid matter of the adhesion layer 19. If it is less than the lower limit, adhesion cannot be obtained, and if it exceeds the upper limit, the conductivity will be poor.
The inorganic particles included in the negative electrode active material layer 16 include at least one element selected from the group consisting of Si, Ge, Mg, Sn, Pb, Ag, Al, Zn, Cd, Sb, Bi, and In alone, and oxidized. It is composed of an alloy with an object or another metal, an alloy of a simple substance and lithium, and a multi-element alloy containing these metals and lithium. These inorganic particles have a function of inserting and extracting lithium. Examples of the inorganic particles based on Si include Si single crystal, polysilicon, amorphous silicon, Si silicide formed with Ca, Fe, Ni, Co, Mn, Mg, etc., silicon oxycarbide, silicon carbide, and the like. The average particle size of the inorganic particles is 0.01 to 50 μm. The preferred average particle size is 0.1 to 20 μm. The ratio of the inorganic particles contained in the negative electrode active material layer 16 is 5 to 95% by weight based on the entire solid material of the active material layer. Preferably, it is 10 to 90% by weight of the whole solid of the active material layer.
[0033]
Next, a procedure for manufacturing the lithium ion polymer secondary battery of the present invention will be described.
First, the modified fluorine-containing polymer compound described above is included in a second binder used for the adhesion layer, and the second binder is dissolved in a solvent to prepare a polymer solution. Disperse to prepare an adhesive layer slurry. As the conductive material, a carbon material having a particle size of 0.5 to 30 μm and a degree of graphitization of 50% or more is used. The slurry of the adhesion layer is prepared by mixing the second binder and the conductive substance so that the weight ratio (third binder / conductive substance) becomes 13/87 to 50/50. As the solvent, dimethylacetamide, acetone, dimethylformamide, and N-methylpyrrolidone (hereinafter, referred to as NMP) are used.
Next, a sheet-like positive electrode current collector and a negative electrode current collector are prepared. The sheet-shaped positive electrode current collector includes an Al foil, and the negative electrode current collector includes a Cu foil. The slurry for the adhesive layer prepared on the negative electrode current collector is applied by a doctor blade method and dried to form a negative electrode current collector having an adhesive layer having a thickness of 0.5 to 30 μm after drying. Here, the doctor blade method precisely adjusts the sheet thickness by adjusting the gap between the knife edge and the carrier, called the doctor blade, by adjusting the thickness of the slip carried on a carrier such as a carrier film or endless belt. How to control. The thickness of the adhesive layer of the negative electrode after drying is preferably 1 to 15 μm.
[0034]
Next, as necessary components for the negative electrode active material layer, inorganic particles such as Si particles, a first binder containing a modified fluorine-containing polymer compound, a conductive auxiliary such as acetylene black (hereinafter, referred to as AB), and NMP. Are mixed to prepare a slurry for coating the negative electrode active material layer. Further, the components required for the positive electrode active material layer and the electrolyte layer are mixed respectively to prepare a slurry for coating the positive electrode active material layer and a slurry for coating the electrolyte layer.
The obtained negative electrode active material layer coating slurry is applied on a negative electrode current collector by a doctor blade method, dried, and rolled to form a negative electrode.
[0035]
Similarly, the positive electrode is formed by applying the obtained slurry for coating a positive electrode active material layer on a positive electrode current collector by a doctor blade method, drying, and rolling to form a positive electrode. The positive electrode or negative electrode active material layer is formed so that the thickness after drying is 20 to 250 μm. The electrolyte layer is formed by applying and drying the obtained slurry for coating an electrolyte layer on release paper by a doctor blade method so that the dry thickness of the electrolyte layer becomes 10 to 150 μm, and peeling off the release paper. Further, the electrolyte layer coating slurry may be applied to the positive electrode surface or the negative electrode surface and dried to form the electrolyte layer. As shown in FIG. 1, each was formed
A positive electrode 13 in which a positive electrode active material layer 12 is formed on a positive electrode current collector 11, an electrolyte layer 18, and a negative electrode active material layer 16 formed on a negative electrode current collector 14 with an adhesion layer 19 interposed therebetween The negative electrodes 17 are sequentially laminated, and the laminate is thermocompression-bonded to form a sheet-like electrode body.
[0036]
Finally, the positive electrode lead and the negative electrode lead made of Ni are welded to the positive electrode current collector and the negative electrode current collector, respectively, and are housed in a bag-shaped laminated package material having an opening. By sealing the opening by thermocompression bonding, a sheet-shaped lithium ion polymer secondary battery can be manufactured.
The lithium ion secondary battery of the present invention thus obtained has excellent adhesion between the current collector and the active material, and can maintain high mechanical strength. In addition, the stress associated with the insertion and desorption reactions of lithium ions during charge and discharge can be controlled, and cycle characteristics can be improved.
[0037]
【Example】
Next, examples of the present invention will be described in detail together with comparative examples.
<Synthesis of modified fluorine-containing polymer compound A>
First, 50 g of PVdF powder was prepared as a material constituting the trunk polymer, and 400 g of a 30% by weight aqueous solution of acrylic acid was prepared as a material constituting the modified substance. The PVdF powder is placed in a polyethylene pack and vacuum-packed so that the absorbed dose to PVdF becomes 30 kGy. ⁶⁰ Irradiation was performed using Co as a γ-ray source. The gamma-irradiated PVdF powder was taken out of the polyethylene pack, transferred to a nitrogen atmosphere, and supplied with 400 g of a 30% by weight aqueous solution of acrylic acid to react with the aqueous solution of acrylic acid. During the reaction, samples of the reaction solution were sequentially sampled, and the sampled solution was titrated with sodium hydroxide to calculate the amount of acrylic acid reacted with PVdF. When the consumption of acrylic acid reaches 20.48% of the weight of PVdF, the reaction is stopped, and the obtained solid product is washed with pure water and dried, and the modified fluorine-containing polymer compound A (hereinafter, referred to as “the modified fluorine-containing polymer compound A”) is used. Modified Compound A) was obtained. The modified substance ratio in the modified compound A was 17% by weight.
[0038]
<Synthesis of modified fluorine-containing polymer compound B>
Synthesis was carried out in the same manner as in the synthesis of the modified compound A, except that methacrylic acid was used as a material constituting the modified substance, to obtain a modified fluorine-containing polymer compound B (hereinafter, referred to as modified compound B).
<Synthesis of modified fluorine-containing polymer compound C>
The synthesis is performed in the same manner as the synthesis of the modified compound A except that the VdF-HFP copolymer powder is used as a material constituting the backbone polymer, and a modified fluorine-containing polymer compound C (hereinafter, referred to as a modified compound C) is obtained. Obtained.
<Synthesis of modified fluorine-containing polymer compound D>
Modified compound A except that it was irradiated with γ rays so that the absorbed dose to PVdF became 50 kGy, and the modified fluorine-containing polymer was synthesized so that the PVdF ratio was 80% by weight and the acrylic acid ratio was 20% by weight. Was synthesized in the same manner as in the synthesis of, to obtain a modified fluorine-containing polymer compound D (hereinafter, referred to as modified compound D).
[0039]
<Synthesis of modified fluorine-containing polymer compound E>
Modified compound A except that it was irradiated with γ-rays so that the absorbed dose to PVdF became 50 kGy, and synthesized so that the ratio of PVdF in the modified fluorine-containing polymer was 75% by weight and the ratio of acrylic acid was 25% by weight. Was synthesized in the same manner as in the synthesis of, to obtain a modified fluorine-containing polymer compound E (hereinafter, referred to as modified compound E).
<Synthesis of modified fluorine-containing polymer compound F>
Modified compound A except that it was irradiated with gamma rays so that the absorbed dose to PVdF became 50 kGy, and the modified fluorine-containing polymer was synthesized so that the PVdF ratio was 95% by weight and the acrylic acid ratio was 5% by weight. Was synthesized in the same manner as in the synthesis of, to obtain a modified fluorine-containing polymer compound F (hereinafter, referred to as modified compound F).
[0040]
<Example 1>
First, 2 g of the obtained modified compound A was collected, 28 g of NMP was added to the sample, and the mixture was stirred to obtain a polymer solution. Specific surface area 150m ^Two / G of graphite powder of 1.2 g / g and 1.2 g of a dispersant for dispersing the graphite powder were added, and 150 g of NMP was further added to prepare an adhesive layer slurry.
Next, an Al foil having a thickness of 20 μm and a width of 250 μm was prepared as a positive electrode current collector, and a Cu foil having a thickness of 14 μm and a width of 250 μm was prepared as a negative electrode current collector. The slurry of the adhesion layer prepared on the surface of the negative electrode current collector was applied and dried by a doctor blade method, and the thickness of the adhesion layer after drying was controlled within a range of 10 ± 1 μm.
[0041]
Next, Si particles having an average particle diameter of 0.4 μm were prepared as inorganic particles, modified compound A as a modified fluorine-containing polymer compound as a resin binder, AB as a conductive auxiliary, and NMP as a solvent. The negative electrode active material layer was prepared by mixing the inorganic particles in an amount of 50 parts by weight, the resin binder in an amount of 10 parts by weight, the conductive additive in an amount of 40 parts by weight, and the solvent in an amount of 50 parts by weight, and mixing with a ball mill for 2 hours. A coating slurry was prepared.
Next, the respective components shown in Table 1 were mixed by a ball mill for 2 hours to prepare a slurry for coating a positive electrode active material layer and a slurry for coating an electrolyte layer, respectively.
[0042]
[Table 1]

[0043]
The obtained negative electrode active material layer coating slurry is coated and dried by a doctor blade method so that the dry thickness of the negative electrode active material layer becomes 80 μm on the surface of the negative electrode current collector having the adhesion layer, and then rolled. A negative electrode was formed. Similarly, the positive electrode active material layer coating slurry was coated on the surface of the positive electrode current collector, dried, and rolled to form a positive electrode. Further, an electrolyte layer coating slurry is applied to the positive electrode by a doctor blade method so that the dry thickness becomes 50 μm, and a positive electrode having an electrolyte layer and a negative electrode are laminated and thermocompression-bonded to form a sheet-like electrode body. Was prepared. The positive electrode lead and the negative electrode lead made of Ni were welded to the positive electrode current collector and the negative electrode current collector, respectively, and housed in a bag-shaped laminated package material having an opening, and the opening was opened under reduced pressure. Was sealed by thermocompression bonding to produce a sheet-shaped battery.
[0044]
<Example 2>
Si particles having an average particle diameter of 1 μm are prepared as inorganic particles, a modified compound A is prepared as a modified fluorine-containing polymer compound as a first binder, and a VdF-HFP copolymer is prepared as a fluorine-containing polymer compound. , 50 parts by weight of the modified compound A, 5 parts by weight of the VdF-HFP copolymer, 35 parts by weight of the conductive additive, and 60 parts by weight of the solvent. A sheet-like battery was produced in the same manner as in Example 1 except that a working slurry was prepared.
<Example 3>
Modified compound B as the modified fluorine-containing polymer compound as the second binder, Si particles having an average particle diameter of 1 μm as the inorganic particles, and modified compound B as the modified fluorine-containing polymer compound as the first binder, respectively Prepare and prepare the adhesive layer slurry so that the modified compound B is 100 parts by weight, the inorganic particles are 40 parts by weight, the modified compound B is 20 parts by weight, the conductive auxiliary is 40 parts by weight, and the solvent is 80 parts by weight. A sheet-like battery was produced in the same manner as in Example 1 except that the respective mixtures were mixed to prepare a slurry for coating the negative electrode active material layer.
[0045]
<Example 4>
Modified compound C as the modified fluorine-containing polymer compound as the second binder, Si particles having an average particle diameter of 1 μm as the inorganic particles, and modified compound C as the modified fluorine-containing polymer compound as the first binder, respectively A slurry for the adhesion layer was prepared so that the modified compound C was 100 parts by weight, the inorganic particles were 60 parts by weight, the modified compound C was 10 parts by weight, the conductive aid was 30 parts by weight, and the solvent was 50 parts by weight. A sheet-like battery was produced in the same manner as in Example 1 except that the respective mixtures were mixed to prepare a slurry for coating the negative electrode active material layer.
<Example 5>
Modified compound D as the modified fluorine-containing polymer compound as the second binder, Si particles having an average particle size of 10 μm as the inorganic particles, and modified compound F as the modified fluorine-containing polymer compound as the first binder, respectively. Prepare and prepare the adhesive layer slurry so that the modified compound D is 100 parts by weight, the inorganic particles are 50 parts by weight, the modified compound F is 10 parts by weight, the conductive auxiliary is 40 parts by weight, and the solvent is 50 parts by weight. A sheet-like battery was produced in the same manner as in Example 1 except that the respective mixtures were mixed to prepare a slurry for coating the negative electrode active material layer.
[0046]
<Example 6>
Modified compound E as a modified fluorine-containing polymer compound, PVdF as a fluorine-containing polymer compound, Si particles having an average particle diameter of 15 μm as inorganic particles, and modified fluorine-containing The modified compound F was prepared as a polymer compound, and the modified compound E was mixed so that 80 parts by weight of the modified compound E and 20 parts by weight of PVdF, respectively, to prepare an adhesive layer slurry, 50 parts by weight of inorganic particles, and 50 parts by weight of the modified compound F Was prepared in the same manner as in Example 1 except that a slurry for coating a negative electrode active material layer was prepared by mixing 20 parts by weight of a conductive auxiliary agent and 50 parts by weight of a solvent, respectively. Produced.
<Example 7>
Modified compound E as the modified fluorine-containing polymer compound as the second binder, Si particles having an average particle size of 1 μm as the inorganic particles, modified compound F as the modified fluorine-containing polymer compound as the first binder, A VdF-HFP copolymer was prepared as a fluorine-containing polymer compound, and an adhesion layer slurry was prepared so that the modified compound E was 100 parts by weight. The inorganic particles were 40 parts by weight, the modified compound F was 10 parts by weight, Same as Example 1 except that the VdF-HFP copolymer was mixed at 5 parts by weight, the conductive additive was mixed at 45 parts by weight, and the solvent was mixed at 50 parts by weight to prepare a slurry for coating the negative electrode active material layer. Thus, a sheet-shaped battery was produced.
[0047]
<Comparative Example 1>
PVdF as a fluorine-containing polymer compound as the second binder, Si particles having an average particle diameter of 10 μm as the inorganic particles, and PVdF as the first binder were prepared so that PVdF was 100 parts by weight. An adhesive layer slurry was prepared, and the inorganic particles were mixed so that 50 parts by weight, PVdF was 10 parts by weight, the conductive additive was 40 parts by weight, and the solvent was 50 parts by weight, to thereby prepare a slurry for coating a negative electrode active material layer. A sheet-like battery was produced in the same manner as in Example 1 except for the preparation.
<Comparative Example 2>
Si particles having an average particle size of 5 μm were prepared as the inorganic particles, and PTFE was prepared as the resin binder. The adhesion layer was not formed, and 50 parts by weight of the inorganic particles, 10 parts by weight of the PTFE, and 40 parts by weight of the conductive assistant were used. Parts and the solvent were mixed so as to be 50 parts by weight, respectively, to prepare a negative electrode active material layer coating slurry, and a sheet battery was produced in the same manner as in Example 1.
<Comparative Example 3>
Si particles having an average particle diameter of 1 μm were prepared as inorganic particles, and a VdF-HFP copolymer was prepared as a resin binder. An adhesion layer was not formed, and 40 parts by weight of inorganic particles and VdF-HFP copolymer were used. A sheet-like battery was prepared in the same manner as in Example 1, except that the slurry was prepared by mixing each of 15 parts by weight, 45 parts by weight of the conductive additive, and 50 parts by weight of the solvent to prepare a slurry for coating the negative electrode active material layer. did.
[0048]
<Comparison test and evaluation>
The following evaluation tests were performed using the sheet batteries obtained in Examples 1 to 7 and Comparative Examples 1 to 3, respectively.
(1) Battery cycle capacity maintenance characteristics test
The obtained sheet-shaped battery is subjected to a charge / discharge cycle test, and a charging step of charging for 10 hours under conditions such that the maximum charging voltage is 4 V and the charging current of the negative electrode active material is 400 mAh / g, and a charging step similar to the charging step. The charge and discharge cycle is repeated with the discharge step of discharging the discharge voltage to 2.0 V at which the discharge voltage becomes the minimum discharge voltage under the current conditions as one cycle. When performing the charge and discharge test, the charge and discharge capacity of each cycle is measured. Thus, the discharge capacity retention rate at the 10th cycle ((discharge capacity at the 10th cycle / initial discharge capacity) × 100) was determined.
[0049]
(2) Adhesion test of adhesion layer to current collector and active material layer
The obtained sheet-shaped battery was subjected to 10 charge / discharge cycles under the same conditions as in the evaluation test (1). Thereafter, the storage package of the sheet-shaped battery after the completion of 10 cycles of charging and discharging was removed, and the positive electrode and the negative electrode of the battery were separated by separating. In the batteries of Examples 1 to 7, the pinch layer of the separated negative electrode was pinched with tweezers and pulled to determine whether or not the adhesion layer was separated from the current collector and whether or not the adhesion layer was separated from the active material layer. Was confirmed respectively. Further, in the batteries of Comparative Examples 1 to 3, it was confirmed whether or not the negative electrode active material layer and the negative electrode current collector were separated when the separated negative electrode was pinched with tweezers and pulled.
[0050]
The compositions of the negative electrode active material layers and the adhesion layers used in the sheet batteries of Examples 1 to 7 and Comparative Examples 1 to 3 are shown in Table 2 below. Table 3 shows the results of the evaluation tests (1) and (2).
The symbols in the evaluation of adhesion in Table 3 have the following meanings. ：: Very good adhesion. ：: There is partial peeling. ×: Completely peeled.
[0051]
[Table 2]

[0052]
[Table 3]

[0053]
As is clear from Table 3, in the cycle capacity maintenance characteristic test of the evaluation test (1), the discharge capacity retention by the sheet batteries of Comparative Examples 1 to 3 was significantly reduced. It is considered that the cause of the decrease in the discharge capacity retention ratio is that the expansion and contraction of the inorganic particles as the active material due to charge and discharge caused peeling occurring at the interface between the active material and the current collector. It is considered that this peeling stopped the flow of electrons between the active material of the battery and the current collector, and thus greatly reduced the capacity retention at the tenth cycle. On the other hand, the discharge capacity retention rates of the sheet batteries of Examples 1 to 7 each showed a high value. This is because the modified fluorine-containing polymer compound contained in the resin binders of Examples 1 to 7 has high adhesion to the inorganic particles, and thus the volume of the inorganic particles due to the volume expansion and contraction of the inorganic particles accompanying charge and discharge is increased. It is considered that peeling was prevented and the cycle capacity was maintained.
[0054]
In the adhesion test after 10 cycles of the evaluation test (2) in the sheet batteries of Comparative Examples 1 to 3, the negative electrode current collector and the negative electrode active material layer were completely peeled off. This is considered to be because the adhesive surface between the inorganic particles and the resin binder was peeled off due to expansion and contraction due to the charge / discharge cycle inside the negative electrode active material layer, and the cohesive strength of the active material layer itself was lost. It is also assumed that the active material layer, which was originally in the form of a sheet, was changed to a fragment-like or sand-like form by several times of charge and discharge. On the other hand, in the sheet batteries of Examples 1 to 7, the negative electrode active material layer was hard to be separated from the negative electrode current collector, and the cohesive strength of the negative electrode active material layer itself was maintained. By including the resin binder containing the modified fluorine-containing polymer compound in the negative electrode material forming the negative electrode active material layer in this way, the adhesion between the negative electrode active material layer and the negative electrode current collector is improved, and the inorganic particles are included. Peeling due to a change in volume of the negative electrode active material layer can be prevented.
[0055]
【The invention's effect】
As described above, the negative electrode for a lithium ion secondary battery of the present invention includes the first binder, Si, Ge, Mg, Sn, Pb, Ag, Al, Zn, Cd, A negative electrode active material layer including both inorganic particles containing at least one element selected from the group consisting of Sb, Bi and In is formed, and a second connection is formed between the negative electrode current collector and the negative electrode active material layer. It is an improvement of a negative electrode having an adhesion layer containing both a bonding agent and a conductive material. The characteristic configuration is such that the first binder and the second binder each include a modified fluorine-containing polymer compound obtained by modifying a fluorine-containing resin with a modifying substance, and the first binder and the second binder are different from each other. The structure of the modified fluorine-containing polymer compound contained in each of the adhesives is the same or different from each other. Such a configuration has the following excellent effects.
(1) It has high chemical stability and excellent adhesion to inorganic particles.
(2) It has excellent adhesion and conductivity between the negative electrode current collector and the negative electrode active material layer, and can improve cycle capacity maintenance characteristics.
(3) High mechanical strength can be maintained.
(4) The adhesion layer is stable with respect to the organic solvent in the electrolytic solution and has excellent long-term storage properties.
(5) Corrosion of the current collector by a strong acid such as hydrofluoric acid generated in the battery can be suppressed.
(6) Cycle characteristics can be improved by controlling the stress associated with the insertion and desorption reactions of lithium ions during charge and discharge.
[Brief description of the drawings]
FIG. 1 is a partial cross-sectional configuration diagram showing an electrode body of a lithium ion secondary battery of the present invention.
FIG. 2 is a schematic view showing the synthesis of a modified fluorine-containing polymer compound.
[Explanation of symbols]
11 positive electrode current collector
12 Positive electrode active material layer
13 Positive electrode
14 Negative electrode current collector
16 Negative electrode active material layer
17 Negative electrode
18 Electrolyte layer

Claims

負極集電体(14)の表面に第１結着剤と、Ｓｉ、Ｇｅ、Ｍｇ、Ｓｎ、Ｐｂ、Ａｇ、Ａｌ、Ｚｎ、Ｃｄ、Ｓｂ、Ｂｉ及びＩｎからなる群より選ばれた少なくとも１種の元素を含む無機質粒子の双方をそれぞれ含む負極活物質層(16)が形成され、
前記負極集電体(14)と前記負極活物質層(16)との間に第２結着剤と導電性物質の双方をそれぞれ含む密着層(19)を有するリチウムイオン二次電池用負極において、
前記第１結着剤及び前記第２結着剤がフッ素含有樹脂を変性物質により変性して得られた変性フッ素含有高分子化合物をそれぞれ含み、
前記第１結着剤及び前記第２結着剤にそれぞれ含まれる変性フッ素含有高分子化合物の構造が互いに同一又は異なる
ことを特徴とするリチウムイオン二次電池用負極。A first binder and at least one selected from the group consisting of Si, Ge, Mg, Sn, Pb, Ag, Al, Zn, Cd, Sb, Bi and In on the surface of the negative electrode current collector (14). A negative electrode active material layer (16) containing both of the inorganic particles containing the element is formed,
In a negative electrode for a lithium ion secondary battery having an adhesion layer (19) containing both a second binder and a conductive material between the negative electrode current collector (14) and the negative electrode active material layer (16). ,
The first binder and the second binder each include a modified fluorine-containing polymer obtained by modifying a fluorine-containing resin with a modifying substance,
A negative electrode for a lithium ion secondary battery, wherein the modified fluorine-containing polymer compounds contained in the first binder and the second binder have the same or different structures.

第１結着剤又は第２結着剤のどちらか一方又はその双方に含まれる変性フッ素含有高分子化合物(24)がフッ素含有樹脂を幹重合体(22)とし、前記幹重合体(22)を変性物質(23)によりグラフト変性させて得られる高分子化合物である請求項１記載の負極。The modified fluorine-containing polymer compound (24) contained in one or both of the first binder and the second binder makes a fluorine-containing resin a trunk polymer (22), and the trunk polymer (22) The negative electrode according to claim 1, wherein the negative electrode is a polymer compound obtained by graft-modifying the compound with a modifying substance (23).

変性フッ素含有高分子化合物(24)を構成する幹重合体(22)が、ポリフッ化ビニリデン、ポリフッ化ビニル、４フッ化エチレンポリマー、３フッ化エチレンポリマー、２フッ化エチレンポリマー、フッ化ビニリデン-ヘキサフルオロプロピレン共重合体、エチレン-４フッ化エチレン共重合体、４フッ化エチレン-６フッ化プロピレン共重合体、３フッ化塩化エチレンポリマー及びポリテトラフルオロエチレンからなる群より選ばれた少なくとも１種のフッ素含有樹脂を含む請求項２記載の負極。The backbone polymer (22) constituting the modified fluorine-containing polymer compound (24) is polyvinylidene fluoride, polyvinyl fluoride, tetrafluoroethylene polymer, trifluoride ethylene polymer, difluoroethylene polymer, vinylidene fluoride- At least one selected from the group consisting of a hexafluoropropylene copolymer, an ethylene tetrafluoroethylene copolymer, a tetrafluoroethylene hexafluoropropylene copolymer, a trifluoroethylene chloride polymer and polytetrafluoroethylene 3. The negative electrode according to claim 2, comprising a kind of fluorine-containing resin.

変性フッ素含有高分子化合物(24)を構成する変性物質(23)が、アクリル酸、アクリル酸メチル、メタクリル酸及びメタクリル酸メチルの少なくとも１種である請求項２記載の負極。3. The negative electrode according to claim 2, wherein the modified substance (23) constituting the modified fluorine-containing polymer compound (24) is at least one of acrylic acid, methyl acrylate, methacrylic acid and methyl methacrylate.

変性フッ素含有高分子化合物(24)を構成する幹重合体(22)がポリフッ化ビニリデン又はフッ化ビニリデン-ヘキサフルオロプロピレン共重合体のいずれか一方又は双方を含む混合物であって、変性物質(23)がアクリル酸である請求項２ないし４いずれか１項に記載の負極。The trunk polymer (22) constituting the modified fluorine-containing polymer compound (24) is a mixture containing one or both of polyvinylidene fluoride and vinylidene fluoride-hexafluoropropylene copolymer, and the modified substance (23 The negative electrode according to any one of claims 2 to 4, wherein) is acrylic acid.

変性フッ素含有高分子化合物(24)はフッ素含有樹脂(22)と変性物質(23)が放射線照射処理によってグラフト化される請求項２ないし５いずれか１項に記載の負極。The negative electrode according to any one of claims 2 to 5, wherein the modified fluorine-containing polymer compound (24) is obtained by grafting a fluorine-containing resin (22) and a modified substance (23) by radiation irradiation treatment.

フッ素含有樹脂(22)への放射線照射処理はγ線照射によって行われ、前記フッ素含有樹脂のγ線の吸収線量が１０〜９０ｋＧｙである請求項６記載の負極。The negative electrode according to claim 6, wherein the irradiation of the fluorine-containing resin (22) with radiation is performed by γ-ray irradiation, and the absorbed dose of γ-rays of the fluorine-containing resin is 10 to 90 kGy.

第１結着剤に含まれる変性フッ素含有高分子化合物が、第２結着剤に含まれる変性フッ素含有高分子化合物を構成する反復単位をその反復単位として含む請求項１ないし７いずれか１項に記載の負極。The modified fluorine-containing polymer compound contained in the first binder contains a repeating unit constituting the modified fluorine-containing polymer compound contained in the second binder as a repeating unit thereof. The negative electrode according to 1.

第１結着剤に変性フッ素含有高分子化合物とは構造の異なるフッ素含有高分子化合物を更に含む請求項１ないし８いずれか１項に記載の負極。The negative electrode according to any one of claims 1 to 8, wherein the first binder further includes a fluorine-containing polymer compound having a structure different from that of the modified fluorine-containing polymer compound.

フッ素含有高分子化合物が、変性フッ素含有高分子化合物を構成するフッ素含有樹脂に含まれる反復単位をその反復単位として含む請求項９記載の負極。The negative electrode according to claim 9, wherein the fluorine-containing polymer compound includes, as a repeating unit, a repeating unit contained in a fluorine-containing resin constituting the modified fluorine-containing polymer compound.

第１結着剤に含まれる変性フッ素含有高分子化合物の含有割合が１０重量％〜１００重量％である請求項１ないし１０いずれか１項に記載の負極。The negative electrode according to any one of claims 1 to 10, wherein the content of the modified fluorine-containing polymer compound contained in the first binder is 10% by weight to 100% by weight.

第２結着剤に含まれる変性フッ素含有高分子化合物の含有割合が１０重量％〜１００重量％である請求項１ないし１０いずれか１項に記載の負極。The negative electrode according to any one of claims 1 to 10, wherein the content ratio of the modified fluorine-containing polymer compound contained in the second binder is 10% by weight to 100% by weight.

第１結着剤に含まれる変性フッ素含有高分子化合物の含有割合が、第２結着剤に含まれる変性フッ素含有高分子化合物の含有割合と同一の割合、又はそれよりも低い割合である請求項１１又は１２記載の負極。The content ratio of the modified fluorine-containing polymer compound contained in the first binder is the same as or lower than the content ratio of the modified fluorine-containing polymer compound contained in the second binder. Item 13. The negative electrode according to item 11 or 12.

第１結着剤に含まれる変性フッ素含有高分子化合物のグラフト重合割合（変性物質の重量／フッ素含有樹脂の重量）が、第２結着剤に含まれる変性フッ素含有高分子化合物のグラフト重合割合と同一の割合、又はそれよりも低い割合である請求項１ないし１３いずれか１項に記載の負極。The graft polymerization ratio of the modified fluorine-containing polymer compound contained in the first binder (the weight of the modified substance / the weight of the fluorine-containing resin) is determined by the graft polymerization ratio of the modified fluorine-containing polymer compound contained in the second binder. The negative electrode according to any one of claims 1 to 13, wherein the negative electrode has the same ratio or a lower ratio.

第１結着剤又は第２結着剤のどちらか一方又はその双方に含まれる変性フッ素含有高分子化合物のグラフト重合割合が、２重量％〜５０重量％である請求項１４記載の負極。The negative electrode according to claim 14, wherein the graft polymerization ratio of the modified fluorine-containing polymer compound contained in one or both of the first binder and the second binder is 2% by weight to 50% by weight.

負極活物質層(16)に含まれる第１結着剤の割合が活物質層固形物全体の１重量％〜５０重量％である請求項１ないし１５いずれか１項に記載の負極。The negative electrode according to any one of claims 1 to 15, wherein a ratio of the first binder contained in the negative electrode active material layer (16) is 1% by weight to 50% by weight of the whole solid material of the active material layer.

密着層(19)に含まれる第２結着剤の割合が密着層固形物全体の５〜４０重量％である請求項１ないし１６いずれか１項に記載の負極。The negative electrode according to any one of claims 1 to 16, wherein the ratio of the second binder contained in the adhesion layer (19) is 5 to 40% by weight of the whole solid matter of the adhesion layer.

無機質粒子はＳｉ、Ｇｅ、Ｍｇ、Ｓｎ、Ｐｂ、Ａｇ、Ａｌ、Ｚｎ、Ｃｄ、Ｓｂ、Ｂｉ及びＩｎからなる群より選ばれた少なくとも１種の元素が単体、酸化物又は他の金属との合金、前記単体とリチウムとの合金、及びこれらの金属、リチウムを含む多元合金で構成される請求項１記載の負極。As the inorganic particles, at least one element selected from the group consisting of Si, Ge, Mg, Sn, Pb, Ag, Al, Zn, Cd, Sb, Bi and In is a simple substance, an oxide or an alloy with another metal. 2. The negative electrode according to claim 1, comprising an alloy of said simple substance and lithium, and a multi-component alloy containing these metals and lithium.

無機質粒子の平均粒径が０．０１〜５０μｍである請求項１又は１８記載の負極。19. The negative electrode according to claim 1, wherein the average particle diameter of the inorganic particles is 0.01 to 50 [mu] m.

負極活物質層(16)に含まれる無機質粒子の割合が活物質層固形物全体の５〜９５重量％である請求項１，１８又は１９いずれか１項に記載の負極。The negative electrode according to any one of claims 1, 18 and 19, wherein a ratio of the inorganic particles contained in the negative electrode active material layer (16) is 5 to 95% by weight of the whole solid of the active material layer.

請求項１ないし２０いずれか１項に記載の負極(17)を用いて作製したリチウムイオン二次電池。A lithium ion secondary battery produced using the negative electrode (17) according to any one of claims 1 to 20.