JP4201517B2

JP4201517B2 - Nonaqueous electrolyte secondary battery

Info

Publication number: JP4201517B2
Application number: JP2002094660A
Authority: JP
Inventors: 洋平廣田
Original assignee: Sanyo Electric Co Ltd
Current assignee: Sanyo Electric Co Ltd
Priority date: 2002-03-29
Filing date: 2002-03-29
Publication date: 2008-12-24
Anticipated expiration: 2022-03-29
Also published as: JP2003297429A

Description

【０００１】
【発明の属する技術分野】
本発明は、正極板と負極板の間にセパレータを介在させて渦巻状に巻回された渦巻状電極体を角形外装缶内に備えた非水電解質二次電池に関する。
【０００２】
【従来の技術】
近年、小型ビデオカメラ、携帯電話、ノートパソコン等の携帯用電子・通信機器等の電源として、小型軽量でかつ高容量な非水電解質二次電池が用いられるようになった。このような非水電解質二次電池は、リチウムイオンの吸蔵・放出が可能な黒鉛を負極活物質として用い、リチウム含有コバルト酸化物（ＬｉＣｏＯ₂）、リチウム含有マンガン酸化物（ＬｉＭｎ₂Ｏ₄）等のリチウム含有遷移金属酸化物を正極活物質として用い、有機溶媒に溶質としてリチウム塩を溶解した電解液を用いて構成される電池である。
【０００３】
ところで、この種の非水電解質二次電池が使用される機器においては、電池を収容するスペースが角形であることが多いことから、発電要素を角形外装缶に収容して形成した角形電池が使用されることが多い。このような角形電池は以下のようにして作製されるのが一般的である。即ち、まず、正極集電体（正極芯体）に正極活物質を含有する正極合剤を塗布して正極板を作製するとともに、負極集電体（負極芯体）に負極活物質を含有する負極合剤を塗布して負極板を作製する。この後、これらの正極板と負極板をセパレータを介して相対向させた後、これらを渦巻状に巻回して渦巻状電極体とする。この渦巻状電極体を加圧成形して、扁平な渦巻状電極体とした後、これを扁平な角形外装缶に収容し、非水電解液を注液して非水電解質二次電池とする。
【０００４】
この場合、扁平な渦巻状電極体を扁平な角形外装缶に挿入できるように加圧する必要があるが、この加圧時にセパレータは圧縮されて、加圧前の厚みに比較して厚みが薄くなる。ところが、渦巻状電極体を強く加圧するほどセパレータが圧縮されて、セパレータのイオン透過性が低下して、電池の放電特性や、高率放電特性を低下させるとともに、充放電サイクル寿命も短くなる。このため、セパレータの圧縮度合が所定の範囲内になるように制御しなければならいこととなる。
【０００５】
【発明が解決しようとする課題】
しかしながら、この種の非水電解質二次電池に対する高容量化の要求は高く、この要求に応えるために、同サイズの電池に充填される活物質の充填量が増大する傾向にある。このため、以前にも増して強い力で渦巻状電極体を加圧しなければ、角形外装缶内に電極体を挿入するのが困難になってきた。ところが、このように渦巻状電極体を強く加圧して作製した電池は、活物質の充填量が多い割には電池容量が低く、充放電サイクル寿命も短くなるという問題を生じた。
【０００６】
そこで、この原因を調査するために、渦巻状電極体を強く加圧して作製した電池から電極体を取り出し、取り出した電極体を分解して調査した結果、以下のことが明らかになった。即ち、分解した電極体から取り出したセパレータの厚みを測定すると、セパレータの厚みが場所によって、薄くなっている所とそうでない所があって、大きく異なっていることが分かった。これは、渦巻状電極体を強く加圧すると、電極体の内部にある集電タブ、極板、セパレータの位置関係により、セパレータが強く圧縮されている部分と、そうでない部分が生じたためと考えられる。そして、セパレータが強く圧縮された部分では、セパレータの厚みが薄くなり、圧縮が強くない部分ではセパレータの厚みがそれほど薄くなっていないことが分かった。
【０００７】
このように、セパレータの厚みが場所によって異なることにより、充放電時の電極反応は不均一になり、電池容量の低下を招く恐れがある。これは、セパレータが強く圧縮されて厚みが薄くなっている部分では、そうでない部分に比べて、多孔度が減少するためにイオンの透過性が低下し、電極反応が不均一になるからである。また、充放電反応での活物質の利用のされ方にも偏りが生じ、電極反応が特定の部分に集中して、充放電サイクルによる劣化も早くなるからである。
そこで、本発明は上記問題点を解消するためになされたものであって、電極体全体が均一に反応できるようにして、高容量で充放電サイクル特性に優れた非水電解質二次電池を提供することを目的とするものである。
【０００８】
【課題を解決するための手段】
上記目的を達成するため、本発明の非水電解質二次電池は、正極板と負極板の間にセパレータを介在させて渦巻状に巻回された渦巻状電極体を角形外装缶内に備えている。そして、渦巻状電極体は加圧成形により扁平状電極体に成形されていて、加圧方向に集電タブが設けられていたり極板の積層枚数が多い部分が存在し、扁平状電極体の正極板と負極板との間に介在するセパレータは、加圧方向に設けられた集電タブや極板の積層枚数が多い部分で強く圧縮されて厚みが最も薄い部分となるとともに、厚みが最も薄い部分の厚みが最も厚い部分の厚みに対して７０％以上で９５％以下になるように成形されていることを特徴とする。
【０００９】
ここで、正極板と負極板の間にセパレータを介在させて渦巻状に巻回した渦巻状電極体の内部は、集電タブが設けられた場所や、極板の積層枚数が多い場所がある。このような渦巻状電極体を加圧成形して扁平状電極体とすると、セパレータが強く圧縮される部分や、そうでない部分が生じる。このため、集電タブの上下に位置するセパレータや、極板の積層枚数が多い場所のセパレータは、それ以外の場所のセパレータに比べてより強く圧縮されることとなる。
【００１０】
この場合、加圧成形時の加圧力は、集電タブが設けられた場所や極板の積層枚数が多い場所に集中し、それ以外の場所のセパレータはそれほど加圧力が集中しないために、セパレータが潰れることはない。そこで、加圧力を種々に変更してセパレータの最大厚みと最小厚みを測定したところ、加圧力によって変動するのは最小厚みであって、最大厚みは大きく変わることがないという実験結果が得られた。したがって、渦巻状電極体を加圧成形するに際して、加圧力を制御することにより、セパレータの最大厚みに対する最小厚みの比率を制御することが可能となる。
【００１１】
そこで、種々の実験を行った結果、渦巻状電極体を加圧成形した際に、セパレータの最大厚みに対する最小厚みの比率が７０％以上であれば、放電容量が向上するとともに、充放電サイクル特性も向上することが分かった。一方、セパレータの加圧力をできるだけ小さくした方が、セパレータ潰れを防止できるようになる。ところが、電極体の加圧力が弱すぎると、スプリングバック現象が生じて、この電極体を角形外装缶へ挿入することが困難になるということが分かった。
【００１２】
そこで、スプリングバック現象が生じない加圧力を求めた結果、セパレータの最大厚みに対する最小厚みの比率が９５％以下になるような加圧力であれば、スプリングバック現象が生じないことが分かった。以上のことから、扁平状電極体内に介在するセパレータは、最も薄い部分の厚みが最も厚い部分の厚みに対して７０％以上で９５％以下になるように成形される必要があるということができる。
【００１３】
この場合、空孔率が４０％未満のセパレータを用いると充放電サイクル特性が低下し、また、空孔率が５５％を超えたセパレータの場合は、ショート発生が起こりやすく、品質面で問題があることが明らかになった。一般にセパレータの空孔率が小さいほどイオンの透過性が低下する傾向があり、セパレータの空孔率が４０％未満の場合、圧縮率が大きい部分でのイオン透過性が著しく小さくなり、電極反応が不均一になって充放電サイクルによる劣化も早くなったものと考えられる。また空孔率が５５％を超えるセパレータは、高分子膜の強度が低下するため、ショートが発生しやすくなり、品質面や安全面で問題がある。このため、空孔率が４０％以上で５５％以下のセパレータを用いるのが好ましい。
【００１４】
また、電極体の中心部に配置される負極板に負極集電タブを設ける場合、この負極集電タブの幅が電極体中心部空間の幅（負極板の巻始め部の端部から数えて第１の折り目から第２の折り目までの長さに等しい）の６０％未満であると、電極体中心部空間の幅に対する負極集電タブの幅は相対的に小さくなる。このため、この部分の極板厚みは他の部分の極板厚みよりも厚くなるため、この部分に配置されるセパレータは強く圧縮されることとなる。
【００１５】
一方、負極集電タブの幅が電極体中心部空間の幅に対して６０％以上になると、電極体中心部空間の幅に対する負極集電タブの幅は相対的に大きくなる。このため、この部分の極板厚みが他の部分の極板厚みと均一化されるようになる。これにより、この部分に配置されるセパレータは均一に圧縮されるようになるので、渦巻状電極体を加圧成形したときにセパレータの厚みの場所による変動が小さくなる。この結果、加圧成形後の電極体内でのセパレータの厚みを均一にすることが可能になる。
【００１６】
【発明の実施の形態】
ついで、本発明の実施の形態を図１〜図３に基づいて説明するが、本発明はこの実施の形態に何ら限定されるものでなく、本発明の目的を変更しない範囲で適宜変更して実施することが可能である。なお、図１は本発明の正極板を示す図であり、図１（ａ）は正面図を示し、図１（ｂ）は断面図を示す。また、図２は本発明の負極板を示す図であり、図２（ａ）は正面図を示し、図２（ｂ）は断面図を示す。さらに、図３は本発明のセパレータを示す図であり、図３（ａ）は正面図を示し、図３（ｂ）は断面図を示す。
【００１７】
１．正極板の作製
まず、正極活物質としてのコバルト酸リチウム（ＬｉＣｏＯ₂）粉末が８５質量部で、導電剤としての人造黒鉛粉末とカーボンブラックをそれぞれ５質量部ずつ混合して正極合剤を調製した。この正極合剤と、ポリビニリデンフルオライド（ＰＶｄＦ）よりなる結着剤をＮ−メチル−２−ピロリドン（ＮＭＰ）からなる有機溶剤に５質量％溶解した結着剤溶液とを、固形分の質量比で９５：５になるように混合、混練して正極活物質スラリーを調製した。
【００１８】
この正極活物質スラリーを正極集電体（例えば、長さが３２４ｍｍで、幅が３７．５ｍｍで、厚みが２０μｍのアルミニウム箔あるいはアルミニウム合金箔）１１の両面に塗布して正極活物質層１２を形成した後、この正極活物質層１２を乾燥させた。その後、乾燥正極板をロールプレス機により圧延して、所定の充填密度の正極板１０を作製した。なお、正極集電体１１の後端部（渦巻状電極体の巻終わり部となる部分）から約５０ｍｍの部分の片面には正極活物質スラリーを塗布せずに、正極集電体１１を露出させた露出部１１ａを形成するようにした。また、後端部から約２５ｍｍの部分の両面には正極活物質スラリーを塗布せずに、正極集電体１１を露出させるようにしている。
【００１９】
２．負極の作製
一方、負極活物質としての天然黒鉛（Ｌｃ値が１０００Åで、ｄ₀₀₂値が３．３５６Åで、平均粒径が２０μｍのもの）粉末９５質量部に、ポリビニリデンフルオライド（ＰＶｄＦ）よりなる結着剤をＮ−メチル−２−ピロリドン（ＮＭＰ）からなる有機溶剤に５質量％溶解した結着剤溶液を、固形分の質量比で９５：５になるように混合、混練して負極活物質スラリーを調製した。得られた負極活物質スラリーを負極集電体（例えば、長さが３２０ｍｍで、幅が３９．５ｍｍで、厚みが１５μｍの銅箔）２１の両面に塗布して負極活物質層２２を形成した後、この負極活物質層２２を乾燥させた。
【００２０】
その後、乾燥負極板をロールプレス機により圧延して、所定の充填密度の負極板２０を作製した。なお、負極集電体２１の先端部（渦巻状電極体の巻始め部となる部分）から２０ｍｍ（ｗ＝２０ｍｍ）の部分の両面には負極活物質スラリーを塗布せずに、負極集電体２１を露出させている。そして、この部分に幅が５ｍｍで、厚みが８０μｍのニッケル金属製の負極集電タブ２３を超音波溶接した。なお、図２（ａ）に示された一点鎖線は、この負極板２０が渦巻状に巻回された後の加圧の際に生じる折り目を表しており、先端部ｔ₀から約等間隔ｗで第１の折り目ｔ₁、第２の折り目ｔ₂となるように折り目が形成されることとなる。
【００２１】
３．電池の作製
（１）電極体の作製
ついで、上述のようにして作製した正極板１０と負極板２０との間に、微多孔のポリエチレン製セパレータ３０を挟み込んで、渦巻状に卷回して渦巻状電極体を作製した。この場合、セパレータ３０としては、幅が４１．５ｍｍで、空孔率が４０％で、加圧前の厚みが３０μｍのものを使用した。そして、渦巻状に卷回する際に正極板１０は負極板２０よりも遅らせて巻き取るようにしている。これにより、正極板１０の先端部（巻始め部）は、負極板２０の先端部（巻始め部）よりも約２０ｍｍ位ずれることとなる。即ち、負極板２０の第１の折曲部ｔ₁（図２参照）に正極板１０が位置しないようにしている。また、正極板１０は正極集電体１１の露出部１１ａを電極体の最外周部に位置させるようにして、後に外装缶の内表面に接触させて電気接続できるようにしている。
【００２２】
この後、上述のように作製した渦巻状電極体の両側から加圧して、角形外装缶の開口部から電極体を挿入できるように成形した。このとき、加圧力を制御して渦巻状電極体を加圧成形すると、極板の積層枚数が多い部分や、負極集電タブ２３がある場所のセパレータ３０は強く圧縮され、加圧後にセパレータ３０の厚みを測定すると、この部分の厚みが最小になっており、その他の部分ではあまり強く圧縮されることがなく、圧縮前の厚み（３０μｍ）とあまり変わらない厚みであることが分かった。このことは、加圧力を変更するだけで、セパレータ３０の最大厚みをほとんど変えることなく、最小厚みを制御できることを意味する。そこで、渦巻状電極体を加圧成形する際の加圧力を変化させて、下記の表１に示すようなセパレータ厚みとなるように加圧して各電極体ａ１〜ａ９を作製した。
【００２３】
【表１】

【００２４】
（２）非水電解液二次電池の作製
ついで、肉厚が０．５ｍｍで、外形寸法の高さが４８ｍｍで、幅が２９ｍｍで、厚みが５ｍｍのアルミニウム製の角形外装缶を用意した。なお、角形外装缶の大きさ、材質、肉厚はこれに限ることはなく、例えば、鉄あるいは鉄合金製のものを用いるようにしてもよい。ついで、これらの各電極体ａ１〜ａ９を、各角形外装缶の開口部から挿入して、各電極体ａ１〜ａ９の正極集電体１１を露出させた露出部１１ａを外装缶の内面に接触させた。
【００２５】
また、各電極体ａ１〜ａ９の負極板２０より延出する負極集電タブ２３を封口体（図示せず）の内底部に設けた端子板（図示せず、封口板とは電気的に絶縁されている）に抵抗溶接した。なお、電池表面の封口板上には、封口板とは電気的に絶縁された負極端子（図示せず）を設け、この端子板は封口板に開けた透孔を通じて負極端子と電気的に接続している。ついで、これらの各角形外装缶内に非水電解液（エチレンカーボネート（ＥＣ）とジエチルカーボネート（ＤＥＣ）を等体積比で混合した溶媒に、六フッ化リン酸リチウム（ＬｉＰＦ₆）からなる電解質を１モル／リットル溶解させた非水溶液）をそれぞれ注入した。この後、外装缶の開口部を封口体で液密に封口して、設計容量が６００ｍＡｈの非水電解質二次電池Ａ１〜Ａ９をそれぞれ作製した。
【００２６】
なお、電極体ａ１を用いて作製した非水電解質二次電池を電池Ａ１とし、電極体ａ２を用いて作製した非水電解質二次電池を電池Ａ２とし、電極体ａ３を用いて作製した非水電解質二次電池を電池Ａ３とし、電極体ａ４を用いて作製した非水電解質二次電池を電池Ａ４とし、電極体ａ５を用いて作製した非水電解質二次電池を電池Ａ５とし、電極体ａ６を用いて作製した非水電解質二次電池を電池Ａ６とし、電極体ａ７を用いて作製した非水電解質二次電池を電池Ａ７とし、電極体ａ８を用いて作製した非水電解質二次電池を電池Ａ８とし、電極体ａ９を用いて作製した非水電解質二次電池を電池Ａ９とした。
【００２７】
４．非水電解液二次電池の負荷特性試験
ついで、上述のように作製した各電池Ａ１〜Ａ９をそれぞれ、室温（約２５℃）で、６００ｍＡ（１ＩｔｍＡ、１Ｉｔは電池容量を１時間で放電する電流値）の定電流で電池電圧が４．１０Ｖになるまで充電し、ついで、４．１０Ｖの定電圧で電流値が１０ｍＡになるまで充電した。この後、６００ｍＡ（１ＩｔｍＡ）の放電電流で電池電圧が２．７５Ｖになるまで放電させて、放電時間から放電容量（ｍＡｈ）を求めると、下記の表２に示すような結果となった。
【００２８】
【表２】

【００２９】
上記表２の結果から明らかなように、電池Ａ６，Ａ７，Ａ８，Ａ９のように、電極体内のセパレータ３０の最大厚みに対する最小厚みの割合が７０％未満であると、放電容量が減少していることが分かる。これに対して、電池Ａ１，Ａ２，Ａ３，Ａ４，Ａ５のように、電極体内のセパレータ３０の最大厚みに対する最小厚みの割合が７０％以上であると、放電容量が増加することが分かる。なお、電池Ａ１のように、電極体内のセパレータ３０の最大厚みに対する最小厚みの割合が９７％の電極体とすると、電極体の作製時のスプリングバック現象が生じて、この電極体を角形外装缶内に挿入するのは困難であった。
【００３０】
このため、スプリングバック現象が生じないようにして、電極体を角形外装缶内に容易に挿入できるようにするためには、電極体内のセパレータ３０の最大厚みに対する最小厚みの割合が９５％以下になるようにする必要がある。これらのことから、電極体内のセパレータ３０の最大厚みに対する最小厚みの割合を７０％以上にする必要があるととともに、電極体内のセパレータ３０の最大厚みに対する最小厚みの割合が９５％以下になるようにする必要があるいうことができる。
【００３１】
５．セパレータの空孔率の検討
ついで、使用するセパレータの空孔率について検討した。ここで、空孔率が約３０％〜約６０％までの間で、約５％おきに空孔率を変化させたセパレータを用いて、上述と同様に渦巻状電極体を作製した。このとき、電極体内のセパレータ３０の最大厚みに対する最小厚みの割合が７０％以上になるような圧力を付加して加圧成形して、上述と同様に設計容量が６００ｍＡｈの非水電解質二次電池Ｂ１〜Ｂ７を作製した。
【００３２】
ここで、空孔率が３０％のセパレータを用いて作製したものを電池Ｂ１とし、空孔率が３５％のセパレータを用いて作製したものを電池Ｂ２とし、空孔率が４０％のセパレータを用いて作製したものを電池Ｂ３とし、空孔率が４５％のセパレータを用いて作製したものを電池Ｂ４とし、空孔率が５０％のセパレータを用いて作製したものを電池Ｂ５とし、空孔率が５５％のセパレータを用いて作製したものを電池Ｂ６とし、空孔率が６０％のセパレータを用いて作製したものを電池Ｂ７とした。
【００３３】
ついで、上述のように作製した各電池Ｂ１〜Ｂ７をそれぞれ、室温（約２５℃）で、６００ｍＡ（１ＩｔｍＡ）の定電流で電池電圧が４．１０Ｖになるまで充電し、ついで、４．１０Ｖの定電圧で電流値が１０ｍＡになるまで充電した。この後、６００ｍＡ（１ＩｔｍＡ）の放電電流で電池電圧が２．７５Ｖになるまで放電させて、放電時間から１サイクル目の放電容量を測定した。同様にこのような充放電サイクルを繰り返して、３００サイクル目の放電容量（ｍＡｈ）を求めた後、１サイクル目の放電容量に対する３００サイクル目の放電容量の割合（％）を求めると、下記の表３に示すような結果となった。
【００３４】
【表３】

【００３５】
上記表３の結果から明らかなように、電池Ｂ１，Ｂ２のようにセパレータの空孔率が小さいセパレータを用いた場合、１サイクル目の放電容量に対する３００サイクル目の放電容量の割合が小さくなることが分かる。これはセパレータの空孔率が４０％未満の場合、圧縮率が大きい部分でのイオン透過性が著しく小さくなり、充放電サイクルによる劣化が早くなったためである。また、空孔率が６０％の電池Ｂ７では充放電サイクルの途中でショートが発生した。これは空孔率が大き過ぎるために、セパレータの強度が低く、サイクル時の耐ショート性が低いためである。このことから、空孔率が４０％以上で５５％以下のセパレータを用いるのが好ましいことが分かる。
【００３６】
６．集電タブの幅の検討
ついで、負極集電タブの幅（以下ではタブ幅（ｍｍ）という）と、電極体中心部空間の幅との関係について検討した。なお、電極体中心部空間の幅は、渦巻状電極体を加圧成形した際に電極体の中心部に形成される空間の幅であって、加圧成形時に極板に形成される折り目間の長さを意味する。具体的には、図２においては、負極板の巻始め部（先端部）（ｔ₀）から数えて第１の折り目（ｔ₁）から第２の折り目（ｔ₂）までの長さ（ｗ＝ｔ₂−ｔ₁）となる。
【００３７】
ここで、図２に示すように、負極集電体２１の先端部（ｔ₀）から第１の折り目（ｔ₁）までの部分（ｗ＝２０ｍｍ）には、上述と同様に負極活物質スラリーを塗布せずに、負極集電体２１が露出した露出部が形成されるようにした。そして、この部分に厚みが８０μｍで、タブ幅を１０ｍｍ〜２０ｍｍ（即ち、電極体中心部空間の幅ｗに対して５０％〜１００％）まで変化させた負極集電タブ２３を超音波溶接して負極板２０を作製した。この場合、負極板２０の巻始め部（ｔ₀）から第１の折り目（ｔ₁）までの長さｗと、第１の折り目（ｔ₁）から第２の折り目（ｔ₂）までの長さｗがほぼ等しくなるようにした。即ち、負極板２０を折りたたんだ際に負極板２０の巻始め部（ｔ₀）と第２の折り目（ｔ₂）が重なるようにした。
【００３８】
そして、タブ幅が５０％（電極体の中心部の負極板の極板幅（ｗ＝ｔ₂−ｔ₁）に対して：以下では同様である）の負極板を用いて作製したものを電池Ｃ１とした。この場合、セパレータとしては空孔率が４０％で、加圧前の厚みが３０μｍのものを使用した。同様に、タブ幅が５５％の負極板を用いて作製したものを電池Ｃ２とし、タブ幅が６０％の負極板を用いて作製したものを電池Ｃ３とし、タブ幅が６５％の負極板を用いて作製したものを電池Ｃ４とし、タブ幅が７０％の負極板を用いて作製したものを電池Ｃ５とし、タブ幅が７５％の負極板を用いて作製したものを電池Ｃ６とした。また、タブ幅が８０％の負極板を用いて作製したものを電池Ｃ７とし、タブ幅が８５％の負極板を用いて作製したものを電池Ｃ８とし、タブ幅が９０％の負極板を用いて作製したものを電池Ｃ９とし、タブ幅が９５％の負極板を用いて作製したものを電池Ｃ１０とし、タブ幅が１００％の負極板を用いて作製したものを電池Ｃ１１とした。
【００３９】
ついで、上述のように作製した各電池Ｃ１〜Ｃ１１をそれぞれ、室温（約２５℃）で、６００ｍＡ（１ＩｔｍＡ）の定電流で電池電圧が４．１０Ｖになるまで充電し、ついで、４．１０Ｖの定電圧で電流値が１０ｍＡになるまで充電した。この後、６００ｍＡ（１ＩｔｍＡ）の放電電流で電池電圧が２．７５Ｖになるまで放電させて、放電時間から放電容量（ｍＡｈ）を求めると、下記の表４に示すような結果となった。
【００４０】
【表４】

【００４１】
上記表４の結果から明らかなように、電池Ｃ１，Ｃ２のようにタブ幅の割合が６０％未満であると放電容量が低く、電池Ｃ３〜Ｃ１１のようにタブ幅の割合が６０％以上になると放電容量が向上することが分かる。これは、タブ幅の割合が６０％未満であると、電極体中心部空間の幅（第１の折り目ｔ₁から第２の折り目ｔ₂までの長さｗ）に対する負極集電体の幅（タブ幅）が相対的に小さくなるため、この部分の極板厚みが他の部分の極板厚みよりも厚くなる。このため、この部分に配置されるセパレータが強く圧縮されるためである。
【００４２】
一方、タブ幅の割合が６０％以上になると、電極体中心部空間の幅（第１の折り目ｔ₁から第２の折り目ｔ₂までの長さｗ）に対する負極集電体の幅（タブ幅）が相対的に大きくなるため、この部分の極板厚みが他の部分の極板厚みと均一化されるようになる。このため、この部分に配置されるセパレータは均一に圧縮されるようになって、渦巻状電極体を加圧成形したときにセパレータの厚みの場所による変動が小さくなる。この結果、加圧成形後の電極体内のセパレータの厚みを均一にすることが可能になったためと考えられる。
【００４３】
上述したように、本発明においては、渦巻状電極体を加圧成形した際に、セパレータの最大厚みに対する最小厚みの比率が７０％以上になるようにしているので、放電容量が向上するとともに、充放電サイクル特性も向上する。また、セパレータの加圧力をできるだけ小さくした方が、セパレータ潰れを防止できるようになるが、電極体の加圧力が弱すぎると、スプリングバック現象が生じて、この電極体を角形外装缶へ挿入することが困難になる。このため、本発明においては、セパレータの最大厚みに対する最小厚みの比率を９５％以下になるような加圧力にして、スプリングバック現象が生じないようにしている。
【００４４】
なお、上述した実施の形態においては、図４（ａ）に示すように、負極板４０ａの巻始め部（ｔ₀）から第１の折り目（ｔ₁）までの長さｗ₁（＝ｔ₁−ｔ₀）と、第１の折り目（ｔ₁）から第２の折り目（ｔ₂）までの長さｗ₂（＝ｔ₂−ｔ₁）がほぼ等しく（ｗ＝ｗ₁＝ｗ₂）なるようにし、この部分に負極集電タブ４３を超音波溶接するようにした。しかしながら、これらの長さｗ₁，ｗ₂は必ずしも等しい長さである必要はない。例えば、図４（ｂ）に示すように、負極板４０ｂの巻始め部（ｔ₀）から第１の折り目（ｔ₁）までの長さｗ₁（＝ｔ₁−ｔ₀）を、第１の折り目（ｔ₁）から第２の折り目（ｔ₂）までの長さｗ₂（＝ｔ₂−ｔ₁）よりも短くなるようにしてもよい。
【００４５】
この場合、巻始め部（ｔ₀）から第１の折り目（ｔ₁）までの長さｗ₁（＝ｔ₁−ｔ₀）の部分に取り付けられる負極集電タブ４３の幅がｗ₁（＝ｔ₁−ｔ₀）よりも大きく、かつ、ｗ₂（＝ｔ₂−ｔ₁）の６０％以上であってもよい。これは、負極集電体４１の厚み（例えば１５μｍ）は負極集電タブ４３の厚み（例えば８０μｍ）に比べて充分に小さいために、負極集電体４１の厚みはセパレータの圧縮厚みに影響を与えないからである。
結局、負極集電タブのタブ幅が、電極体の中心部に配置される負極板の第１の折り目から第２の折り目までの長さ（図２、図４におけるｔ₂−ｔ₁間の距離）、即ち、電極体中心部空間の幅に対して６０％以上であり、且つ負極集電タブと負極集電体が電気的に十分接合されていればよい。
【００４６】
また、上述した実施の形態においては、電極体の中心部に配置される極板を負極板としてこの負極板に負極集電タブを設け、電極体の最外周に配置される極板を正極板としてこの部分の正極集電体を外装缶の表面に接触するように露出させる例について説明したが、この関係が逆になるようにしてもよい。即ち、電極体の中心部に配置される極板を正極板としてこの正極板に正極集電タブを設け、電極体の最外周に配置される極板を負極板としてこの部分の負極集電体を外装缶の表面に接触するように露出させる。そして、正極集電タブのタブ幅の割合、即ち、電極体の中心部に配置される正極板の幅（即ち、渦巻状電極体を加圧成形した際に形成れる正極板の第１の折り目から第２の折り目までの長さ）に対する割合が６０％以上になるようにすればよい。
【００４７】
また、上述した実施の形態においては、負極活物質として天然黒鉛を用いた例について説明したが、天然黒鉛以外に、リチウムイオンを吸蔵・放出し得るカーボン系材料、例えば、カーボンブラック、コークス、ガラス状炭素、炭素繊維、またはこれらの焼成体、人造黒鉛、非晶質酸化物等の公知のものを用いてもよい。また、リチウム、リチウムを主体とする合金を負極に用いるときには、正極に本発明を適用できるのは勿論である。
【００４８】
また、上述した実施の形態においては、正極活物質にコバルト酸リチウムを用いた例について説明したが、コバルト酸リチウム以外に、ニッケル酸リチウム、マンガン酸リチウム等のリチウム含有遷移金属複合酸化物あるいは二酸化マンガン（ＭｎＯ₂）、五酸化バナジウム、五酸化ニオブなどの金属酸化物、二硫化チタン、二硫化モリブデンなどの金属カルコゲン化物等も使用できる。
また、上述した実施の形態においては、セパレータとしてポリエチレン製の微多孔膜を用いた例について説明したが、セパレータとしては、ポリプロピレン製の微多孔膜など、ポリオレフィン系の微多孔膜も使用できる。さらに、ポリオレフィン系の繊維を使用した不織布セパレータも使用できる。
【図面の簡単な説明】
【図１】本発明の正極板を示す図であり、図１（ａ）は正面図を示し、図１（ｂ）は断面図を示す。
【図２】本発明の負極板を示す図であり、図２（ａ）は正面図を示し、図２（ｂ）は断面図を示す。
【図３】本発明のセパレータを示す図であり、図３（ａ）は正面図を示し、図３（ｂ）は断面図を示す。
【図４】負極板に負極集電タブを溶接した状態を説明する図であり、図４（ａ）は、巻始め部から第１の折り目までの長さと、第１の折り目から第２の折り目までの長さが等しい負極板に負極集電タブを溶接した状態を示す正面図であり、図４（ｂ）は、巻始め部から第１の折り目までの長を、第１の折り目から第２の折り目までの長さよりも短くした負極板に負極集電タブを溶接した状態を示す正面図である。
【符号の説明】
１０…正極板、１１…正極集電体、１１ａ…露出部、１２…正極活物質層、２０…負極板、２１…負極集電体、２２…負極活物質層、２３…負極集電タブ、３０…セパレータ、４０ａ，４０ｂ…負極板、４１…負極集電体、４２…負極活物質層、４３…負極集電タブ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous electrolyte secondary battery including a spiral electrode body wound in a spiral shape with a separator interposed between a positive electrode plate and a negative electrode plate.
[0002]
[Prior art]
In recent years, non-aqueous electrolyte secondary batteries that are small and light and have a high capacity have come to be used as power sources for portable electronic and communication devices such as small video cameras, mobile phones, and notebook computers. Such a non-aqueous electrolyte secondary battery uses, as a negative electrode active material, graphite capable of occluding and releasing lithium ions, and a lithium-containing cobalt oxide (LiCoO). ₂ ), Lithium-containing manganese oxide (LiMn) ₂ O _Four ) Or the like as a positive electrode active material, and a battery configured using an electrolyte solution in which a lithium salt is dissolved as a solute in an organic solvent.
[0003]
By the way, in a device in which this type of non-aqueous electrolyte secondary battery is used, since the space for storing the battery is often rectangular, a rectangular battery formed by accommodating a power generation element in a rectangular outer can is used. Often done. Such a prismatic battery is generally manufactured as follows. That is, first, a positive electrode mixture containing a positive electrode active material is applied to a positive electrode current collector (positive electrode core) to produce a positive electrode plate, and a negative electrode current collector (negative electrode core) contains a negative electrode active material. A negative electrode mixture is applied to produce a negative electrode plate. Then, after making these positive electrode plates and negative electrode plates oppose each other via a separator, they are spirally wound to form a spiral electrode body. After forming this spiral electrode body into a flat spiral electrode body, the spiral electrode body is housed in a flat rectangular outer can, and a non-aqueous electrolyte is injected to form a non-aqueous electrolyte secondary battery. .
[0004]
In this case, it is necessary to pressurize the flat spiral electrode body so that it can be inserted into the flat rectangular outer can. At this pressurization, the separator is compressed, and the thickness becomes thinner than the thickness before pressurization. . However, the stronger the spiral electrode body is pressed, the more the separator is compressed, the ion permeability of the separator is lowered, and the discharge characteristics and high-rate discharge characteristics of the battery are lowered, and the charge / discharge cycle life is shortened. For this reason, it is necessary to control the degree of compression of the separator to be within a predetermined range.
[0005]
[Problems to be solved by the invention]
However, there is a high demand for high capacity for this type of nonaqueous electrolyte secondary battery, and in order to meet this demand, the amount of active material filled in batteries of the same size tends to increase. For this reason, unless the spiral electrode body is pressurized with a stronger force than before, it has become difficult to insert the electrode body into the rectangular outer can. However, the battery produced by strongly pressing the spiral electrode body in this way has a problem that the battery capacity is low and the charge / discharge cycle life is shortened for a large amount of the active material filled.
[0006]
Therefore, in order to investigate the cause, the electrode body was taken out from the battery produced by strongly pressing the spiral electrode body, and the taken-out electrode body was disassembled and investigated. As a result, the following became clear. That is, when the thickness of the separator taken out from the decomposed electrode body was measured, it was found that the thickness of the separator was greatly different depending on the location, where it was thinned and not. This is thought to be because when the spiral electrode body was pressed strongly, there were parts where the separator was strongly compressed and parts that were not, due to the positional relationship of the current collector tab, electrode plate, and separator inside the electrode body. It is done. Then, it was found that the thickness of the separator was reduced in the portion where the separator was strongly compressed, and the thickness of the separator was not so thin in the portion where the compression was not strong.
[0007]
Thus, when the thickness of a separator changes with places, the electrode reaction at the time of charging / discharging becomes non-uniform | heterogenous, and there exists a possibility of causing the fall of battery capacity. This is because, in the portion where the separator is strongly compressed and thinned, the porosity is reduced and the ion permeability is lowered and the electrode reaction becomes non-uniform compared to the portion where the separator is not. . Further, the use of the active material in the charge / discharge reaction is also biased, and the electrode reaction is concentrated on a specific portion, so that deterioration due to the charge / discharge cycle is accelerated.
Therefore, the present invention has been made to solve the above problems, and provides a non-aqueous electrolyte secondary battery having a high capacity and excellent charge / discharge cycle characteristics so that the entire electrode body can react uniformly. It is intended to do.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, a nonaqueous electrolyte secondary battery of the present invention includes a spiral electrode body wound in a spiral shape with a separator interposed between a positive electrode plate and a negative electrode plate in a rectangular outer can. The spiral electrode body is formed into a flat electrode body by pressure molding. And there are parts where current collecting tabs are provided in the pressurizing direction or the number of stacked electrode plates is large, The separator interposed between the positive electrode plate and the negative electrode plate of the flat electrode body is The current collecting tab or electrode plate provided in the pressing direction is strongly compressed at the part where the number of stacked layers is large, and the thickness becomes the thinnest part. It is characterized by being molded so as to be 70% or more and 95% or less with respect to the thickness of the thickest part.
[0009]
Here, inside the spiral electrode body wound in a spiral shape with a separator interposed between the positive electrode plate and the negative electrode plate, there are places where current collecting tabs are provided and places where the number of stacked electrode plates is large. When such a spiral electrode body is pressure-molded into a flat electrode body, a portion where the separator is strongly compressed and a portion where the separator is not formed are generated. For this reason, the separators located above and below the current collecting tab and the separators where the number of stacked electrode plates is large are more strongly compressed than the separators in other places.
[0010]
In this case, the pressing force at the time of pressure molding is concentrated in the place where the current collecting tab is provided or where the number of stacked electrode plates is large, and the pressing force is not concentrated so much in the separators in other places. Will not be crushed. Therefore, when the maximum thickness and the minimum thickness of the separator were measured by changing the applied pressure in various ways, an experimental result was obtained that the minimum thickness varied with the applied pressure and the maximum thickness did not change greatly. . Therefore, the ratio of the minimum thickness to the maximum thickness of the separator can be controlled by controlling the pressing force when the spiral electrode body is pressure-molded.
[0011]
Accordingly, as a result of various experiments, when the ratio of the minimum thickness to the maximum thickness of the separator is 70% or more when the spiral electrode body is pressure-molded, the discharge capacity is improved and the charge / discharge cycle characteristics are increased. Was also found to improve. On the other hand, the separator can be prevented from being crushed by reducing the pressure applied to the separator as much as possible. However, it has been found that if the pressure applied to the electrode body is too weak, a springback phenomenon occurs, making it difficult to insert the electrode body into the rectangular outer can.
[0012]
Therefore, as a result of obtaining a pressing force that does not cause the springback phenomenon, it was found that if the pressing force is such that the ratio of the minimum thickness to the maximum thickness of the separator is 95% or less, the springback phenomenon does not occur. From the above, it can be said that the separator interposed in the flat electrode body needs to be molded such that the thickness of the thinnest part is 70% or more and 95% or less with respect to the thickness of the thickest part. .
[0013]
In this case, when a separator having a porosity of less than 40% is used, the charge / discharge cycle characteristics are deteriorated. In addition, in the case of a separator having a porosity of more than 55%, short-circuiting is likely to occur, and there is a problem in terms of quality. It became clear that there was. In general, the smaller the porosity of the separator, the lower the permeability of ions. When the separator has a porosity of less than 40%, the ion permeability at the portion where the compressibility is large is significantly reduced, and the electrode reaction is reduced. It is considered that the deterioration due to the charge / discharge cycle was accelerated due to nonuniformity. Further, a separator having a porosity exceeding 55% has a problem in terms of quality and safety because the strength of the polymer film is lowered and short circuit is likely to occur. For this reason, it is preferable to use a separator having a porosity of 40% or more and 55% or less.
[0014]
It is also arranged at the center of the electrode body Negative When the negative electrode current collecting tab is provided on the electrode plate, the width of the negative electrode current collecting tab is equal to the width of the electrode body central space (from the first fold to the second fold when counted from the end of the winding start portion of the negative electrode plate). The width of the negative electrode current collecting tab is relatively small with respect to the width of the electrode body central space. For this reason, since the electrode plate thickness of this part becomes thicker than the electrode plate thickness of another part, the separator arrange | positioned at this part will be compressed strongly.
[0015]
On the other hand, when the width of the negative electrode current collecting tab is 60% or more with respect to the width of the electrode body central space, the width of the negative current collector tab is relatively large with respect to the width of the electrode body central space. For this reason, the electrode plate thickness of this part becomes uniform with the electrode plate thickness of another part. Thereby, since the separator arrange | positioned in this part comes to be compressed uniformly, when the spiral electrode body is pressure-molded, the fluctuation | variation by the place of the thickness of a separator becomes small. As a result, the thickness of the separator in the electrode body after pressure molding can be made uniform.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Next, an embodiment of the present invention will be described with reference to FIGS. 1 to 3. However, the present invention is not limited to this embodiment, and may be changed as appropriate without departing from the scope of the present invention. It is possible to implement. FIG. 1 is a view showing a positive electrode plate of the present invention, FIG. 1 (a) shows a front view, and FIG. 1 (b) shows a cross-sectional view. Moreover, FIG. 2 is a figure which shows the negative electrode plate of this invention, FIG. 2 (a) shows a front view, FIG.2 (b) shows sectional drawing. Furthermore, FIG. 3 is a figure which shows the separator of this invention, FIG. 3 (a) shows a front view, FIG.3 (b) shows sectional drawing.
[0017]
1. Production of positive electrode plate
First, lithium cobalt oxide (LiCoO as a positive electrode active material) ₂ ) The powder was 85 parts by mass, and 5 parts by mass of artificial graphite powder and carbon black as a conductive agent were mixed to prepare a positive electrode mixture. The positive electrode material mixture and a binder solution obtained by dissolving 5% by mass of a binder made of polyvinylidene fluoride (PVdF) in an organic solvent made of N-methyl-2-pyrrolidone (NMP) are used as a solid mass. A positive electrode active material slurry was prepared by mixing and kneading so that the ratio was 95: 5.
[0018]
This positive electrode active material slurry is applied to both surfaces of a positive electrode current collector (for example, an aluminum foil or aluminum alloy foil having a length of 324 mm, a width of 37.5 mm, and a thickness of 20 μm) 11 to form a positive electrode active material layer 12. After the formation, this positive electrode active material layer 12 was dried. Then, the dry positive electrode plate was rolled with the roll press machine, and the positive electrode plate 10 of the predetermined filling density was produced. In addition, the positive electrode current collector 11 is exposed without applying the positive electrode active material slurry to one surface of a portion about 50 mm from the rear end portion (the portion that becomes the winding end portion of the spiral electrode body) of the positive electrode current collector 11. The exposed portion 11a was formed. Further, the positive electrode current collector 11 is exposed without applying the positive electrode active material slurry to both surfaces of a portion about 25 mm from the rear end.
[0019]
2. Production of negative electrode
On the other hand, natural graphite as an anode active material (Lc value is 1000%, d ₀₀₂ (With a value of 3.356 mm and an average particle size of 20 μm) In 95 parts by mass of powder, a binder composed of polyvinylidene fluoride (PVdF) is added to an organic solvent composed of N-methyl-2-pyrrolidone (NMP). The binder solution dissolved in mass% was mixed and kneaded so that the mass ratio of the solid content was 95: 5 to prepare a negative electrode active material slurry. The obtained negative electrode active material slurry was applied to both surfaces of a negative electrode current collector (for example, a copper foil having a length of 320 mm, a width of 39.5 mm, and a thickness of 15 μm) to form a negative electrode active material layer 22. Thereafter, the negative electrode active material layer 22 was dried.
[0020]
Thereafter, the dried negative electrode plate was rolled by a roll press to produce a negative electrode plate 20 having a predetermined packing density. It should be noted that the negative electrode current collector was not applied to both surfaces of a portion 20 mm (w = 20 mm) from the tip of the negative electrode current collector 21 (the portion that becomes the winding start portion of the spiral electrode body). 21 is exposed. Then, a nickel metal negative electrode current collecting tab 23 having a width of 5 mm and a thickness of 80 μm was ultrasonically welded to this portion. Note that the alternate long and short dash line shown in FIG. 2A represents a crease that occurs during the pressurization after the negative electrode plate 20 is spirally wound, and the tip t ₀ 1st crease t at approximately equal interval w from ₁ , Second crease t ₂ A crease is formed so that
[0021]
3. Battery fabrication
(1) Production of electrode body
Next, a microporous polyethylene separator 30 was sandwiched between the positive electrode plate 10 and the negative electrode plate 20 manufactured as described above, and the spiral electrode body was manufactured by winding in a spiral shape. In this case, a separator having a width of 41.5 mm, a porosity of 40%, and a thickness before pressing of 30 μm was used as the separator 30. Then, when winding in a spiral shape, the positive electrode plate 10 is wound later than the negative electrode plate 20. As a result, the tip end portion (winding start portion) of the positive electrode plate 10 is displaced by about 20 mm from the tip end portion (winding start portion) of the negative electrode plate 20. That is, the first bent portion t of the negative electrode plate 20 ₁ The positive electrode plate 10 is prevented from being positioned at (see FIG. 2). In addition, the positive electrode plate 10 is configured such that the exposed portion 11a of the positive electrode current collector 11 is positioned on the outermost peripheral portion of the electrode body, and then brought into contact with the inner surface of the outer can so as to be electrically connected.
[0022]
Then, it pressed from both sides of the spiral electrode body produced as mentioned above, and it shape | molded so that an electrode body could be inserted from the opening part of a square-shaped exterior can. At this time, when the spiral electrode body is pressure-molded by controlling the applied pressure, the separator 30 where the number of stacked electrode plates is large or where the negative electrode current collecting tab 23 is located is strongly compressed. The thickness of this part was measured, and the thickness of this part was minimized, and the other parts were not compressed so strongly, and it was found that the thickness was not much different from the thickness before compression (30 μm). This means that the minimum thickness can be controlled with little change in the maximum thickness of the separator 30 only by changing the applied pressure. Therefore, the electrode bodies a1 to a9 were produced by changing the pressure applied when the spiral electrode body was pressure-formed and applying pressure to the separator thickness as shown in Table 1 below.
[0023]
[Table 1]

[0024]
(2) Production of non-aqueous electrolyte secondary battery
Next, an aluminum square outer can having a thickness of 0.5 mm, a height of 48 mm, a width of 29 mm, and a thickness of 5 mm was prepared. Note that the size, material, and thickness of the rectangular outer can are not limited to this, and for example, iron or iron alloy may be used. Then, each of these electrode bodies a1 to a9 is inserted from the opening of each rectangular outer can, and the exposed portion 11a where the positive electrode current collector 11 of each electrode body a1 to a9 is exposed is brought into contact with the inner surface of the outer can. I let you.
[0025]
Further, a terminal plate (not shown, electrically insulated from the sealing plate) provided with a negative electrode current collecting tab 23 extending from the negative electrode plate 20 of each electrode body a1 to a9 at the inner bottom portion of the sealing body (not shown). Resistance welded). A negative electrode terminal (not shown) that is electrically insulated from the sealing plate is provided on the sealing plate on the surface of the battery, and this terminal plate is electrically connected to the negative electrode terminal through a through hole formed in the sealing plate. is doing. Next, a non-aqueous electrolyte (ethylene carbonate (EC) and diethyl carbonate (DEC) mixed in an equal volume ratio in each of these rectangular outer cans is mixed with lithium hexafluorophosphate (LiPF). ₆ The non-aqueous solution in which 1 mol / liter of the electrolyte made of) was dissolved was injected. Then, the opening part of the armored can was sealed liquid-tightly with the sealing body, and nonaqueous electrolyte secondary battery A1-A9 with a design capacity of 600 mAh was produced, respectively.
[0026]
The nonaqueous electrolyte secondary battery produced using the electrode body a1 is designated as battery A1, the nonaqueous electrolyte secondary battery produced using the electrode body a2 is designated as battery A2, and the nonaqueous electrolyte produced using the electrode body a3. The electrolyte secondary battery is designated as battery A3, the nonaqueous electrolyte secondary battery produced using electrode body a4 is designated as battery A4, the nonaqueous electrolyte secondary battery produced using electrode body a5 is designated as battery A5, and electrode body a6. A non-aqueous electrolyte secondary battery manufactured using the battery is A6, a non-aqueous electrolyte secondary battery manufactured using the electrode body a7 is battery A7, and a non-aqueous electrolyte secondary battery manufactured using the electrode body a8 is used. The nonaqueous electrolyte secondary battery manufactured using the electrode body a9 as the battery A8 was referred to as the battery A9.
[0027]
4). Load characteristics test of non-aqueous electrolyte secondary battery
Next, each of the batteries A1 to A9 produced as described above is a constant current of 600 mA (1 ItmA, 1 It is a current value for discharging the battery capacity in 1 hour) at room temperature (about 25 ° C.), and the battery voltage is 4. The battery was charged until it reached 10 V, and then charged at a constant voltage of 4.10 V until the current value reached 10 mA. Thereafter, the battery was discharged at a discharge current of 600 mA (1 ItmA) until the battery voltage reached 2.75 V, and the discharge capacity (mAh) was determined from the discharge time. The results shown in Table 2 below were obtained.
[0028]
[Table 2]

[0029]
As is clear from the results in Table 2 above, when the ratio of the minimum thickness to the maximum thickness of the separator 30 in the electrode body is less than 70% as in the batteries A6, A7, A8, A9, the discharge capacity is reduced. I understand that. On the other hand, the discharge capacity increases when the ratio of the minimum thickness to the maximum thickness of the separator 30 in the electrode body is 70% or more as in the batteries A1, A2, A3, A4, and A5. If the ratio of the minimum thickness to the maximum thickness of the separator 30 in the electrode body is 97% as in the battery A1, a springback phenomenon occurs during the production of the electrode body, and this electrode body is formed into a rectangular outer can. It was difficult to insert inside.
[0030]
Therefore, in order to prevent the springback phenomenon from occurring and to allow the electrode body to be easily inserted into the rectangular outer can, the ratio of the minimum thickness to the maximum thickness of the separator 30 in the electrode body is 95% or less. It is necessary to be. For these reasons, the ratio of the minimum thickness to the maximum thickness of the separator 30 in the electrode body needs to be 70% or more, and the ratio of the minimum thickness to the maximum thickness of the separator 30 in the electrode body is 95% or less. You can say that you need to.
[0031]
5. Examination of separator porosity
Next, the porosity of the separator used was examined. Here, a spiral electrode body was produced in the same manner as described above, using a separator in which the porosity was changed between about 30% and about 60% at intervals of about 5%. At this time, a non-aqueous electrolyte secondary battery having a design capacity of 600 mAh as described above is formed by applying pressure so that the ratio of the minimum thickness to the maximum thickness of the separator 30 in the electrode body is 70% or more. B1 to B7 were prepared.
[0032]
Here, a battery B1 is prepared using a separator having a porosity of 30%, a battery B2 is manufactured using a separator having a porosity of 35%, and a separator having a porosity of 40% is used. A battery B3 was prepared using a separator having a porosity of 45%, a battery B4 was manufactured using a separator having a porosity of 50%, and a battery B5 was prepared using a separator having a porosity of 50%. A battery B6 was prepared using a separator having a rate of 55%, and a battery B7 was prepared using a separator having a porosity of 60%.
[0033]
Next, each of the batteries B1 to B7 manufactured as described above was charged at room temperature (about 25 ° C.) with a constant current of 600 mA (1 ItmA) until the battery voltage reached 4.10 V, and then 4.10 V The battery was charged at a constant voltage until the current value reached 10 mA. Thereafter, the battery was discharged at a discharge current of 600 mA (1 ItmA) until the battery voltage reached 2.75 V, and the discharge capacity at the first cycle was measured from the discharge time. Similarly, after repeating such charge / discharge cycles to determine the discharge capacity (mAh) at the 300th cycle, the ratio (%) of the discharge capacity at the 300th cycle to the discharge capacity at the first cycle is calculated as follows. The results shown in Table 3 were obtained.
[0034]
[Table 3]

[0035]
As is clear from the results in Table 3 above, when a separator having a low porosity such as batteries B1 and B2 is used, the ratio of the discharge capacity at the 300th cycle to the discharge capacity at the first cycle is reduced. I understand. This is because when the porosity of the separator is less than 40%, the ion permeability at the portion where the compressibility is large is remarkably reduced, and deterioration due to the charge / discharge cycle is accelerated. Further, in the battery B7 having a porosity of 60%, a short circuit occurred during the charge / discharge cycle. This is because the porosity is too high, the strength of the separator is low, and the short circuit resistance during cycling is low. This shows that it is preferable to use a separator having a porosity of 40% or more and 55% or less.
[0036]
6). Examining current collector tab width
Next, the relationship between the width of the negative electrode current collector tab (hereinafter referred to as tab width (mm)) and the width of the electrode body central space was examined. The width of the electrode body central space is the width of the space formed at the center of the electrode body when the spiral electrode body is pressure-molded, and between the creases formed on the electrode plate during pressure molding. Means the length of Specifically, in FIG. 2, the winding start portion (tip portion) (t ₀ ) From the first fold (t ₁ ) To the second crease (t ₂ ) Length (w = t) ₂ -T ₁ )
[0037]
Here, as shown in FIG. 2, the tip of the negative electrode current collector 21 (t ₀ ) To the first fold (t ₁ ) (W = 20 mm), the exposed portion where the negative electrode current collector 21 was exposed was formed without applying the negative electrode active material slurry in the same manner as described above. Then, the negative electrode current collector tab 23 having a thickness of 80 μm and a tab width of 10 mm to 20 mm (that is, 50% to 100% with respect to the width w of the electrode body central space) is ultrasonically welded to this portion. Thus, the negative electrode plate 20 was produced. In this case, the winding start portion (t ₀ ) To the first fold (t ₁ ) And the first fold (t ₁ ) To the second crease (t ₂ ) To be substantially equal. That is, when the negative electrode plate 20 is folded, the winding start portion (t ₀ ) And the second crease (t ₂ ) Overlapped.
[0038]
The tab width is 50% (the electrode plate width of the negative electrode plate at the center of the electrode body (w = t ₂ -T ₁ The battery C1 was prepared using the negative electrode plate of (the same as below). In this case, a separator having a porosity of 40% and a thickness before pressing of 30 μm was used. Similarly, a battery prepared using a negative electrode plate having a tab width of 55% is referred to as battery C2, a battery prepared using a negative electrode plate having a tab width of 60% is referred to as battery C3, and a negative electrode plate having a tab width of 65% is prepared. A battery C4 was prepared using the negative electrode plate having a tab width of 70%, and a battery C6 was prepared using a negative electrode plate having a tab width of 75%. Also, a battery C7 was prepared using a negative electrode plate having a tab width of 80%, a battery C8 was prepared using a negative electrode plate having a tab width of 85%, and a negative electrode plate having a tab width of 90% was used. A battery C9 was prepared using a negative electrode plate having a tab width of 95%, a battery C10 was prepared using a negative electrode plate having a tab width of 100%, and a battery C11 was prepared.
[0039]
Next, each of the batteries C1 to C11 manufactured as described above was charged at room temperature (about 25 ° C.) with a constant current of 600 mA (1 ItmA) until the battery voltage reached 4.10 V, and then 4.10 V The battery was charged at a constant voltage until the current value reached 10 mA. Thereafter, the battery was discharged at a discharge current of 600 mA (1 ItmA) until the battery voltage reached 2.75 V, and the discharge capacity (mAh) was determined from the discharge time. The results shown in Table 4 below were obtained.
[0040]
[Table 4]

[0041]
As is clear from the results of Table 4 above, the discharge capacity is low when the tab width ratio is less than 60% as in the batteries C1 and C2, and the tab width ratio is 60% or more as in the batteries C3 to C11. It can be seen that the discharge capacity is improved. When the tab width ratio is less than 60%, the width of the electrode body central space (the first fold t ₁ To second crease t ₂ Since the width (tab width) of the negative electrode current collector is relatively small with respect to the length w), the thickness of the electrode plate in this portion is larger than the thickness of the electrode plate in other portions. For this reason, the separator disposed in this portion is strongly compressed.
[0042]
On the other hand, when the tab width ratio is 60% or more, the width of the electrode body central space (the first fold t ₁ To second crease t ₂ Since the width (tab width) of the negative electrode current collector is relatively large with respect to the length w), the thickness of the electrode plate in this portion is made uniform with the thickness of the electrode plate in other portions. For this reason, the separator arrange | positioned in this part comes to be compressed uniformly, and the fluctuation | variation by the place of the thickness of a separator becomes small when a spiral electrode body is pressure-molded. As a result, it is considered that the thickness of the separator in the electrode body after pressure molding can be made uniform.
[0043]
As described above, in the present invention, when the spiral electrode body is pressure-molded, the ratio of the minimum thickness to the maximum thickness of the separator is 70% or more, so that the discharge capacity is improved, Charge / discharge cycle characteristics are also improved. In addition, it is possible to prevent the separator from being crushed by reducing the applied pressure of the separator as much as possible. However, if the applied pressure of the electrode body is too weak, a springback phenomenon occurs, and this electrode body is inserted into the rectangular outer can. It becomes difficult. For this reason, in the present invention, the applied pressure is such that the ratio of the minimum thickness to the maximum thickness of the separator is 95% or less so that the springback phenomenon does not occur.
[0044]
In the above-described embodiment, as shown in FIG. 4A, the winding start portion (t ₀ ) To the first fold (t ₁ ) Length w ₁ (= T ₁ -T ₀ ) And the first fold (t ₁ ) To the second crease (t ₂ ) Length w ₂ (= T ₂ -T ₁ ) Is almost equal (w = w ₁ = W ₂ The negative electrode current collecting tab 43 was ultrasonically welded to this portion. However, these lengths w ₁ , W ₂ Need not be of equal length. For example, as shown in FIG. 4B, the winding start portion (t ₀ ) To the first fold (t ₁ ) Length w ₁ (= T ₁ -T ₀ ) To the first fold (t ₁ ) To the second crease (t ₂ ) Length w ₂ (= T ₂ -T ₁ ) May be shorter.
[0045]
In this case, the winding start part (t ₀ ) To the first fold (t ₁ ) Length w ₁ (= T ₁ -T ₀ ) The width of the negative electrode current collecting tab 43 attached to the portion of w) is w ₁ (= T ₁ -T ₀ ) Larger than and w ₂ (= T ₂ -T ₁ ) May be 60% or more. This is because the thickness of the negative electrode current collector 41 (for example, 15 μm) is sufficiently smaller than the thickness of the negative electrode current collector tab 43 (for example, 80 μm), and thus the thickness of the negative electrode current collector 41 affects the compression thickness of the separator. It is because it does not give.
Eventually, the tab width of the negative electrode current collector tab is the length from the first fold line to the second fold line of the negative electrode plate arranged at the center of the electrode body (t in FIGS. 2 and 4). ₂ -T ₁ Distance), that is, 60% or more with respect to the width of the electrode body central space, and the negative electrode current collector tab and the negative electrode current collector may be sufficiently electrically connected.
[0046]
In the above-described embodiment, the electrode plate arranged at the center of the electrode body is used as a negative electrode plate, and a negative electrode current collecting tab is provided on the negative electrode plate. As described above, the example in which the positive electrode current collector in this portion is exposed so as to be in contact with the surface of the outer can, but this relationship may be reversed. That is, the electrode plate arranged at the center of the electrode body is used as a positive electrode plate, and a positive electrode current collecting tab is provided on the positive electrode plate, and the electrode plate arranged on the outermost periphery of the electrode body is used as the negative electrode plate. Is exposed to contact the surface of the outer can. And the ratio of the tab width of the positive electrode current collector tab, that is, the width of the positive electrode plate arranged at the center of the electrode body (that is, the first fold of the positive electrode plate formed when the spiral electrode body is pressure-formed) To the second crease) may be 60% or more.
[0047]
In the embodiment described above, an example in which natural graphite is used as the negative electrode active material has been described. However, in addition to natural graphite, a carbon-based material capable of occluding and releasing lithium ions, such as carbon black, coke, and glass. Known carbon, carbon fiber, or a fired body thereof, artificial graphite, amorphous oxide, or the like may be used. In addition, when lithium or a lithium-based alloy is used for the negative electrode, it is needless to say that the present invention can be applied to the positive electrode.
[0048]
In the above-described embodiment, the example in which lithium cobaltate is used as the positive electrode active material has been described. In addition to lithium cobaltate, lithium-containing transition metal composite oxides such as lithium nickelate and lithium manganate, or dioxide dioxide Manganese (MnO ₂ ), Metal oxides such as vanadium pentoxide and niobium pentoxide, metal chalcogenides such as titanium disulfide and molybdenum disulfide, and the like can also be used.
In the above-described embodiment, an example in which a polyethylene microporous film is used as a separator has been described. However, a polyolefin microporous film such as a polypropylene microporous film can also be used as the separator. Furthermore, a nonwoven fabric separator using polyolefin fibers can also be used.
[Brief description of the drawings]
FIG. 1 shows a positive electrode plate of the present invention, FIG. 1 (a) shows a front view, and FIG. 1 (b) shows a cross-sectional view.
FIG. 2 is a view showing a negative electrode plate of the present invention, FIG. 2 (a) shows a front view, and FIG. 2 (b) shows a cross-sectional view.
FIG. 3 is a view showing a separator of the present invention, FIG. 3 (a) shows a front view, and FIG. 3 (b) shows a cross-sectional view.
FIG. 4 is a diagram for explaining a state in which a negative electrode current collecting tab is welded to a negative electrode plate. FIG. 4 (a) shows a length from a winding start portion to a first fold, and a first fold to a second fold. FIG. 4B is a front view showing a state in which the negative electrode current collecting tab is welded to the negative electrode plate having the same length to the crease, and FIG. 4B shows the length from the winding start portion to the first crease from the first crease. It is a front view which shows the state which welded the negative electrode current collection tab to the negative electrode plate shortened rather than the length to a 2nd crease | fold.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Positive electrode plate, 11 ... Positive electrode collector, 11a ... Exposed part, 12 ... Positive electrode active material layer, 20 ... Negative electrode plate, 21 ... Negative electrode collector, 22 ... Negative electrode active material layer, 23 ... Negative electrode current collection tab, 30 ... Separator, 40a, 40b ... Negative electrode plate, 41 ... Negative electrode current collector, 42 ... Negative electrode active material layer, 43 ... Negative electrode current collector tab

Claims

A non-aqueous electrolyte secondary battery including a spiral electrode body wound in a spiral shape with a separator interposed between a positive electrode plate and a negative electrode plate,
The spiral electrode body is formed into a flat electrode body by pressure molding, and there is a portion where a current collecting tab is provided in the pressing direction or the number of stacked electrode plates is large,
The separator interposed between the positive electrode plate and the negative electrode plate of the flat electrode body is strongly compressed at a portion where the number of stacked current collecting tabs and electrode plates provided in the pressurizing direction is large and has the largest thickness. As it becomes thin,
A non-aqueous electrolyte secondary battery, wherein the thickness of the thinnest portion is 70% or more and 95% or less with respect to the thickness of the thickest portion.

The nonaqueous electrolyte secondary battery according to claim 1, wherein the separator has a porosity of 40% or more and 55% or less.

A negative electrode current collecting tab is disposed in the center of the flat electrode body,
The width of the negative electrode current collector tabs, claim 1, characterized in that the first fold counted from the winding start end portion of the negative electrode in the flat-shaped electrode body is the second least 60% of the length to the fold Or the non-aqueous electrolyte secondary battery of Claim 2 .