JP4719982B2

JP4719982B2 - Non-aqueous electrolyte secondary battery and manufacturing method thereof

Info

Publication number: JP4719982B2
Application number: JP2001021156A
Authority: JP
Inventors: 哲志梶川; 義幸尾崎; 敬介大森
Original assignee: Panasonic Corp; Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Corp; Panasonic Holdings Corp
Priority date: 2001-01-30
Filing date: 2001-01-30
Publication date: 2011-07-06
Anticipated expiration: 2021-01-30
Also published as: JP2002231316A

Description

【０００１】
【発明の属する技術分野】
本発明は非水電解液二次電池とその製造方法に関するものである。
【０００２】
【従来の技術】
近年、携帯電話やノート型パソコン、ビデオカムコーダーなどのポータブル電子機器の駆動用電源としての小型電池をリードする電池系としてリチウム二次電池が急速な成長を遂げている。また、環境問題、エネルギー問題等の観点から電気自動車用あるいは夜間電力貯蔵用の大型電池の開発も盛んに行われ、より高容量、高エネルギー密度で、充放電サイクル特性に優れ、しかも経済性に優れるリチウム二次電池の実現への要望が強い。リチウム二次電池は高い作動電圧と高エネルギー密度を有する点が他の電池系に比較して優れているが、非水電解液を用いるため、水溶液電解液と比較し、イオン伝導度が低く電流が取り出しにくいという欠点を有していた。これを改良するものとして、薄いアルミニウム箔芯体上にＬｉＣｏＯ₂、ＬｉＮｉＯ₂あるいはＬｉＭｎ₂Ｏ₄などのリチウム含有複合酸化物からなる薄い合剤層を形成した正極と薄い銅箔芯体上に炭素材料からなる薄い合剤層を形成した負極とをセパレータを介して捲回もしくは積層することにより、大面積化し電流を取り出しやすくした電池が実用に供されている。但し、ニカド電池やニッケル水素電池で実用化されている電動工具用途やハイブリッド電気自動車用途で求められている様な数秒から数十秒間のパルスによる大電流放電を可能とする高出力密度の電池としては、不充分な点があり、ほとんど実用化には至っていない。
【０００３】
一方、リチウム二次電池は充放電を行うと、リチウム含有複合酸化物からなる正極はリチウムを放出、吸蔵、逆に負極はリチウムを吸蔵、放出するが、その際、正、負極いずれも結晶構造へのリチウムの挿入、脱離によって結晶構造が膨張あるいは収縮し、合剤層の膨張、収縮を引き起こす。従って、充放電を繰り返すことにより、合剤層自体すなわち活物質間の導電性が低下し、集電体と合剤層の密着性が低下し、また極板の多孔度の減少により電解液が極板外に押し出され、局部的な電解液の枯渇が起こり、その結果、内部抵抗が上昇するため、サイクル寿命特性、高率放電特性や出力が低下するという課題を有していた。
【０００４】
特に負極である炭素材料が黒鉛の場合にはリチウムの未挿入時は格子面間隔（ｄ００２）が０．３３５ｎｍであるが、Ｃ₆Ｌｉまでのリチウム挿入を行うと０．３７２ｎｍまでカーボンの層間が広がり、膨張度合いが大きい。また格子面間隔（ｄ００２）が０．３７２ｎｍ以上である低結晶性炭素材料については、結晶構造へのリチウムの挿入、脱離によって結晶構造が膨張、収縮することがほとんどないが、ＬｉＣｏＯ₂、ＬｉＮｉＯ₂、ＬｉＭｎ₂Ｏ₄などの正極活物質が膨張、収縮を繰り返すために電極群の加圧力が次第に低下し、正、負極とセパレータの密着性やさらには正、負極の合剤層の導電性も低下する。
【０００５】
そこで特開２０００―１３８０６２号公報では、サイクル寿命特性の改良を目的として、リチウムマンガン複合酸化物を正極に用い、非晶質炭素材料に繊維状または針状の形状をした気相法で作製した炭素繊維または導電性セラミック繊維を導電剤として含有させた負極が報告されている。しかしながら、サイクル後の内部抵抗、出力や高率放電特性についての記載はなく、さらに、電極群の加圧力についての規定もなされていない。
【０００６】
他方、特開２０００−２１４４１号公報には、正極活物質にスピネル構造のマンガン酸リチウム、負極に炭素材を用い、非水電解液中のリチウム塩にＬｉＰＦ₆、ＬｉＢＦ₄の少なくとも１種を用い、前記リチウム塩の濃度を０．４〜０．８ｍｏｌ／ｌとして、高温寿命特性を向上させるという記載がある。
【０００７】
【発明が解決しようとする課題】
しかしながら、従来の技術では低レート電流の充放電による高温サイクル寿命特性を改良することができても、高出力充放電では、サイクル経過後には電池の内部抵抗が上昇し、出力や高率放電特性が低下するという課題を有している。
【０００８】
本発明はこのような従来の課題を解決するもので、初期から高出力密度で、充放電サイクル特性にも優れ、しかもサイクル経過による内部抵抗の上昇が少ないため、サイクル経過後も高出力や高率放電特性を維持することが可能な非水電解液二次電池を提供することを目的とする。
【０００９】
【課題を解決するための手段】
本発明は上記目的を達成するため、リチウム含有遷移金属酸化物からなる正極とリチウムを吸蔵、放出し得る炭素材料からなる負極とセパレータと非水電解液とを備え、前記炭素材料は０．３７２ｎｍ〜０．４００ｎｍの格子面間隔（ｄ００２）を有する低結晶性炭素材料であり、前記負極には前記炭素材料の質量に対して２〜６質量％の前記炭素材料より高い導電性を有する導電剤が添加され、前記非水電解液中のリチウム塩の濃度が０．７〜０．９ｍｏｌ／ｌであることを特徴とする非水電解液二次電池とするものである。
【００１０】
本発明によれば、上記炭素材料は電池の充放電に従い、リチウムの挿入、脱離が繰り返されても、黒鉛にリチウムを挿入した場合の格子面間隔（ｄ００２）０．３７２ｎｍ以上の結晶構造を最初から有するために膨張、収縮を繰り返すことなく、良好なサイクル寿命特性が得られる。
【００１１】
また、黒鉛のような層構造の発達した炭素材料では、リチウムはインターカレーション反応によって黒鉛層間にインターカレートされ、ステージ構造と呼ばれるきわめて異方性が大きい状態でリチウムがイオン状態で格納される。一方、格子面間隔（ｄ００２）０．３７２ｎｍ以上の結晶構造をもつ低結晶性炭素材料は、リチウムがインターカレーション反応による層間への格納よりも炭素結晶構造の空隙部分へ格納される割合が圧倒的に多く、リチウムは等方的に均一に挿入されるため、負極の活物質内部へのリチウムイオンの移動が速い。このため、大電流での放電でも放電電圧が低下しにくいため、高出力可能な非水電解液二次電池を得ることができる。
【００１２】
加えて、電解液中のリチウム塩濃度を０．７〜０．９ｍｏｌ／ｌとすることにより、電解液の粘度を低下させ、電解液のイオン伝導度を上げることにより、リチウムイオンの移動をより速くすることが可能である。
【００１３】
ここで、低結晶性炭素材料は黒鉛に比べて真密度が低いため活物質自体の導電性は黒鉛よりも低くなるが、炭素材料の質量に対して前記炭素材料より高い導電性を有する導電剤を２〜６質量％含有することにより、負極の活物質間の導電性が高くなり、サイクル経過後にも負極の活物質間の導電性が保持され、内部抵抗の上昇が少なく、初期と同等の高出力や高率放電特性が得られる。
【００１４】
【発明の実施の形態】
本発明の請求項１に記載の発明は、リチウム含有遷移金属酸化物からなる正極とリチウムを吸蔵、放出し得る炭素材料からなる負極とセパレータと非水電解液とを備え、前記炭素材料は０．３７２ｎｍ〜０．４００ｎｍの格子面間隔（ｄ００２）を有する低結晶性炭素材料であり、前記負極には前記炭素材料の質量に対して２〜６質量％の前記炭素材料より高い導電性を有する導電剤が添加され、前記非水電解液中のリチウム塩の濃度が０．７〜０．９ｍｏｌ／ｌであることを特徴とする非水電解液二次電池であり、初期から高出力密度であり、しかもリチウムの挿入、脱離による膨張、収縮がなく、充放電を繰り返しても内部抵抗の上昇が少ないため、サイクル経過後も高出力、高率放電特性を維持することができるという作用を有する。
【００１５】
格子面間隔（ｄ００２）の値は炭素材料粉末のＸ線回折法によって、ＣｕＫα線をターゲットとした場合、２θ＝２３〜２７度付近に００２回折線が得られる。高純度ケイ素粉末を内部標準試料として加え、角度を補正することによってより精密な値が得られる。炭素材料については上限が特に限定されるものではないが、０．４００ｎｍまでであれば高容量、高エネルギー密度の点から好ましい。
【００１６】
導電剤の添加量についても上限が特に限定されるものではないが、６質量％以下であれば高容量、高エネルギー密度の点から好ましく、２質量％未満では効果が少なすぎる。
【００１７】
非水電解液については、式ＬｉＰＦ₆、ＬｉＢＦ₄、ＬｉＣｌＯ₄、ＬｉＣＦ₃ＳＯ₃、ＬｉＮ（ＣＦ₃ＳＯ₂）₂などで示される無機塩の一種もしくは二種以上をプロピレンカーボネート、エチレンカーボネート、ビニレンカーボネート、γ−ブチロラクトンなどの環状エステル、ジエチルカーボネート、ジメチルカーボネート、エチルメチルカーボネート、メチルプロピオネート、エチルプロピオネートなどの直鎖状エステル、１，２−ジメトキシエタン、テトラヒドロフラン、２−メチルテトラヒドロフラン、グライム、１，３−ジオキソランなどのエーテルからなる群から選ばれた一種もしくは二種以上の溶媒に溶解させたものを用いることができる。
【００１８】
また、リチウム塩濃度については、通常は伝導度の高い１．０〜１．２ｍｏｌ／ｌとされており、０．７〜０．９ｍｏｌ／ｌでは伝導度が若干低下するが、電解液の粘度は低い方がリチウムイオンの移動が速いため、数秒〜数十秒間のパルス電流充放電では放電電圧が高くなり、出力も高い。但し、０．７ｍｏｌ／ｌ未満では、リチウムイオン量が少なすぎるので好ましくない。
【００１９】
請求項２に記載の発明は、捲回前の前記セパレータの空孔率が４０〜７０％であり、かつ捲回後の厚みが捲回前の９０％以下であるとともに捲回後のセパレータの空孔率が１４〜６７％であることを特徴とする請求項１に記載の非水電解液二次電池であり、充放電により極板の膨張、収縮が起こる場合においても、正極および負極とセパレータとの密着性を保つことができるという作用を有する。
【００２０】
通常、正極、負極とも極板の作製法としては、活物質と耐有機電解質性で結着性の強固なものでＳＢＲ（スチレンーブタジエンーゴム）、ＣＭＣ（カルボキシメチルセルロース）、水ガラス、ポリエチレン、あるいはポリ四フッ化エチレン、ポリフッ化ビニリデンなどのフッ素樹脂系から選ばれる結着剤とを水もしくはＮ−メチル−２−ピロリドンのような有機溶媒で分散、練合してペーストを作製し、薄い箔などの金属芯体に塗工、乾燥、圧延し、成型体を短冊状に裁断する。
【００２１】
また、活物質の形状は、球形、鱗片状、塊状など様々ではあるが、数μｍ〜数十μｍの大きさであり、圧延ロールの表面の凹凸が０．１μｍ以下であっても、圧延後の極板表面には１〜３μｍ程度の凹凸は残るため、この凹凸を持つ極板表面とセパレータを捲回後さらには充放電の繰り返し後にも密着性を保つためには、４０〜７０％の空孔率をもつセパレータを捲回後の厚みが捲回前の９０％以下になるとともに空孔率が１４〜６７％になるまで極板表面に押し付けて捲回することが好ましい。
【００２２】
捲回前のセパレータの空孔率が４０％未満であれば、電解液の保液性が低下し、充放電繰り返し後には電解液の枯渇が起こり、７０％を超えると空間が多すぎるため強度、特に突き刺し強度が弱くなってしまう。また捲回後のセパレータの厚みが捲回前の９０％より大きいと正極の膨張、収縮によって電極群の加圧力が低下し、正、負極とセパレータの密着性が低下して正、負極の合剤層の導電性が低下するため出力が低下し、捲回後のセパレータの空孔率が６７％を超えると充放電の繰り返し後に密着性が弱くなり極板自体の導電性が低下し、１４％未満であればセパレータが保持できる電解液量が少なくなり出力や高率放電特性が低下してしまう。
【００２３】
なお捲回後のセパレータの空孔率は、セパレータの厚みの変化率より換算されるものであり、また捲回によりセパレータの合成樹脂部分が圧縮されることはなく、空孔部のみが圧縮されるものである。
【００２４】
【実施例】
以下、本発明の実施の形態について図１および図２を用いて説明する。
【００２５】
（実施例１）
図１に本発明の実施例に用いた円筒形電池の断面図を示す。
【００２６】
図１において、負極３はＸ線広角回折法による格子面間隔（ｄ００２）が０．３８ｎｍの低結晶性炭素（呉羽化学工業株式会社製カーボトロンＰＳ（Ｆ））を活物質とし、結着剤としてのポリフッ化ビニリデン（呉羽化学工業株式会社製ＫＦポリマーＬ１３２０）と導電剤としての気相成長炭素繊維（昭和電工株式会社製）を活物質／結着剤／導電剤＝８８／８／４（質量比）で混合し、Ｎ−メチル−２−ピロリドンに分散したペーストを、厚み１４μｍの銅箔芯体の両面に塗工、乾燥、圧延し、所定の寸法に裁断して、銅製の負極リード板４を超音波接合した。なお、気相成長炭素繊維は繊維径が０．１〜０．５μｍであり、繊維長が１〜３０μｍであるものを使用した。
【００２７】
正極１は活物質としてのＬｉＭｎ₂Ｏ₄に、結着剤としてのポリフッ化ビニリデンと導電剤としてのアセチレンブラックを活物質／結着剤／導電剤＝９４／４／２（質量比）で混合し、Ｎ−メチル−２−ピロリドンに分散したペーストを、厚み２０μｍのアルミニウム箔芯体の両面に塗工、乾燥、圧延した後、所定の寸法に裁断し、アルミニウム製の正極リード板２を超音波接合したものである。上記正極活物質のＬｉＭｎ₂Ｏ₄は、電解二酸化マンガン（ＭｎＯ₂）と炭酸リチウム（Ｌｉ₂ＣＯ₃）とをＬｉ／Ｍｎ＝１／２となるように混合し、８００℃２０時間、大気中で焼成したものを用いた。
【００２８】
正極としてはアルミニウム箔芯体を含めて９０μｍの厚みとし、負極は銅箔芯体を含めて９５μｍの厚みとした。
【００２９】
５はポリエチレン製フィルムからなる厚み４０μｍで空孔率４５％のセパレータであり、正極１と負極３とをセパレータ５を介して渦巻き状に捲回して電極群を構成する。電極群の構成方法としては、直径４ｍｍでセパレータを２枚分挟めるようなスリットを入れてある金属製の巻芯を用いて、この巻芯に巻き付けるように捲回し、正極１、負極３、セパレータ５を１周分以上巻いた後に、正、負極、セパレータのもう片方の端部に巻芯とは反対方向に引っ張り張力を加えた。引っ張り張力は正極に４００ｇ／ｃｍ、負極に４００ｇ／ｃｍ、セパレータに２００ｇ／ｃｍを加えた。巻芯と正極、負極、セパレータの外側には直径１００ｍｍの樹脂製のローラーを配置し、電極群に５００ｇ／ｃｍの加圧力を加えた。捲回後には金属製の巻芯は引き抜いた。この電極群のセパレータの厚みの変化率は８５％、空孔率は３５．３％であった。
【００３０】
この電極群の上下それぞれにポリエチレン製の上部絶縁板６、下部絶縁板７を配してステンレス製のケース８に挿入し、正極リード板２を封口板１０に、負極リード板４をケース８の底部にそれぞれ溶接した後、電解液を注入し、ガスケット９を介して電池を封口して完成電池とする。この電池の寸法は直径３３ｍｍ、高さ６１．５ｍｍである。１１は電池の正極端子であり、負極端子は電池ケース８がこれを兼ねている。
【００３１】
電解液はエチレンカーボネート（ＥＣ）とエチルメチルカーボネート（ＥＭＣ）とを体積比で１：３に混合した混合溶媒に、ＬｉＰＦ₆を０．８ｍｏｌ／ｌの濃度に溶解したものを用いた。この電池を電池Ａとする。
【００３２】
比較例として、上記実施例の電池作製において、Ｘ線広角回折法による格子面間隔（ｄ００２）が０．３３８ｎｍのカーボンを活物質としたものを電池Ｂ、負極に気相成長炭素繊維などの導電剤を含まないものを電池Ｃ、電解液中の電解質塩ＬｉＰＦ₆の濃度を１．２ｍｏｌ／ｌとしたものを電池Ｄ、捲回前後のセパレータ厚みの変化率を９５％としたものを電池Ｅ、捲回前のセパレータ空孔率を３０％としたものを電池Ｆとする。これらを下記表１にまとめる。
【００３３】
【表１】

【００３４】
電池の初期放電容量の評価試験は、２５℃において、充放電電流を４００ｍＡ（０．２ＣｍＡ相当）とし、充電終止電圧４．３Ｖ、放電終止電圧２．５Ｖの条件とした。また、直流内部抵抗の評価試験は、２５℃において、完全放電状態から０．２ＣｍＡで３時間充電した状態（６０％の充電状態）から４０００ｍＡ（２ＣｍＡ相当）、８０００ｍＡ（４ＣｍＡ相当）、１２０００ｍＡ（６ＣｍＡ相当）、１６０００ｍＡ（８ＣｍＡ相当）、２００００ｍＡ（１０ＣｍＡ相当）の電流値で各１０秒間、放電を行い、その時の１０秒目の電圧値と電流値との関係（直線の傾き）から算出した。さらに電圧値と電流値との関係式（近似式）において、２．５Ｖまで外挿した時の電流値（Ａ）×２．５Ｖ＝出力（Ｗ）とした。その結果を表２に示す。また、２５℃において、充放電電流を２０００ｍＡ（１ＣｍＡ相当）とし、充電終止電圧４．２Ｖ、放電終止電圧２．５Ｖの条件として、電池Ａ〜Ｆの初期放電容量を１００としたときのサイクルに伴う放電容量の維持率を図２に示す。さらに１０００サイクル後に直流内部抵抗の試験を再度行い、電池Ａ〜Ｆの初期出力に対する比を表３に示す。
【００３５】
【表２】

【００３６】
表２において、電池Ａは格子面間隔（ｄ００２）が０．３８ｎｍであり、気相成長炭素繊維が添加され、ＬｉＰＦ₆濃度が０．８ｍｏｌ／ｌであるために、初期の出力が高い。電池Ｂについては、格子面間隔（ｄ００２）が０．３３８ｎｍであるので、格子面間隔（ｄ００２）が０．３８ｎｍのものに比べると、初回の充放電時に起こる不可逆容量が少ないために高容量な電池であるが、炭素材料の結晶構造上、リチウムの移動が遅くなるので出力が低い。電池Ｃは負極に導電剤を含まないため負極の導電性が低く、電池Ｄは電解液の粘度が高いためリチウムの移動が遅いので、電池Ａと比較すると出力が低い。電池Ｆは捲回前のセパレータの空孔率が３０％と低く、電解液を保持できる量が少ないため、出力が低下している。
【００３７】
図２において、電池Ｂ、Ｆはサイクルに伴い、放電容量が減少するが、電池Ａは減少が少ないことが明白である。
【００３８】
【表３】

【００３９】
表３において、電池Ｂは負極の炭素材料が格子面間隔（ｄ００２）０．３３８ｎｍであるので、炭素材料が膨張、収縮を繰り返すために電解液が枯渇し、放電による電圧低下が大きくなるため出力が低くなり、電池Ｆも空孔率３０％のセパレータでは電解液を保持できる量が少ないため、１０００サイクル後にはさらに内部抵抗が上昇して出力が低下する。電池Ｃは、負極に気相成長炭素繊維がないために負極の導電性が低下する。電池Ｅは、捲回後のセパレータの厚みが捲回前の厚みの９５％であるため、初期の出力は他の比較例電池よりも高かったが、正極の膨張、収縮によって電極群の加圧力が低下し、正、負極とセパレータの密着性が低下して正、負極の合剤層の導電性が低下するため出力が低下する。
【００４０】
したがって、格子面間隔（ｄ００２）０．３８ｎｍの低結晶性炭素材料に気相成長炭素繊維が添加されており、電解液のＬｉＰＦ₆濃度が０．８ｍｏｌ／ｌで、かつセパレータの空孔率が４５％であっても、充放電サイクル経過後の出力低下を少なくするためには、捲回後のセパレータ厚みを捲回前のセパレータ厚みの９０％以下とし、かつ捲回後のセパレータの空孔率を１４〜６７％にすることが好ましい。
【００４１】
本実施例では円筒形電池を用いて説明したが、電池形状については、電極を楕円体状の芯に巻き付けるように捲回し、その芯と電極群を角形の電池ケースに収納した角形電池を用いても、同様の効果が得られる。
【００４２】
また、正極活物質をＬｉＣｏＯ₂またはＬｉＮｉＯ₂とした場合でもほぼ同様の効果が得られる。
【００４３】
また、本発明は正、負極をセパレータを介して捲回する電池であるが、薄型の電極を複数枚積層して加圧力を加え、セパレータ厚みを圧縮して角形の電池ケースに収納した角形電池を用いても、同様の効果が得られることは容易に推測できる。
【００４４】
【発明の効果】
以上のように本発明によれば、初期から高い出力が得られ、充放電サイクル特性に優れ、充放電サイクル経過後にも高出力、高率放電特性を維持できる非水電解液二次電池を得ることができる。
【図面の簡単な説明】
【図１】本発明の円筒形電池の断面図
【図２】本発明の実施例電池と比較例電池のサイクル特性図
【符号の説明】
１正極
２正極リード板
３負極
４負極リード板
５セパレータ
６上部絶縁板
７下部絶縁板
８ケース
９ガスケット
１０封口板
１１正極端子[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a non-aqueous electrolyte secondary battery and a method for manufacturing the same.
[0002]
[Prior art]
In recent years, lithium secondary batteries have rapidly grown as battery systems that lead small batteries as power sources for driving portable electronic devices such as mobile phones, notebook computers, and video camcorders. In addition, large batteries for electric vehicles or nighttime power storage have been actively developed from the viewpoint of environmental problems, energy problems, etc., with higher capacity, higher energy density, excellent charge / discharge cycle characteristics, and economical efficiency. There is a strong demand for the realization of excellent lithium secondary batteries. Lithium secondary batteries are superior to other battery systems in that they have a high operating voltage and high energy density, but use non-aqueous electrolytes, so they have lower ionic conductivity and less current than aqueous electrolytes. Had the disadvantage of being difficult to remove. To improve this, a positive electrode in which a thin mixture layer made of a lithium-containing composite oxide such as LiCoO ₂ , LiNiO ₂ or LiMn ₂ O ₄ is formed on a thin aluminum foil core and carbon on a thin copper foil core. Batteries that are large in area and easy to extract current by winding or laminating a negative electrode formed with a thin material mixture layer made of materials through a separator have been put to practical use. However, as a battery with a high output density that enables a large current discharge by a pulse of several seconds to several tens of seconds as required for a power tool application or a hybrid electric vehicle application that is put to practical use in a nickel-cadmium battery or a nickel metal hydride battery. Has insufficient points and has hardly been put into practical use.
[0003]
On the other hand, when a lithium secondary battery is charged and discharged, the positive electrode made of a lithium-containing composite oxide releases and occludes lithium, and conversely, the negative electrode occludes and releases lithium. The crystal structure expands or contracts due to the insertion and desorption of lithium into the material, causing the mixture layer to expand and contract. Therefore, by repeating charge and discharge, the mixture layer itself, that is, the conductivity between the active materials is lowered, the adhesion between the current collector and the mixture layer is lowered, and the electrolyte solution is reduced by reducing the porosity of the electrode plate. As a result of being pushed out of the electrode plate and causing local depletion of the electrolyte, the internal resistance is increased. As a result, the cycle life characteristics, the high rate discharge characteristics, and the output are reduced.
[0004]
In particular, when the carbon material as the negative electrode is graphite, the lattice spacing (d002) is 0.335 nm when lithium is not inserted. However, when lithium insertion up to C ₆ Li is performed, the carbon interlayer increases to 0.372 nm. Spreads and expands greatly. In addition, for the low crystalline carbon material having a lattice spacing (d002) of 0.372 nm or more, the crystal structure hardly expands or contracts due to the insertion or desorption of lithium into the crystal structure, but LiCoO ₂ , LiNiO ₂ , because the positive electrode active material such as LiMn ₂ O ₄ repeatedly expands and contracts, the applied pressure of the electrode group gradually decreases, the adhesion between the positive and negative electrodes and the separator, and further the conductivity of the positive and negative electrode mixture layers Also decreases.
[0005]
Therefore, in Japanese Patent Application Laid-Open No. 2000-138062, for the purpose of improving cycle life characteristics, lithium manganese composite oxide was used for the positive electrode, and the amorphous carbon material was produced by a vapor phase method having a fibrous or needle shape. A negative electrode containing carbon fiber or conductive ceramic fiber as a conductive agent has been reported. However, there is no description of the internal resistance, output, and high rate discharge characteristics after the cycle, and further, there is no provision for the applied pressure of the electrode group.
[0006]
On the other hand, JP 2000-21441 uses a spinel-type lithium manganate as a positive electrode active material, a carbon material as a negative electrode, and uses at least one of LiPF ₆ and LiBF ₄ as a lithium salt in a non-aqueous electrolyte. There is a description that the concentration of the lithium salt is 0.4 to 0.8 mol / l to improve the high-temperature life characteristics.
[0007]
[Problems to be solved by the invention]
However, even though the conventional technology can improve the high-temperature cycle life characteristics due to low-rate current charging / discharging, the high-power charging / discharging increases the internal resistance of the battery after the cycle, and the output and high-rate discharging characteristics. Has the problem of decreasing.
[0008]
The present invention solves such a conventional problem. It has a high output density from the beginning, is excellent in charge / discharge cycle characteristics, and has a small increase in internal resistance due to the cycle. An object of the present invention is to provide a non-aqueous electrolyte secondary battery capable of maintaining rate discharge characteristics.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, the present invention comprises a positive electrode made of a lithium-containing transition metal oxide, a negative electrode made of a carbon material capable of occluding and releasing lithium, a separator, and a non-aqueous electrolyte, and the carbon material is 0.372 nm. A conductive agent having a low crystalline carbon material having a lattice spacing (d002) of ˜0.400 nm, and having a conductivity higher than that of the carbon material in an amount of 2 to 6% by mass relative to the mass of the carbon material in the negative electrode. Is added, and the concentration of the lithium salt in the non-aqueous electrolyte is 0.7 to 0.9 mol / l.
[0010]
According to the present invention, the carbon material has a crystal structure with a lattice spacing (d002) of 0.372 nm or more when lithium is inserted into graphite even when lithium insertion and desorption are repeated according to charge and discharge of the battery. Since it has from the beginning, good cycle life characteristics can be obtained without repeating expansion and contraction.
[0011]
In a carbon material with a layered structure such as graphite, lithium is intercalated between graphite layers by an intercalation reaction, and lithium is stored in an ionic state in a highly anisotropic state called a stage structure. . On the other hand, in the low crystalline carbon material having a crystal structure with a lattice spacing (d002) of 0.372 nm or more, the proportion of lithium stored in the void portion of the carbon crystal structure is overwhelming than that stored in the interlayer by the intercalation reaction. Because lithium is isotropically and uniformly inserted, lithium ions move quickly into the negative electrode active material. For this reason, since the discharge voltage does not easily decrease even when discharging with a large current, a non-aqueous electrolyte secondary battery capable of high output can be obtained.
[0012]
In addition, the lithium salt concentration in the electrolytic solution is 0.7 to 0.9 mol / l, thereby reducing the viscosity of the electrolytic solution and increasing the ionic conductivity of the electrolytic solution, thereby further increasing the movement of lithium ions. It can be fast.
[0013]
Here, since the low crystalline carbon material has a lower true density than graphite, the conductivity of the active material itself is lower than that of graphite. However, the conductive agent has higher conductivity than the carbon material with respect to the mass of the carbon material. 2-6 mass%, the conductivity between the active materials of the negative electrode is increased, the conductivity between the active materials of the negative electrode is maintained even after the cycle, and the increase in internal resistance is small, equivalent to the initial value. High output and high rate discharge characteristics can be obtained.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
The invention according to claim 1 of the present invention comprises a positive electrode made of a lithium-containing transition metal oxide, a negative electrode made of a carbon material capable of occluding and releasing lithium, a separator, and a non-aqueous electrolyte, wherein the carbon material is 0 A low crystalline carbon material having a lattice spacing (d002) of 372 nm to 0.400 nm, and the negative electrode has higher conductivity than 2 to 6 mass% of the carbon material with respect to the mass of the carbon material. A non-aqueous electrolyte secondary battery, wherein a conductive agent is added, and the concentration of the lithium salt in the non-aqueous electrolyte is 0.7 to 0.9 mol / l. In addition, there is no expansion or contraction due to insertion or extraction of lithium, and there is little increase in internal resistance even after repeated charging and discharging, so that high output and high rate discharge characteristics can be maintained even after cycling. Have.
[0015]
With respect to the value of the lattice spacing (d002), a 002 diffraction line is obtained in the vicinity of 2θ = 23 to 27 degrees by the X-ray diffraction method of the carbon material powder when the CuKα ray is targeted. By adding high purity silicon powder as an internal standard sample and correcting the angle, a more precise value can be obtained. The upper limit of the carbon material is not particularly limited, but a carbon material up to 0.400 nm is preferable from the viewpoint of high capacity and high energy density.
[0016]
The upper limit of the addition amount of the conductive agent is not particularly limited, but 6 mass% or less is preferable from the viewpoint of high capacity and high energy density, and if it is less than 2 mass%, the effect is too small.
[0017]
For non-aqueous electrolytes, one or more of inorganic salts represented by the formulas LiPF ₆ , LiBF ₄ , LiClO ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) _2, etc. are used as propylene carbonate, ethylene carbonate, vinylene. Cyclic esters such as carbonate and γ-butyrolactone, linear esters such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propionate and ethyl propionate, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, Those dissolved in one or two or more solvents selected from the group consisting of ethers such as glyme and 1,3-dioxolane can be used.
[0018]
The lithium salt concentration is usually 1.0 to 1.2 mol / l, which is high in conductivity, and the conductivity slightly decreases at 0.7 to 0.9 mol / l, but the viscosity of the electrolyte solution Is lower, the lithium ions move faster, so that the discharge voltage becomes higher and the output is high in pulse current charging / discharging for several seconds to several tens of seconds. However, less than 0.7 mol / l is not preferable because the amount of lithium ions is too small.
[0019]
In the invention according to claim 2, the porosity of the separator before winding is 40 to 70%, the thickness after winding is 90% or less before winding, and the separator after winding is 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the porosity is 14 to 67%, and even when the electrode plate expands or contracts due to charge and discharge, It has the effect | action that adhesiveness with a separator can be maintained.
[0020]
Usually, the positive electrode and the negative electrode are prepared by using an active material and an organic electrolyte resistant and strong binding material such as SBR (styrene-butadiene rubber), CMC (carboxymethylcellulose), water glass, polyethylene, Alternatively, a paste is prepared by dispersing and kneading a binder selected from a fluororesin system such as polytetrafluoroethylene and polyvinylidene fluoride with water or an organic solvent such as N-methyl-2-pyrrolidone. A metal core such as foil is coated, dried and rolled, and the molded body is cut into strips.
[0021]
Further, the active material has various shapes such as a spherical shape, a scale shape, and a lump shape. However, the active material has a size of several μm to several tens of μm, and even if the surface unevenness of the rolling roll is 0.1 μm or less, after rolling Since the unevenness of about 1 to 3 μm remains on the surface of the electrode plate, 40 to 70% of the electrode plate surface having this unevenness and 40% to 70% can be maintained after winding the separator and further after repeated charging and discharging. It is preferable to wind a separator having a porosity by pressing it against the surface of the electrode plate until the thickness after winding is 90% or less before winding and the porosity is 14 to 67%.
[0022]
If the porosity of the separator before winding is less than 40%, the liquid retainability of the electrolytic solution decreases, and after repeated charge and discharge, the electrolyte is depleted. In particular, the piercing strength is weakened. If the thickness of the separator after winding is greater than 90% before winding, the pressure applied to the electrode group decreases due to the expansion and contraction of the positive electrode, and the adhesion between the positive and negative electrodes and the separator decreases. Since the conductivity of the agent layer is reduced, the output is reduced. When the porosity of the separator after winding exceeds 67%, the adhesion becomes weak after repeated charging and discharging, and the conductivity of the electrode plate itself is reduced. If it is less than%, the amount of electrolyte solution that can be held by the separator decreases, and the output and high rate discharge characteristics deteriorate.
[0023]
Note that the porosity of the separator after winding is calculated from the rate of change of the thickness of the separator, and the synthetic resin portion of the separator is not compressed by winding, and only the pore portion is compressed. Is.
[0024]
【Example】
Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1 and 2.
[0025]
Example 1
FIG. 1 shows a cross-sectional view of a cylindrical battery used in an example of the present invention.
[0026]
In FIG. 1, the negative electrode 3 is composed of low crystalline carbon (Carbotron PS (F) manufactured by Kureha Chemical Industries, Ltd.) having a lattice spacing (d002) of 0.38 nm by an X-ray wide angle diffraction method as an active material. Polyvinylidene Fluoride (KF Polymer L1320 manufactured by Kureha Chemical Industry Co., Ltd.) and vapor grown carbon fiber (made by Showa Denko Co., Ltd.) as a conductive agent, active material / binder / conductive agent = 88/8/4 (mass) Ratio) and a paste dispersed in N-methyl-2-pyrrolidone is coated on both sides of a copper foil core having a thickness of 14 μm, dried, rolled, cut into a predetermined size, and a copper negative electrode lead plate 4 was ultrasonically bonded. The vapor grown carbon fiber used had a fiber diameter of 0.1 to 0.5 μm and a fiber length of 1 to 30 μm.
[0027]
In the positive electrode 1, LiMn ₂ O ₄ as an active material is mixed with polyvinylidene fluoride as a binder and acetylene black as a conductive agent in an active material / binder / conductive agent = 94/4/2 (mass ratio). Then, the paste dispersed in N-methyl-2-pyrrolidone is coated on both sides of an aluminum foil core having a thickness of 20 μm, dried and rolled, and then cut to a predetermined size to exceed the positive electrode lead plate 2 made of aluminum. It has been sonic bonded. LiMn ₂ O _{4 as the} positive electrode active material is obtained by mixing electrolytic manganese dioxide (MnO ₂ ) and lithium carbonate (Li ₂ CO ₃ ) so that Li / Mn = 1/2, and at 800 ° C. for 20 hours in the air. What was baked with was used.
[0028]
The positive electrode had a thickness of 90 μm including the aluminum foil core, and the negative electrode had a thickness of 95 μm including the copper foil core.
[0029]
5 is a separator made of a polyethylene film having a thickness of 40 μm and a porosity of 45%. The positive electrode 1 and the negative electrode 3 are wound in a spiral shape through the separator 5 to constitute an electrode group. As a method for forming the electrode group, a metal core having a slit of 4 mm in diameter and sandwiching two separators is used and wound so as to be wound around the core, and the positive electrode 1, the negative electrode 3, and the separator After winding 5 for one or more turns, tensile tension was applied to the other end of the positive electrode, the negative electrode, and the separator in the direction opposite to the core. The tensile tension was 400 g / cm for the positive electrode, 400 g / cm for the negative electrode, and 200 g / cm for the separator. A resin roller having a diameter of 100 mm was disposed outside the core, the positive electrode, the negative electrode, and the separator, and a pressure of 500 g / cm was applied to the electrode group. After winding, the metal core was pulled out. The change rate of the thickness of the separator of this electrode group was 85%, and the porosity was 35.3%.
[0030]
An upper insulating plate 6 and a lower insulating plate 7 made of polyethylene are arranged on the upper and lower sides of this electrode group, inserted into a stainless steel case 8, the positive electrode lead plate 2 is used as the sealing plate 10, and the negative electrode lead plate 4 is used as the case 8. After welding each to the bottom, an electrolyte solution is injected, and the battery is sealed through the gasket 9 to obtain a completed battery. This battery has a diameter of 33 mm and a height of 61.5 mm. Reference numeral 11 denotes a positive terminal of the battery, and the battery case 8 also serves as the negative terminal.
[0031]
The electrolytic solution used was a solution of LiPF ₆ dissolved at a concentration of 0.8 mol / l in a mixed solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1: 3. This battery is referred to as battery A.
[0032]
As a comparative example, in the production of the battery of the above example, the battery B was obtained by using carbon having a lattice plane distance (d002) of 0.338 nm by the X-ray wide angle diffraction method as the active material. Battery C without the agent, Battery D with the electrolyte salt LiPF ₆ concentration of 1.2 mol / l in the electrolyte, and Battery E with 95% change in separator thickness before and after winding A battery F having a separator porosity of 30% before winding is designated as battery F. These are summarized in Table 1 below.
[0033]
[Table 1]

[0034]
In the evaluation test of the initial discharge capacity of the battery, the charge / discharge current was set to 400 mA (equivalent to 0.2 CmA) at 25 ° C., and the charge end voltage was 4.3 V and the discharge end voltage was 2.5 V. In addition, the evaluation test of the DC internal resistance was performed at 25 ° C. from a state of charging at 0.2 CmA for 3 hours from a fully discharged state (60% charged state) to 4000 mA (equivalent to 2 CmA), 8000 mA (equivalent to 4 CmA), 12000 mA (6 CmA Equivalent), 16000 mA (equivalent to 8 CmA), and 20000 mA (equivalent to 10 CmA) for 10 seconds each, and was calculated from the relationship (straight line) between the voltage value and current value at the 10th second. Furthermore, in the relational expression (approximate expression) between the voltage value and the current value, the current value (A) × 2.5 V = output (W) when extrapolated to 2.5 V was used. The results are shown in Table 2. In addition, at 25 ° C., the charge / discharge current is 2000 mA (corresponding to 1 CmA), the charge end voltage is 4.2V, and the discharge end voltage is 2.5V. The accompanying discharge capacity retention rate is shown in FIG. Further, after 1000 cycles, the test of the DC internal resistance was performed again, and the ratio to the initial output of the batteries A to F is shown in Table 3.
[0035]
[Table 2]

[0036]
In Table 2, the cell A has a lattice spacing (d002) of 0.38 nm, a vapor-grown carbon fiber is added, and the LiPF ₆ concentration is 0.8 mol / l, so the initial output is high. For battery B, since the lattice spacing (d002) is 0.338 nm, the battery B has a high capacity because the irreversible capacity that occurs at the first charge / discharge is less than that of the lattice spacing (d002) of 0.38 nm. Although it is a battery, its output is low because of the slow movement of lithium due to the crystal structure of the carbon material. Battery C does not contain a conductive agent in the negative electrode, so the conductivity of the negative electrode is low, and battery D has a low output compared to battery A because the viscosity of the electrolyte is high and lithium moves slowly. Since the battery F has a low porosity of 30% before winding and the amount of electrolyte that can be retained is small, the output is low.
[0037]
In FIG. 2, it is clear that the batteries B and F have a reduced discharge capacity with the cycle, but the battery A has a small decrease.
[0038]
[Table 3]

[0039]
In Table 3, since the carbon material of the negative electrode of the battery B has a lattice spacing (d002) of 0.338 nm, the carbon material repeatedly expands and contracts, so that the electrolyte is depleted and the voltage drop due to the discharge increases. In the separator with the porosity of 30%, the battery F also has a small amount of electrolyte that can be retained, so that the internal resistance further increases and the output decreases after 1000 cycles. In the battery C, the conductivity of the negative electrode is reduced because the negative electrode has no vapor-grown carbon fiber. In the battery E, the separator output after winding was 95% of the thickness before winding, so the initial output was higher than that of the other comparative batteries. Decreases, the adhesion between the positive and negative electrodes and the separator decreases, and the conductivity of the positive and negative electrode mixture layers decreases, resulting in a decrease in output.
[0040]
Therefore, the vapor-grown carbon fiber is added to the low crystalline carbon material having a lattice spacing (d002) of 0.38 nm, the LiPF ₆ concentration of the electrolyte is 0.8 mol / l, and the porosity of the separator is Even if it is 45%, in order to reduce the output decrease after the charge / discharge cycle, the separator thickness after winding is 90% or less of the separator thickness before winding, and the separator pores after winding The rate is preferably 14 to 67%.
[0041]
In this embodiment, the cylindrical battery is used for explanation, but the battery shape is a rectangular battery in which the electrode is wound around an ellipsoidal core and the core and the electrode group are housed in a rectangular battery case. However, the same effect can be obtained.
[0042]
Even when the positive electrode active material is LiCoO ₂ or LiNiO ₂ , substantially the same effect can be obtained.
[0043]
Further, the present invention is a battery in which the positive and negative electrodes are wound through a separator, but a rectangular battery in which a plurality of thin electrodes are stacked and applied with pressure to compress the separator thickness and accommodated in a rectangular battery case. It can be easily estimated that the same effect can be obtained even when using.
[0044]
【The invention's effect】
As described above, according to the present invention, it is possible to obtain a non-aqueous electrolyte secondary battery that can obtain high output from the beginning, has excellent charge / discharge cycle characteristics, and can maintain high output and high rate discharge characteristics even after the charge / discharge cycle has elapsed. be able to.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a cylindrical battery of the present invention. FIG. 2 is a cycle characteristic diagram of an example battery and a comparative battery of the present invention.
DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Positive electrode lead plate 3 Negative electrode 4 Negative electrode lead plate 5 Separator 6 Upper insulating plate 7 Lower insulating plate 8 Case 9 Gasket 10 Sealing plate 11 Positive electrode terminal

Claims

A positive electrode made of a lithium-containing transition metal oxide, a negative electrode made of a carbon material capable of occluding and releasing lithium, a separator, and a nonaqueous electrolyte solution, the carbon material having a lattice spacing of 0.372 nm to 0.400 nm (d002 ), A vapor-grown carbon fiber having a conductivity higher than that of the carbon material of 2 to 6% by mass with respect to the mass of the carbon material is added as a conductive agent to the negative electrode, The non-aqueous electrolyte secondary battery, wherein the concentration of the lithium salt in the non-aqueous electrolyte is 0.7 to 0.9 mol / l.

The porosity of the separator before winding is 40 to 70%, the thickness after winding is 90% or less before winding, and the porosity of the separator after winding is 14 to 67%. The nonaqueous electrolyte secondary battery according to claim 1, wherein:

In an electrode plate group including a positive electrode and a negative electrode via a separator, while winding the positive electrode, the negative electrode, and one end of the separator while applying tensile tension, and applying pressure from the outside of the electrode group with a roller-shaped tool The method for producing a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein winding is performed.

The non-aqueous electrolyte according to claim 3, wherein a tensile tension to the positive electrode, the negative electrode, and the separator is 10 to 1000 g / cm, and a pressure from the outside of the electrode group is 100 to 1000 g / cm. A method for manufacturing a secondary battery.