JP2004127913A - Lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve charge and discharge cycle characteristics of a high energy-density lithium secondary battery, and improve or maintain discharge rate characteristics, low-temperature discharge characteristics, and safety (heat resistance). <P>SOLUTION: The lithium secondary battery is prepared by using a negative electrode wherein active material composed of a mixture consisting of an artificial graphite particle A that has been obtained by kneading and granulating the base material made by crushing a bulk mesophase pitch, a pitch, and/or a thermosetting resin in a softened state and by making it carbonized/graphitized, and a sherical graphite particle B of which circularity is large, is fixed on the copper core material. This enables to improve the charge and discharge cycle characteristics of the high energy-density lithium secondary battery, and at the same time to provide the battery that is superior in the discharge rate characteristics, the low-temperature discharge characteristics, and safety (heat resistance). <P>COPYRIGHT: (C)2004,JPO

Description

【０１０５】
ここに示した黒鉛粒子を用いてリチウム二次電池の負極作製を行う場合、いずれか１種を単独で負極活物質に用いることもできるし、２種以上を所定の比率で混合して負極活物質に用いることもできる。
【０１０６】
本発明者等は、これまでに種々の黒鉛粒子に関して検討してきた。その経験則によれば、銅芯材上に塗布した黒鉛合剤層をロールプレス等で圧延して、合剤密度が１．６ｇ／ｃｍ^３を超える高密度な負極を作製する場合、Ｄ、ＥおよびＦのメソフェーズ炭素に由来する特殊人造黒鉛粒子を単独で活物質として用いた場合には、所定の密度にまで高密度化が出来ない場合が多い。これは、これらの材料はいずれも、製造工程の炭素化・黒鉛化工程の前段階で、実質上、メソフェーズ粒子表層が不融化（緩い酸化処理）を受けているためと考えられる。すなわち、粒子表層は、黒鉛化のあまり進行していない非晶質に近い状態となっている。層状構造を持たない非晶質炭素は、黒鉛層状構造に特有の粒子間の静電反発（π電子の相互作用）が少ないため、滑り性に乏しくなる。
【０１０７】
また、本発明で用いる黒鉛粒子Ａ１およびＡ２の予備検討において、簡易的に銅芯材上に作製した塗膜に対して、圧縮試験を実施した。その結果、Ｄ、ＥおよびＦほどではないが、比較的高密度化が困難であるというデータが得られた。従って、黒鉛粒子Ａ１、Ａ２、Ｄ、ＥおよびＦを活物質として用いる場合には、これらに円形度の大きい球状黒鉛粒子Ｂ１ないしはＢ２を添加することが必須と考えられた。そこで、表２に示すような配合比率（重量比）の負極活物質ａ〜ｖ（合計２２種）に関して、検討を実施した。
【０１０８】
なお、黒鉛粒子Ａ１またはＡ２のみを単独で用いた場合、仮に高密度な負極を作製できたとしても、充放電サイクルの繰り返しに伴う粒子の極板からの脱落が比較的起こりやすく、サイクル特性が劣化し易いと考えられる。なぜなら、粒子が比較的硬いため、充放電に伴う負極合剤層の膨張・収縮の際に、応力を分散させることができない、と予想されるからである。
一方、球状黒鉛粒子Ｂ１またはＢ２を単独で用いた場合には、充放電サイクルの繰り返しに伴う容量劣化や、表面積が大きいことに起因する安全性の問題が生じるものと考えられる。
【０１０９】
【表２】

【０１１０】
（負極の作製）
１００重量部の負極活物質ａに、カルボキシメチルセルロース（ＣＭＣ）の１重量％水溶液を１００重量部および結着剤であるスチレンブタジエンゴム（ＳＢＲ）の水性ディスパージョンを加え、十分に混練して、合剤スラリを作製した。ここで、ＳＢＲの添加量に関しては、負極活物質ａの１００重量部に対する固形分（ゴム成分）の比率が２重量部となるように、その添加量を調整した。
【０１１１】
こうして作製したスラリを、銅箔（厚み１０μｍ）の両面に塗工機を用いて一定厚みに塗布し、１００℃の熱風で乾燥させ、その後、ロールプレスを用いて圧延した。ここでは、合剤層の密度（ＣＭＣ、ＳＢＲの重量も含めた値）が１．７ｇ／ｃｍ^３で、その厚みが７０μｍ（電極の厚みは約１５０μｍ）となるように調整した。そして、これを所定の大きさに裁断加工して、集電のためのニッケル製リードを取りつけて、負極ａとした。また、負極活物質ｂ〜ｖに関しても、すべて上記と同様の条件で、各負極活物質の符号に対応する負極ｂ〜ｖを作製した。
【０１１２】
（正極の作製）
本検討においては、正極の活物質として、Ｃｏ_３Ｏ_４とＬｉ_２ＣＯ_３との混合物を大気雰囲気下９５０℃で焼成後、粉砕・粒度調整して作製したＬｉＣｏＯ_２を使用した。正極板の作製に際しては、１００重量部の正極活物質に、導電材としてアセチレンブラック（ＡＢ）を３重量部加えて、乾式ミキサー内で十分に混合分散した後、結着剤としてのポリフッ化ビニリデン（ＰＶＤＦ）を５重量部添加し、溶剤のＮ−メチル−２−ピロリドン（ＮＭＰ）を適宜加えながら混練して、合剤スラリを作製した。
【０１１３】
こうして作製したスラリを、アルミニウム箔（厚み２０μｍ）の両面に塗工機を用いて一定厚みに塗布し、１００℃のドライエアで乾燥させ、ロールプレスを用いて圧延した。ここでは、合剤層の密度（ＡＢとＰＶＤＦの重量も含めた値）が３．７ｇ／ｃｍ^３で、その厚みが７０μｍ（電極の厚みは約１６０μｍ）となるように調整した。そして、これを所定の大きさに裁断加工して、集電のためのアルミニウム製リードを取りつけて正極とした。
【０１１４】
（リチウム二次電池の作製）
上記のような手順で作製した負極ａ、ＬｉＣｏＯ_２正極、および両者を物理的に隔絶するためのポリエチレン製多孔膜セパレータ（厚み２５μｍ）の真空乾燥を、余分な水分を除去する目的で、それぞれ実施した。負極と正極は１００℃で８時間、セパレータは５０℃で１２時間の真空乾燥を行った。
【０１１５】
続いて、負極ａと正極とを、セパレータを挟持して捲回し、図１中に示したような概四角柱状（横断面形状がおよそ長方形状）の極板群１を形成した。この概四角柱状の極板群１を、５３３０４８サイズ（厚さ５．３ｍｍ×幅３０ｍｍ×高さ４８ｍｍ）の角型アルミニウム合金製電池ケース４に挿入した。そして、上部の封口板５に正極リード２を溶接し、絶縁性ガスケットにより封口板とは電気的に隔絶された負極端子６に負極リード３を溶接した。その後、封口板５を、レーザー溶接によって電池ケース４に接合した。続いて、封口板５が具備する注入口から非水電解液を注入し、極板群１に真空含浸させた。
【０１１６】
そして、注入口が開いたままの状態で、初回の部分充電を施した。初回充電の初期段階に、負極上で皮膜形成に伴って電解液分解等が起こり、ガスが発生する。このガスは十分に拡散除去させた。その後、注入口を、アルミニウム合金製の封栓７で塞ぎ、これをレーザーで溶融し、次いで固化させて、注入口を封止した。こうして、負極ａを用いたリチウム二次電池ａ（設計容量が８００ｍＡｈ）を得た。
【０１１７】
また、負極ａの代わりに負極ｂ〜ｖを用いること以外、すべて上記と同じ条件で、それぞれの負極に対応するリチウム二次電池ｂ〜ｖを作製した。ここで、極板群の構成、正・負極リードの溶接、封口板のケースへの接合、電解液の注入・含浸、初回の部分充電、封栓による密閉化、の各工程は、すべて露点が−４０℃以下のドライエア雰囲気下で実施した。
【０１１８】
また、非水電解液には、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）とジエチルカーボネート（ＤＥＣ）とを、体積比１：２：１で混合した溶媒に、１．０Ｍ（Ｍ：モル／Ｌ）の濃度となるようにＬｉＰＦ_６を溶解させた溶液を使用した。電解液の注液後、電池の初回の部分充電は、２０℃雰囲気下で、充電レート０．１Ｃ（ここでは１Ｃ＝８００ｍＡと仮定して８０ｍＡ）で２時間実施した。
【０１１９】
（電池特性評価）
上記で作製したリチウム二次電池に対し、以下の電池特性の評価を実施した。▲１▼不可逆容量の測定
上記で作製した２２種のリチウム二次電池に対して、以下のパターンで、充放電サイクルを３サイクル行なった。
・充電：　定電流方式　０．２Ｃ（１６０ｍＡ）、終止電圧４．１Ｖ
・放電：　定電流　０．２Ｃ（１６０ｍＡ）、放電カット電圧３．０Ｖ
・雰囲気温度：　２０℃
そして、以下の計算によって、各電池の初期不可逆容量を算出した。
初期不可逆容量＝｛（電池作製時の初回部分充電容量：１６０ｍＡｈ）＋（上記３サイクルの合計充電容量）−（上記３サイクルの合計放電容量）｝／３
【０１２０】
▲２▼高率放電特性
不可逆容量の測定が終わった２２種のリチウム二次電池に、以下に示す充放電試験を行って、放電容量の比率（２Ｃ放電容量Ｃ_２と０．２Ｃ放電容量Ｃ_０．２との比率：Ｃ_２／Ｃ_０．２）を算出して、各電池の高率放電特性を評価した。ここで、試験に際しての雰囲気温度は２０℃とした。
【０１２１】
第１サイクル（０．２Ｃ放電）
・充電：　定電流定電圧方式　０．７Ｃ（５６０ｍＡ）、充電制御電圧４．２Ｖ、合計充電時間２．５時間
・放電：　定電流　０．２Ｃ（１６０ｍＡ）、放電カット電圧３．０Ｖ
第２サイクル（２Ｃ放電）
・充電：　定電流定電圧方式　０．７Ｃ（５６０ｍＡ）、充電制御電圧４．２Ｖ、合計充電時間２．５時間
・放電：　定電流　２Ｃ（１６００ｍＡ）、放電カット電圧３．０Ｖ
【０１２２】
▲３▼低温放電特性
不可逆容量の測定が終わった２２種のリチウム二次電池に対して、▲２▼とは異なる以下に示す充放電試験を行って、放電容量の比率（−１０℃下における１Ｃでの放電容量Ｃ_−１０と、２０℃下における１Ｃでの放電容量Ｃ_２０との比率：Ｃ_−１０／Ｃ_２０）を算出することにより、低温放電特性を評価した。
【０１２３】
第１サイクル（２０℃）
・充電：　定電流定電圧方式　０．７Ｃ（５６０ｍＡ）、充電制御電圧４．２Ｖ、合計充電時間２．５時間、雰囲気温度２０℃
・放電：　定電流　１Ｃ（８００ｍＡ）、放電カット電圧２．５Ｖ
（容量は３．０Ｖまでの放電量で算出）
雰囲気温度２０℃
【０１２４】
第２サイクル（−１０℃）
・充電：　定電流定電圧方式　０．７Ｃ（５６０ｍＡ）、充電制御電圧４．２Ｖ、合計充電時間２．５時間、雰囲気温度２０℃
・放電：　定電流　１Ｃ（８００ｍＡ）、放電カット電圧２．５Ｖ
（容量は３．０Ｖまでの放電量で算出）
雰囲気温度　−１０℃
【０１２５】
▲４▼サイクル寿命特性
不可逆容量の測定が終わった２２種のリチウム二次電池に対して、以下の充放電を５００サイクル繰り返した。そして、５００サイクル時の容量Ｃ_５００と初回サイクルの容量Ｃ_ｉｎｉとを比較して、容量の維持率（Ｃ_５００／Ｃ_ｉｎｉ）を求めた。また、上記のような角型リチウム二次電池に特有の現象として現れる、サイクルに伴う厚み方向への電池ケースの膨れ（膨張）についても、初期からのケースの膨れ量（ｍｍ）として測定した。
【０１２６】
・充電：　定電流定電圧方式　０．７Ｃ（５６０ｍＡ）、充電制御電圧４．２Ｖ、合計充電時間２．５時間
・充電後休止：　３０分
・放電：　定電流　０．７Ｃ（５６０ｍＡ）、放電カット電圧３．０Ｖ
・放電後休止：　３０分
・評価雰囲気温度：　２０℃
【０１２７】
以上の電池評価の結果を表３にまとめて示す。表３では、電池ａの評価結果の値を１００として規格化し、各電池ｂ〜ｖの性能を相対的に比較した。表３において、本発明における電池ａ〜ｄは、天然黒鉛を主体とした負極を用いた電池ｑ〜ｖに比べると、すべての特性において明らかに優れている。また、人造黒鉛を主体とした負極を用いた電池ｅ〜ｐと比較しても、若干不可逆容量が大きいという欠点はあるが、放電レート比率、低温放電特性、５００サイクル時容量維持率といった他特性においては、電池ａ〜ｄが優れており、電池膨れも十分に抑制されている。
【０１２８】
【表３】

【０１２９】
このように、本発明の電池ａ〜ｄが他より優れるという傾向が得られた理由は、負極の主活物質である人造黒鉛粒子Ａの粉末物性に基づく部分が大きいと考えられる。
まず、高率放電特性が高い点については、人造黒鉛粒子Ａ１およびＡ２が、その製造プロセスに依存して、十分に発達した黒鉛結晶がランダムな方向に配向した粒子になっているため（粉末物性値：Ｉ_００２／Ｉ_１１０値が十分に小さいため）と考えられる。すなわち、本実施例のように合剤密度（ＣＭＣ、ＳＢＲの重量も含めた値）が１．７０ｇ／ｃｍ^３に達するような高密度負極内においては、一部の黒鉛粒子が銅芯材の面方向に配向すると考えられる。しかしながら、粒子内のランダムに存在する黒鉛結晶子は、その配向の影響を受けず、黒鉛粒子と電解液との間で、Ｌｉイオンの吸蔵・放出が円滑に進行し得ると推察される。
【０１３０】
低温放電特性に関しては、負極合剤の電子伝導性（極板抵抗）が大きく影響していると推測される。
本発明の電池ａ〜ｄで用いた人造黒鉛粒子Ａ１およびＡ２の基材であるバルクメソフェーズピッチは、針状コークスよりも易黒鉛化性であり、黒鉛化によって黒鉛結晶構造が十分に発達している。そのため、針状コークスを基材に用いた黒鉛粒子Ｃ１およびＣ２（電池ｅ〜ｊ）に比べると、粒子自体の電子伝導性が高い。
【０１３１】
また、電池ｋおよびｌに用いたメソフェーズ炭素由来の他の人造黒鉛粒子Ｄ（黒鉛化ミルドＭＣＦ：細長い柱状）、電池ｍおよびｎに用いた黒鉛粒子Ｅ（黒鉛化ＭＣＭＢ：真球形状）に比べると、粒子Ａ１およびＡ２は形状が適度な塊状である。そのため、粒子Ａ１およびＡ２は、黒鉛粒子Ｂ１およびＢ２と多数の接触点を確保でき、負極合剤全体の電子伝導性が高くなる。従って、低温放電に際しての放電電圧の低下の度合いが少なくなって、優れた低温放電特性を確保したと考えられる。また、人造黒鉛粒子Ｆ（黒鉛化バルクメソフェーズ）を用いた電池ｏおよびｐとの低温放電特性の差に関しては、先述のように、負極黒鉛粒子Ａ１およびＡ２内では黒鉛結晶がランダムな方向に配向して存在する点が影響したものと考えられる。
【０１３２】
ところで、表３から明らかなように、例えば比較人造黒鉛粒子Ｃ１だけを単独で用いた電池ｅと、比較人造黒鉛粒子Ｃ１および球状黒鉛粒子Ｂ１もしくはＢ２との混合物を用いた電池ｆおよびｇとを比較すると、性能上、大きな差異は見られない。このことは、人造黒鉛粒子Ａ１またはＡ２の代わりに比較人造黒鉛粒子Ｃ１を用いたとしても、本発明と同様の効果が得られないことを示している。つまり人造黒鉛粒子Ａ１またはＡ２と、球状黒鉛粒子Ｂ１またはＢ２との組み合わせにおいて、特に優れた特性の負極や電池を得ることができると言える。
【０１３３】
なお、比較人造黒鉛粒子Ｃ１だけを単独で用いた電池ｅの負極では、圧延の際に、人造黒鉛の微粉を生成し、これが球状黒鉛粒子Ｂ１またはＢ２と同様の作用を担っていると考えられる。そのため、電池ｅと電池ｆおよびｇとの間に特性上の差異が現れないものと推測される。負極ｅの圧延の際に人造黒鉛の微粉が生成すると考えられるのは、比較人造黒鉛粒子Ｃ１の製造工程が、黒鉛の粉砕工程を有するため、一次粒子の結合が弱くなっていると考えられるからである。
【０１３４】
本発明の電池ａ〜ｄでは、充放電サイクル特性が他より優れ、充放電に伴う電池膨れの程度も少ないという点に関しては、以下が主な要因と考えられる。
（１）本発明の電池で用いたような負極活物質粒子ａ〜ｄでは、人造黒鉛粒子Ａ１ないしはＡ２の空隙を埋めるように、球状黒鉛粒子Ｂ１ないしはＢ２が、最適に配置している。そのため、合剤密度を１．７０ｇ／ｃｍ^３に達するほど高くしても、合剤層の表面付近にある黒鉛粒子が破砕・崩壊等を起こして銅芯材の面方向に配向することが防がれる。そして、合剤内への電解液の浸透性（含浸性）が妨げられることもない。つまり、合剤層内部においても、高い電解液の浸透性（含浸性）が確保されるので、長期のサイクルで電解液の分解・減少が部分的に起こっても、円滑な充放電反応は確保される。
【０１３５】
（２）主活物質の人造黒鉛粒子Ａ１およびＡ２の黒鉛結晶子が、ランダムな方向に配向しているため、充放電サイクルの繰り返し（Ｌｉイオンの挿入・脱離）に伴う粒子の膨張・収縮の程度が少なく、負極の厚みの増加（膨潤）の程度が少ない。
【０１３６】
（３）前記（２）にも関連するが、人造黒鉛粒子Ａ１およびＡ２は、充放電サイクルの繰り返しに伴う粒子の膨張・収縮の程度が少ないため、サイクルの進行に伴う黒鉛活物質粒子の割れが発生しにくい。従って、黒鉛活物質粒子の割れ（新規黒鉛エッジ面の露出）によって引き起こされる、ガス発生を伴った電解液の分解消費反応が抑制される。
【０１３７】
（４）メソフェーズ炭素由来の人造黒鉛粒子では、一般に、高率充電時のＬｉイオンの受け入れ性能が、低い傾向にある。しかしながら、人造黒鉛粒子Ａ１ないしはＡ２の表面の濡れ性（表面官能基の種類や濃度に依存する）は、バルクメソフェーズピッチ粉砕粒とピッチないしは熱硬化性樹脂との造粒・黒鉛化によって、変化しており、比較的高い水準にまで改善されている。そのため、充放電サイクルの進行に伴う負極表面への金属リチウムの析出現象が抑制される。
特に、本発明の電池ａ〜ｄは、人造黒鉛粒子Ａ１およびＡ２と製法的に類似した人造黒鉛粒子Ｃ１およびＣ２を用いた電池ｅ〜ｊに比べて、充放電サイクル特性に優れている。その理由については、詳細なメカニズムは解明できないが、以下のような点が考えられる。
【０１３８】
（５）人造黒鉛粒子Ａ１およびＡ２は、炭素化・黒鉛化の後に粉砕工程を行なわないため、円形度が大きくて、タップ密度の高い粒子となっている。従って、これを用いて作製した高密度負極中の粒子の破砕（崩壊）の程度は、人造黒鉛粒子Ｃ１およびＣ２を用いて作製した高密度負極ｅ〜ｊに比較して少ない。
【０１３９】
（６）人造黒鉛粒子を作製する際に使用した基材炭素源の差、すなわちバルクメソフェーズピッチ粉砕粒と針状コークスとの差により影響されて、充放電サイクルによる黒鉛粒子の割れの進行度合いは、人造黒鉛粒子Ａ１およびＡ２の方が、人造黒鉛粒子Ｃ１およびＣ２よりも少なくなる。ここで、初期１０サイクルの充放電を繰り返した電池（放電状態）と、５００サイクル後の電池（放電状態）とを、分解して、負極合剤を抽出・洗浄し、ＢＥＴ法によって活物質粒子の比表面積を測定した。その結果、電池ａ〜ｄ（負極ａ〜ｄ）の方が、電池ｅ〜ｊ（負極ｅ〜ｊ）よりも、初期から粒子の比表面積が小さく、また、サイクルに伴う粒子の比表面積の増加度合いも小さいことが実際に確認された。
【０１４０】
（安全性試験）
リチウム二次電池の黒鉛負極は、一般に、電池の熱安定性との相関が強いと考えられている。ここで、リチウム二次電池の熱安定性（熱暴露）の評価法・評価基準等に関しては、各種規格・ガイドラインが存在するが、統一されたものではない。そこで、本検討では、比較的厳しく、かつ負極種の違いがなるべく明確に反映される条件として、以下の条件を採用し、電池の耐熱試験を行なった。
【０１４１】
まず、上記の負極ａ〜ｖに対応する２２種のリチウム二次電池を、２０℃雰囲気下、充電レート０．１Ｃ（８０ｍＡ）の定電流および２時間の定電圧保持で４．３Ｖまで充電した。そして、電池の表面温度をモニターできるように、電池に熱電対をとりつけて、２０℃雰囲気の恒温槽内で宙づりとした。そして、恒温槽の温度を５℃／分で１６５℃まで昇温した後に、１６５℃で保持した。
【０１４２】
この試験においては、恒温槽の温度を１６５℃に保持しても、充電状態にある負極黒鉛活物質粒子の一部が電解液もしくは結着剤と反応したり、黒鉛表面の被膜が分解したりして、反応熱を発生する。従って、電池表面温度は１６５℃以上の温度にまで到達する。そして、この際の最高到達温度が極端に高いと、電池内部の正極（ないしは負極の）連鎖的な発熱反応（熱暴走）もしくは急激なセパレータ収縮に伴う内部短絡を引き起こしてしまう。電池の最高到達温度が低いものほど、電池の安全性が高いと言える。結果をまとめて表４に示す。
【０１４３】
【表４】

【０１４４】
この結果から、耐熱試験での優劣は、負極を形成する黒鉛粒子のＢＥＴ比表面積と非常に相関性が高いことが理解される。本発明の電池ａ〜ｄで用いた負極に関しては、主黒鉛活物質粒子Ａ１およびＡ２のＢＥＴ比表面積が、０．４〜０．５ｍ^２／ｇと、いずれも１ｍ^２／ｇ以下の低い水準に抑制されている。このことが、耐熱性という観点では、非常に有利であることが解る。また、電池ａ〜ｄで用いている負極の球状黒鉛粒子Ｂ１およびＢ２のＢＥＴ比表面積は、６．９〜７．２ｍ^２／ｇと、かなり大きい値である。これに関しては、可能な限りＢＥＴ比表面積を低減したものを用いることが、より一層好ましいと言える。
【０１４５】
以上の検討結果から、本発明の電池ａ〜ｄは、高い放電特性と優れたサイクル寿命特性を持ち合わせると同時に、高い安全性も確保していることが確認できる。
ここで、電池の形態に関して、実施例中では、概四角柱状の電極群を角型アルミニウム合金製電池ケースに挿入する形態（図１）としたが、本発明はこれに限定されるものではない。例えば、概四角柱状ないしは楕円柱状に捲回してなる電極群を、アルミニウム箔と樹脂膜とのラミネートシートからなるケースに封入し、非水電解液を注入した形態のリチウム二次電池としても、同様に放電特性、サイクル寿命特性および安全性に優れ、高エネルギー密度を有し、しかも薄型かつ軽量のリチウム二次電池を得ることができる。
【０１４６】
同様に、負極合剤層の密度を１．６〜１．８ｇ／ｃｍ^３と高く設定した負極と、高密度に充填した正極と、セパレータとを組み合わせて、円柱（スパイラル）状に構成した電極群を作製し、ニッケルめっき鋼板製の円筒ケース内に収容し、非水電解液を注入した形態としても、やはり放電特性、サイクル寿命特性および安全性に優れた４００Ｗｈ／Ｌレベルの高容量リチウム二次電池を得ることができる。
【０１４７】
また、人造黒鉛粒子Ａ１ないしはＡ２の作製に際して、平均粒子径が１３μｍのバルクメソフェーズピッチ粉砕粒を基材として用いたが、これに限定されるものではない。平均粒子径が７〜２０μｍ程度のものであれば、同様の人造黒鉛粒子を作製することができる。
【０１４８】
また、バインダーとして、コールタールピッチ（石炭ピッチ）およびフェノール樹脂を用いたが、石油ピッチ、ナフタレンピッチ等のピッチ、ポリイミド樹脂、ポリ塩化ビニル樹脂、セルロース樹脂、フルフリルアルコール樹脂等の熱硬化性樹脂を用いることも可能である。
【０１４９】
また、混練・造粒において、基材とバインダーとの配合比を８５：１５（重量比）、温度を２００℃、時間を１２０分、混練装置をＺ型のニーダとしたが、これらの条件に限定されるものではない。得られる造粒物の円形度が０．８５〜０．９５、平均粒子径Ｄ_５０が１５〜３０μｍ、Ｄ_１０／Ｄ_９０の値が０．２〜０．５となるように各条件を調整すればよい。
【０１５０】
また、造粒物の炭素化（焼成）の温度を８００℃としたが、７００〜１５００℃の非酸化性雰囲気であればよい。また、黒鉛化の温度を２９５０℃としたが、２５００〜３０００℃の非酸化性雰囲気で加熱して、十分に黒鉛化を進行させれば、同様の人造黒鉛粒子を得ることができる。
【０１５１】
また、球状黒鉛粒子として、粉砕した鱗片状天然黒鉛をカウンター式のジェットミル内で衝撃を与えて球形化・分級した粒子Ｂ１、ないしは針状コークス粉末を黒鉛化した後、同様の球形化・分級を行った粒子Ｂ２を用いたが、これに限定されるものではない。特に、天然黒鉛を原料とする方が、黒鉛粒子から最も大きな可逆容量を得ることができるとともに、黒鉛化工程が省けるため、安価な粒子とすることができる。
【０１５２】
この球形化手段としては、カウンター式のジェットミル内での衝撃法に限らず、原鉱からの粉砕設備・条件に、各種改善を加えること等によっても、同様にＤ_５０が５〜１５μｍで、円形度が０．８８〜１と大きい球状黒鉛粒子を得ることができる。
【０１５３】
また、球状黒鉛粒子は、黒鉛負極の安全性（耐熱性）という観点から、ＢＥＴ比表面積をできるだけ低くしたものであることが好ましく、このための表面改質（被覆処理等）を行ったものが最も好適である。
【０１５４】
また、負極活物質の作製に際して、球状黒鉛粒子Ｂ１ないしはＢ２の活物質全体に対する配合比を２５重量％としたが、５〜４５重量％の範囲であれば同様の効果を得ることができる。
【０１５５】
また、負極合剤の作製に際して、ゴム状結着剤にスチレンブタジエンゴム（ＳＢＲ）を用いたが、類似のブタジエン誘導体からなるゴム状結着剤として、ブタジエンと、芳香族ビニルモノマーと、エチレン性不飽和カルボン酸エステルモノマーとの共重合体からなるゴム状高分子を結着剤に用いても、同様の負極を作製することができる。ここで、芳香族ビニルモノマーには、スチレン、α−メチルスチレン等を用いることができ、エチレン性不飽和カルボン酸エステルモノマーには、アクリル酸エステル（アクリル酸メチル、アクリル酸エチル、アクリル酸プロピル等）やメタクリル酸エステル（メタクリル酸メチル、メタクリル酸エチル、メタクリル酸プロピル等）を用いることができる。
【０１５６】
また、ゴム状結着剤の添加量を、負極活物質重量に対して２重量％としたが、例えば３重量％以下であれば、負極特性を損なうことなく、電池を作製することができる。
【０１５７】
さらに、合剤層の密度が１．７ｇ／ｃｍ^３で、その厚みが７０μｍとなるように圧延を調整して負極を作製したが、合剤密度が１．６〜１．８ｇ／ｃｍ^３であり、合剤厚みが例えば４０〜１００μｍであれば、同様の優れた特性を有するリチウム二次電池を作製することができる。
【０１５８】
さらに、非水電解液には、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、ジエチルカーボネート（ＤＥＣ）とを、体積比１：２：１で混合した溶媒に、１．０Ｍの濃度となるようにＬｉＰＦ_６を溶解させた溶液を用いたが、これに限定されるものではない。
【０１５９】
【発明の効果】
以上のように、本発明によれば、高エネルギー密度のリチウム二次電池の充放電サイクル特性を大幅に改善することができ、同時に放電レート特性、低温放電特性および安全性（耐熱性）にも優れた電池を提供することが可能となる。従って、産業上の価値は非常に大きい。
【図面の簡単な説明】
【図１】実施例で作製したリチウム二次電池の一部を切り欠いた斜視図である。
【符号の説明】
１　極板群
２　正極リード
３　負極リード
４　電池ケース
５　封口板
６　負極端子
７　封栓[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a lithium ion secondary battery including a lithium secondary battery, particularly a negative electrode including an active material including graphite, a positive electrode including an active material including a lithium-containing transition metal oxide, a separator, and a nonaqueous electrolyte. Battery.
[0002]
[Prior art]
2. Description of the Related Art As electronic devices become more portable and cordless, lithium secondary batteries having a small size, light weight, and high energy density are promising as power sources for driving them. For example, it is composed of a negative electrode using a carbon material capable of reversibly storing and releasing lithium ions as an active material, a positive electrode using a transition metal composite oxide containing lithium as an active material, a separator, and a non-aqueous electrolyte. In addition, a rocking chair type so-called lithium ion secondary battery has already been put into practical use and is rapidly spreading.
[0003]
Here, regarding the negative electrode, graphite (graphite) particles having high crystallinity among various carbon materials have recently become mainstream. The reasons are as follows. Graphite particles have (1) high electron conductivity and excellent discharge performance at large current, (2) small change in potential due to discharge, suitable for applications such as constant power discharge, and (3) true density. It is obtained as particles having a large bulk density because of its large size, and is suitable for increasing the energy density of a battery.
[0004]
At present, as graphite for the negative electrode of lithium secondary batteries,
I. Agglomerated natural graphite, which is a natural graphite particle obtained by subjecting flake-like particles to agglomeration (spheroidization) in the process of pulverizing raw ore,
II. Artificial graphite particles, a material obtained by graphitizing granulated particles of certain coke or coke and various pitches, and
III. Special artificial graphite particles utilizing mesophase carbon (a type of liquid crystal) generated when pitch and tar are heated are mainly used.
[0005]
Special artificial graphite particles include
(1) Carbonized and graphitized material (graphitized MCMB) of mesophase microspheres separated and extracted,
(2) A melted mesophase pitch formed by polymerization growth of mesophase spheres is spun, infusibilized by surface oxidation, then carbonized, further cut and crushed, and graphitized material (graphite fiber mill, Or graphitized milled MCF),
(3) Materials obtained by carbonizing and graphitizing pulverized particles of bulk mesophase pitch with low meltability formed by polymerization growth of mesophase small spheres (graphitized bulk mesophase).
[0006]
In response to the recent increase in energy density of lithium secondary batteries, attempts have been made to improve the performance of the above graphites I to III.
Since natural graphite particles (I) have a reversible capacity that is close to the theoretical capacity of graphite (372 mAh / g), high-density filling of electrodes is being studied. For example, accumulation of techniques for adjusting the particle shape so as to be suitable for high-density packing is active. Further, it has been proposed to coat an edge surface exposed on the surface of graphite particles with amorphous carbon classified as graphitizable carbon (Patent Document 1). According to this proposal, it is possible to suppress the decomposition reaction of the electrolytic solution on the surface of the graphite particles that occurs at the time of initial charging, and to reduce the irreversible capacity accompanying the decomposition reaction.
[0007]
At present, the artificial graphite particles (II) and the special artificial graphite particles (III) do not have a reversible capacity close to the theoretical capacity of graphite (the reversible capacity is inferior to natural graphite). For this reason, studies have been made to increase the purity of coke, pitch or tar as a raw material, to optimize graphitization conditions according to the material, and to add a catalyst species for promoting graphitization. That is, studies to increase the degree of graphitization of the particles to improve the reversible capacity have been actively made. In such artificial graphite, since the proportion of the graphite edge surface exposed on the particle surface is small, the irreversible capacity at the time of initial charging is generally smaller than that of natural graphite (I).
[0008]
When producing the negative electrode of the lithium secondary battery, one of the above graphite types may be used alone or two or more types may be used as a mixture as the active material.
[0009]
The step of preparing the negative electrode generally includes a step of preparing an aqueous paste or an organic paste containing graphite active material particles. The aqueous paste is a mixture of graphite active material particles, SBR (styrene-butadiene copolymer rubber) as a binder, CMC (carboxymethylcellulose) as a thickener, and an appropriate amount of water. is there. The organic paste is a mixture of PVDF (polyvinylidene fluoride) or the like as a binder and a thickener, and an appropriate amount of NMP (N-methyl-2-pyrrolidone) or the like as a dispersion medium.
[0010]
These pastes are applied on a copper core material, dried and then rolled to a desired thickness and density, cut and processed, and lead welding to a current collector is performed to form a negative electrode plate. . During rolling, the density of the negative electrode mixture layer is about 1.6 g / cm. ³ In many cases, the upper limit is set. This is because when the rolling is performed to a very high density, the crushing and collapsing of the negative electrode active material particles, the falling off and separation of the particles from the core material, and the like occur.
[0011]
However, even in the case where the upper limit is set as described above, similarly, LiCoO rolled to a high density is used. ₂ And a high-energy-density lithium secondary battery having a volume energy density exceeding 350 Wh / L by using a positive electrode mainly composed of, and a thin polyolefin microporous membrane separator having appropriate mechanical strength and porosity. It is possible to obtain batteries.
[0012]
In recent years, from the viewpoint of facilitating the miniaturization and thinning of portable devices, there is an increasing need from the market for high-energy-density lithium secondary batteries that are “thin and lightweight” and have added value. In these batteries, together with the nonaqueous electrolyte, the electrode group is housed in a square metal case or a case made of a laminate sheet of an aluminum foil and a resin film. The electrode plate group often uses a negative electrode, a positive electrode, and a separator wound in a substantially rectangular column shape or an elliptical column shape.
[0013]
In the above lithium secondary batteries, there are many required performances.
First, with the aim of further increasing the energy density of the battery, studies have been made to further increase the density of the negative electrode mixture layer fixed on the copper core material. Specifically, the density of the negative electrode mixture layer including the binder and the like is set to 1.6 to 1.8 g / cm. ³ It is desired to be on the order. However, the true density of the graphite material is 2.22 to 2.24 g / cm. ³ 1.6 g / cm ³ Above corresponds to a very high packing state. Therefore, in the process of rolling the negative electrode mixture layer by a roll press or the like, the production of the mixture layer cannot be compressed to a predetermined thickness, or the separation and detachment of the mixture layer from the core material become apparent. The above problems are likely to occur.
[0014]
These problems are often governed mainly by the type of graphite particles that are the negative electrode active material.
According to the studies by the present inventors so far, the above-mentioned special artificial graphite particles (III) derived from mesophase carbon tend to cause the former problem that the mixture layer cannot be compressed to a predetermined thickness.
[0015]
This is because the special artificial graphite particles (III) have poor slipperiness between the particles. For the same material, it is necessary to make the surface layer of the mesophase particles infusible (loose oxidation treatment) as a pretreatment in order to alleviate the occurrence of fusion between particles in the carbonization / graphitization process, which is the manufacturing process. . For this reason, the obtained particle surface layer is in a state close to amorphous where graphitization has not progressed much. That is, in the negative electrode mixture layer manufactured using the same material, the contact between the active material particles is substantially the contact between the amorphous carbons.
[0016]
Amorphous carbon that does not have a layered structure is poor in slipperiness because there is little electrostatic repulsion (interaction of π electrons) between particles peculiar to the graphite layered structure. Therefore, when this material is used for the negative electrode active material, the problem that the mixture layer cannot be compressed to a predetermined thickness during high-density rolling tends to occur. As a countermeasure against such a problem, for example, it has been proposed to add agglomerated natural graphite or flaky natural graphite particles to graphitized MCMB as an auxiliary material to form a negative electrode mixture layer (Patent Document 2). ).
[0017]
Further, in the case of artificial graphite particles (II) derived from coke or the like, the latter problem such as falling off and peeling of the mixture layer from the core material tends to occur.
The reason for this is that artificial graphite particles (II) derived from coke and the like are generally subjected to pulverization and particle size adjustment after graphitization, so that particles having a high bulk density (or tap density) and specific surface area Obtaining small particles can be difficult. This is considered to be one of the causes that the mixture layer is likely to peel or fall off during high-density rolling. That is, since the particles are bulky, the particles are likely to be crushed or collapsed during high-density rolling of the mixture layer. In addition, since the specific surface area of the particles is large, most of the binder added to the mixture layer is adsorbed on the particle surface, and the binding property between the core material and the particles or between the particles is maintained. Becomes difficult. For this reason, it is presumed that the mixture layer easily falls off or peels off during high-density rolling.
[0018]
On the other hand, in comparison with these, the natural graphite particles (I) have been sufficiently graphitized to the surface of the particles. Therefore, the electrostatic repulsion between the particles is strong, and the sliding property is very large. Therefore, the mixture density is 1.6 g / cm ³ High-density rolling is relatively easy, and a problem in production hardly occurs.
[0019]
However, even if the squamous particles are subjected to agglomeration (spheronization) treatment (Patent Document 3), it is very difficult to control the shape of all the particles to a shape close to a perfect sphere. Actually, a large number of spindle-shaped (flat) particles having a considerably large aspect ratio are mixed. Therefore, depending on the degree of shape control, the density of the mixture layer is 1.6 g / cm. ³ When strong rolling is performed, the spindle-shaped particles are oriented in the plane direction of the core material with some deformation of the particles. This phenomenon is a well-known phenomenon with conventional flaky natural graphite particles.
[0020]
When this happens,
{Circle around (1)} The edge surfaces of the graphite particles that occlude and release Li ions are less likely to be exposed to the electrolytic solution, the diffusion of Li ions is reduced, and the high-rate discharge characteristics are reduced.
{Circle around (2)} During charge / discharge, expansion / contraction of graphite particles in the c-axis direction tends to be reflected as a change in the thickness of the mixture layer, and the degree of expansion / contraction of the electrode increases, resulting in a problem in characteristics. .
[0021]
As described above, the natural graphite particles have a problem that the orientation of the particles (in other words, graphite crystals) occurs at the time of high-density rolling, and the electrode performance is reduced.
Based on this, it is possible to mix a graphitizable substrate (mainly coke, etc.) with a graphitizable binder (tar, pitch, etc.), carbonize it, pulverize it, and then graphitize it. It has been proposed (Patent Documents 4 and 5). According to this method, it is possible to produce artificial graphite in which graphite structure or graphite crystals are oriented in random directions in the particles.
[0022]
Further, as a similar technique, there is a technique in which quiche graphite (recrystallized graphite) obtained from an iron making process is granulated using a binder, and a graphitized material is used for a negative electrode (Patent Document 6).
[0023]
When these artificial graphite particles are used, in the high-density rolling described above, a problem in the process of falling off and peeling of the mixture layer is likely to occur, but even if the particles are oriented in the plane direction of the core material, random particles are generated in the particles. Are not affected by such an orientation. Therefore, the problems described in (1) and (2) can be relatively easily avoided.
[0024]
Further, in a recent lithium ion secondary battery with a high energy density design exceeding 350 Wh / L, it is necessary to fill a battery case with a predetermined volume with more negative electrode active material and more positive electrode active material. Therefore, the remaining space inside the battery (in this case, the space obtained by subtracting the volumes of the components such as the positive electrode, the negative electrode, and the separator from the internal volume of the battery case) is reduced. Then, the ratio of the amount of the electrolyte to the battery design capacity (cc / mAh) tends to be extremely small. As a result, the following problems occur, which have not been observed in conventional batteries having a relatively large amount of electrolyte.
[0025]
First, since the electrolyte cannot sufficiently penetrate or impregnate into the inside of the negative electrode mixture layer that has been rolled at high density, problems such as deterioration in charge / discharge characteristics at high rates and discharge characteristics at low temperatures are likely to occur. . As an improvement measure, it is effective to use a graphite material capable of maintaining an appropriate degree of particle circularity (sphericity) even after rolling (Patent Document 7). This graphite material has a specific average particle size (10 to 35 μm), has a relatively sharp particle size distribution, and does not contain much fine powder of 4 μm or less. Therefore, it is necessary to use natural graphite particles or artificial graphite particles in which graphite crystals are oriented in random directions, which have been subjected to the above-mentioned agglomeration (spheronization) treatment, and adjust the particle size to an optimum size, and use the charge / discharge characteristics at a high rate. It is considered to be effective in improving the discharge characteristics at low and low temperatures.
[0026]
However, the lithium secondary battery of the high energy density design has a problem that the capacity deterioration accompanying the progress of the charge / discharge cycle is larger than that of the conventional battery. This is because, as the charge / discharge cycle progresses, the graphite active material particles are cracked or collapsed, and the newly formed graphite edge surface is exposed to the electrolytic solution. Along with this, the electrolytic solution having a small absolute amount is decomposed and consumed from the beginning, and the internal resistance of the battery increases. It is also considered that the main factor is that the decomposition product of the electrolytic solution is deposited as a film on the surface of the negative electrode, thereby lowering the charge and discharge efficiency of the negative electrode. In addition, a rectangular metal case or a case formed of a laminate sheet of an aluminum foil and a resin film, which is used for a recent lithium secondary battery, generally has low strength. Therefore, when the decomposition reaction of the electrolytic solution occurs with the progress of the charge / discharge cycle, the generated decomposition gas increases the internal pressure of the battery and deforms (swells) the battery in the thickness direction. Furthermore, the electrode group wound into a substantially square column or an elliptical column used in such a battery has a larger expansion and contraction of the negative electrode mixture layer than the column (spiral) electrode group used in a cylindrical battery. Accompanying deformation is likely to occur. It is considered that these factors combine to significantly reduce the cycle life characteristics.
[0027]
Therefore, as an improvement measure from the negative electrode side,
{Circle around (1)} In order to suppress the decomposition and consumption of the electrolytic solution, graphite particles that are less likely to be cracked or disintegrated with the progress of the charge / discharge cycle (have low reactivity with the electrolyte during the charge / discharge cycle) are used. Used for substances,
{Circle around (2)} It is easy to take measures such as using graphite particles having a small degree of expansion and contraction due to charge and discharge.
[0028]
The present inventors have conducted intensive studies on various graphite materials and found that agglomerated natural graphite particles (or agglomerated natural graphite having been subjected to surface modification, surface coating, etc.) were used as the main active material of the negative electrode. In this case, the degree of cracking / collapse of the particles accompanying the progress of the charge / discharge cycle was generally larger than that of the artificial graphite particles. Even when various known additives for forming a negative electrode protective film are added to an electrolytic solution, satisfactory cycle life characteristics have not been obtained. Here, the additive for forming a protective film forms a protective film on the negative electrode graphite particles at the time of initial charging, and suppresses the decomposition reaction of the electrolytic solution accompanying the cycle, and typical examples thereof include vinylene carbonate. No.
[0029]
On the other hand, as described above, artificial graphite particles in which graphite crystals are oriented in random directions have a small degree of cracking / collapse of particles as the charge / discharge cycle proceeds, and a relatively small degree of expansion / shrinkage due to charge / discharge. It has been found to be small and suitable.
[0030]
However, on the other hand, in the artificial graphite particles produced by the methods disclosed in Patent Documents 4 and 5, the particles are firmly fused during carbonization and graphitization in the production process. Therefore, it is necessary to perform strong pulverization after graphitization. As a result, the specific surface area of the obtained graphite particles increases. It is known that the specific surface area of the negative graphite particles has a correlation with the initial irreversible capacity and thermal stability of the negative electrode (such as heat resistance of the charged negative electrode). If the specific surface area of the particles is large, the initial irreversible capacity becomes large and the thermal stability tends to decrease, which is not preferable from the viewpoints of both high battery capacity and safety.
[0031]
In view of the above, as a measure for improving artificial graphite particles, a graphitizable base material (coke) is mixed with a graphitizable binder (tar, pitch, etc.), then carbonized, and lightly pulverized. Thus, a method for producing artificial graphite which is graphitized in powder form has been proposed (Patent Document 8). That is, here, the material is not pulverized after the graphitization.
[0032]
In Patent Document 8,
(1) oxidize the binder to make it infusible before carbonization,
(2) By adding a thermosetting resin to a binder, fusion during carbonization is suppressed.
(3) By coating the mixture obtained by mixing the base material and the binder with a thermosetting resin to suppress fusion during carbonization, the specific surface area of the particles is reduced to 1.0 to 3.0m ² / G. In the examples, the average particle diameter (D ₅₀ ) Is 25 to 30 μm, and the specific surface area by the BET method is 1.8 to 2.2 m. ² / G artificial graphite particles have been produced.
[0033]
However, as long as specific starting materials (coke, tar, and pitch in Patent Document 8) are used, there is a limit in reducing the specific surface area of the particles. For example, to reduce the sedimentation of the negative electrode mixture paste, to facilitate the handling of the paste in the manufacturing process, and to increase the yield, etc. ₅₀ When the particle size is adjusted to be about 20 μm, the BET specific surface area is 3 m ² / G. As a result, the initial irreversible capacity of the negative electrode increases, and the thermal stability (heat resistance) of the negative electrode deteriorates. Further, the graphite particles described in the above publication have a lower bulk density (or tap density) than other graphite particles. Therefore, when the electrode is rolled so as to have a high density, there is a disadvantage that the mixture layer is likely to fall off.
[0034]
[Patent Document 1]
JP-A-11-54123
[Patent Document 2]
JP 2001-236950 A
[Patent Document 3]
JP-A-11-263612
[Patent Document 4]
JP-A-2001-89118
[Patent Document 5]
JP-A-2002-50346
[Patent Document 6]
JP 2001-357849 A
[Patent Document 7]
JP 2000-90930 A
[Patent Document 8]
JP-A-11-199213
[0035]
[Problems to be solved by the invention]
In view of the above problems, the present invention is to improve or maintain discharge rate characteristics, low-temperature discharge characteristics, and safety (heat resistance) at the same time as significantly improving the charge / discharge cycle characteristics of a high energy density lithium secondary battery. Aim.
[0036]
[Means for Solving the Problems]
In the present invention, a negative electrode in which an active material composed of a mixture of artificial graphite particles A and spherical graphite particles B having a large circularity is fixed on a copper core material is used.
That is, the present invention is a lithium secondary battery comprising a positive electrode, a negative electrode and a non-aqueous electrolyte, wherein the negative electrode comprises a copper core material and a negative electrode mixture layer fixed on the core material, The agent layer contains an active material composed of a mixture of artificial graphite particles A and spherical graphite particles B.
[0037]
The artificial graphite particles A are isotropic artificial graphite particles in which a graphite structure (graphite crystal) is randomly oriented in the particles, and (1) a plane distance d of (002) plane obtained by powder X-ray diffraction method. ₀₀₂ Is 3.362% or less, and (2) density is 1.6 g / cm. ³ Intensity I attributed to the (002) plane in the diffraction pattern when pellets are formed and X-ray diffraction measurement is performed ₀₀₂ And peak intensity I attributed to the (110) plane ₁₁₀ Ratio with: I ₀₀₂ / I ₁₁₀ Is 1000 or less, (3) average particle circularity is 0.85 to 0.95, and (4) particle diameter D at a volume fraction of 50% measured using a laser diffraction particle size distribution analyzer. ₅₀ Is 15 to 30 μm and the particle diameter D at a volume fraction of 10%. ₁₀ And the particle diameter D at a volume fraction of 90% ₉₀ Ratio with: D ₁₀ / D ₉₀ Is 0.2 to 0.5, and (5) tap density is 1 g / cm. ³ As described above, (6) the specific surface area measured by the BET method is 1 m ² / G or less.
[0038]
The spherical graphite particles B have (1) an average particle circularity of 0.88 to 1, and (2) a particle diameter D at a volume fraction of 50% measured using a laser diffraction type particle size distribution analyzer. ₅₀ Is 5 to 15 μm, and (3) the spacing d of the (002) plane determined by powder X-ray diffraction method ₀₀₂ Is less than or equal to 3.357 ° and (4) the specific surface area measured by the BET method is 8 m ² / G or less.
[0039]
The artificial graphite particles A are obtained by kneading and granulating a base material prepared by pulverizing a bulk mesophase pitch, and a pitch and / or thermosetting resin in a softened state, and obtaining the granulated product in a range of 700 to 1500. C. and carbonized at 2500-3000C.
[0040]
The mixing ratio of the spherical graphite particles B to the entire active material is preferably 5 to 45% by weight.
[0041]
The positive electrode and the negative electrode are wound around a separator to form an electrode group, and are sealed in a square metal case or a case made of a laminate sheet of an aluminum foil and a resin film. preferable.
[0042]
It is preferable that the negative electrode mixture layer further contains a rubbery binder containing a butadiene unit and a cellulose-based thickener.
[0043]
The spherical graphite particles B are preferably natural graphite particles and / or natural graphite particles that have been subjected to a modification treatment for partially amorphizing only the surface.
[0044]
The addition amount of the rubbery binder is 3 parts by weight or less based on 100 parts by weight of the active material, and the density of the negative electrode mixture layer is 1.6 to 1.8 g / cm. ³ And the thickness of the negative electrode mixture layer is preferably 40 to 100 μm.
[0045]
As described above, in the present invention, with respect to the artificial graphite particles A used as the main graphite active material of the negative electrode, the pulverized particles of the bulk mesophase pitch having low fusibility formed by the polymerization growth of mesophase spheres are used as the base material (graphitizable Base material). In this regard, the present invention is significantly different from the artificial graphite particles disclosed in the aforementioned Patent Documents 4, 5 and 8.
[0046]
As described in, for example, Japanese Patent Application Laid-Open No. 2001-23635, if pulverized particles of bulk mesophase pitch formed so as to have a low volatile content are carbonized and graphitized, the particles are fused at the time of carbonization. Does not occur. Therefore, a pulverizing step in the middle can be omitted, and graphite particles for a negative electrode having a small specific surface area can be produced with a high yield.
[0047]
The artificial graphite particles A of the negative electrode used in the lithium secondary battery of the present invention, in addition to the use of such fusible bulk mesophase pitch pulverized particles as a base material, as a binder for kneading and granulation. Also, a pitch and / or thermosetting resin having low meltability in the post-process is used as a starting material. Also in this case, during carbonization and graphitization, strong fusion of the particles does not occur, and it is possible to omit a pulverizing step in the middle.
[0048]
Therefore, the artificial graphite particles A thus obtained are considered to be the most suitable from the viewpoint of improving the cycle life characteristics of the lithium secondary battery of the high energy density design as described above and improving the safety (negative electrode heat resistance). . That is, in the particles of the artificial graphite particles A, the graphite crystals are oriented in random directions, the particles have a small specific surface area, and the degree of cracking / collapse of the particles accompanying the progress of the charge / discharge cycle is small. In addition, the bulk mesophase pitch of the base material is easily graphitizable, and the graphite layer structure is more easily developed when graphitized than coke. This is the same in comparison with needle coke, which is particularly likely to be graphitized. Therefore, the artificial graphite particles A can be used as a graphite active material capable of realizing higher capacity.
[0049]
Such artificial graphite particles A tend to have a higher bulk density (or tap density) than the graphite particles prepared using coke as a base material. The artificial graphite particles A are relatively large, particularly by granulation. Therefore, an appropriate amount of spherical graphite particles B having a large circularity is added so as to fill the voids of the graphite particles A, and a negative electrode active material is produced. And the paste containing this is apply | coated on a copper core material. In this way, the particles can be packed in the closest packing, and the graphite particles B also improve the slipperiness between the particles. Therefore, 1.6 g / cm ³ Even if the material is rolled to a high density up to the maximum value, the mixture layer hardly falls off. In addition, an extremely good high-density negative electrode can be obtained also in terms of permeation (impregnation) of the electrolyte.
[0050]
In addition, the artificial graphite particles A used in the present invention accept Li ions during high-rate charging or low-temperature charging, compared to graphite particles produced using coke as a base material and other general artificial graphite particles derived from mesophase carbon. Performance tends to be high. Therefore, the lithium secondary battery of the present invention can also be expected to have a secondary improvement effect such as excellent high-rate charge / discharge cycle characteristics at low temperatures.
[0051]
Next, the artificial graphite particles A that provide a negative electrode mixture that can be rolled with high density will be described. Such artificial graphite particles A satisfy predetermined physical properties required by a micro compression tester. Its physical properties can be determined as follows.
[0052]
First, artificial graphite particles A, PVDF, and NMP are mixed to prepare a slurry. It is preferable that the content of artificial graphite particles A in the slurry is 40 to 60% by weight, the content of PVDF is 2 to 12% by weight, and the content of NMP is 38 to 58% by weight. Next, the slurry is applied to the base material with a doctor blade having a predetermined gap, and the obtained coating film of the mixture is compressed by a micro compression tester. It can be said that the larger the amount of displacement of the coating thickness at that time, the easier the graphite can be rolled.
[0053]
An example of a method for measuring the amount of displacement will be described in more detail.
First, 45 parts by weight of artificial graphite particles A, 5 parts by weight of PVDF, and 50 parts by weight of NMP are mixed to prepare a slurry. Next, this slurry is applied on an electrolytic copper foil (thickness: 10 μm) spread on a glass plate with a doctor blade having a gap of 135 μm. Then, the coating film is dried in a dryer at 80 ° C. The thickness of the dried coating film is, for example, about 100 μm. Subsequently, an indenter having a diameter of 500 μm is attached to the micro compression tester, a load of 200 gf is applied to the dried coating film, and the displacement (compression amount) of the coating film thickness is measured. The artificial graphite particles A having a displacement of the coating film thickness measured by such a method of 25 μm or more give a negative electrode mixture which can be rolled particularly at high density.
In addition, as a micro compression tester, MCTM-500 made by Shimadzu Corporation can be used.
[0054]
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention relates to a lithium secondary battery using a negative electrode in which an active material composed of a mixture of artificial graphite particles A and spherical graphite particles B having a large circularity is fixed on a copper core material. The artificial graphite particles A are obtained by kneading and granulating a base material prepared by pulverizing a bulk mesophase pitch and a pitch and / or thermosetting resin in a softened state, and obtaining the granulated material at 700 to 1500 ° C. And carbonized at 2,500-3000 ° C.
[0055]
In the method for producing the artificial graphite particles A, it is preferable that the average particle diameter of the base material produced by pulverizing the bulk mesophase pitch is 7 to 20 μm. The pitch and / or the amount of the thermosetting resin is preferably 8 to 25 parts by weight per 100 parts by weight of the base material.
[0056]
According to the present invention, the graphite structure of the artificial graphite particles A as the main active material of the negative electrode is sufficiently developed, the graphite crystals are oriented in random directions, the BET specific surface area is small, and the charge / discharge cycle is small. It is possible to design a lithium secondary battery having a high energy density that greatly exceeds 350 Wh / L due to the fact that the degree of cracking and collapsing of particles accompanying the progress of particles is small. At the same time, there is provided a lithium secondary battery in which the deterioration of the battery capacity due to the decomposition of the electrolytic solution on the negative electrode due to the progress of the charge / discharge cycle is very small, and the safety (heat resistance performance, etc.) is secured at a high level. It becomes possible.
[0057]
As a preferred embodiment of the lithium secondary battery of the present invention, a negative electrode, a positive electrode, an electrode group formed by winding a substantially square pillar or an elliptical pillar via a separator, a square metal case or aluminum foil and a resin film And a non-aqueous electrolyte injected into the case. Also in the lithium secondary battery of such a form, due to the characteristics of the artificial graphite particles A, the lithium secondary battery has a high energy density and suppresses the deterioration of the battery capacity due to the progress of the charge / discharge cycle. Further, a phenomenon that is specific to such a battery, such as a phenomenon in which a battery case is deformed in a thickness direction due to a gas generated by decomposition of an electrolytic solution, and a deformation of an electrode group caused by expansion and contraction of a negative electrode mixture layer ( Group buckling) can be suppressed at a high level. At the same time, the safety (heat resistance and the like) of the battery can be secured at a high level.
[0058]
In the lithium secondary battery of the present invention, it is preferable to use a rubbery binder containing a butadiene unit as the binder of the negative electrode mixture. In addition, it is preferable to use a cellulose-based thickener at the time of kneading the negative electrode mixture. That is, a negative electrode in which an active material composed of a mixture of artificial graphite particles A and spherical graphite particles B having a large circularity is fixed on a copper core material by the action of a rubbery binder and a cellulose-based thickener is used. Is preferred.
[0059]
As the cellulose-based thickener, a polymer composed of cellulose or various cellulose derivatives can be used. Among them, carboxymethylcellulose (CMC) is preferred because a small amount can give high viscoelasticity. Further, it is preferable to use styrene-butadiene copolymer rubber (SBR) or the like as the rubbery binder containing a butadiene unit.
[0060]
In general, there are two methods for manufacturing a negative electrode of a lithium secondary battery. One is a method in which an aqueous paste obtained by adding SBR or the like as a binder, CMC or the like as a thickener, and an appropriate amount of water to graphite active material particles is applied to a core material. The other is an organic paste in which graphite active material particles are added with polyvinylidene fluoride (PVDF) or the like as a binder or a thickener and an appropriate amount of N-methyl-2-pyrrolidone (NMP) or the like as a dispersion medium. Is applied to the core material.
[0061]
The former aqueous paste has relatively poor rheological stability during kneading and coating. However, many studies have revealed that when a lithium secondary battery is manufactured using the obtained negative electrode, the reductive decomposition reaction of the electrolytic solution on the graphite active material particles is suppressed. For example, it is very effective to use an aqueous paste to reduce the initial irreversible capacity of the negative electrode. In addition, when the battery in a charged state is stored at a high temperature, reductive decomposition (gas generation) of the electrolytic solution on the negative electrode and gas generation reaction accompanying the charge / discharge cycle of the battery are suppressed. It is very effective to use a paste. Related descriptions are also found in JP-A-2001-076731 and the like. Therefore, when the battery characteristics are more important than the productivity, it is preferable to use an aqueous paste for producing the negative electrode.
[0062]
Next, the physical properties of the artificial graphite particles A and the spherical graphite particles B used in the present invention will be described.
First, a substrate prepared by pulverizing bulk mesophase pitch and a pitch and / or thermosetting resin in a softened state are kneaded and granulated, and the obtained granulated material is carbonized at 700 to 1500 ° C. The artificial graphite particles A obtained by further graphitizing at 2500 to 3000 ° C. will be described.
[0063]
The artificial graphite particles A are isotropic artificial graphite particles in which a graphite structure is randomly oriented in the particles. And the physical properties are
(1) Interplanar spacing d of (002) plane obtained by powder X-ray diffraction method ₀₀₂ Is less than 3.362Å,
(2) Density 1.6 g / cm ³ Intensity I attributed to the (002) plane in the diffraction pattern when pellets are formed and X-ray diffraction measurement is performed ₀₀₂ And peak intensity I attributed to the (110) plane ₁₁₀ Ratio with: I ₀₀₂ / I ₁₁₀ Is less than 1000,
(3) an average particle circularity of 0.85 to 0.95,
(4) Particle size D at a volume fraction of 50% measured using a laser diffraction type particle size distribution analyzer ₅₀ Is 15 to 30 μm and the particle diameter D at a volume fraction of 10%. ₁₀ And the particle diameter D at a volume fraction of 90% ₉₀ Ratio with: D ₁₀ / D ₉₀ Is 0.2-0.5,
(5) Tap density is 1 g / cm ³ that's all,
(6) Specific surface area measured by BET method is 1 m ² / G or less.
[0064]
Here, the particle diameter D when the volume fraction is x% _x Is determined from a volume-based particle size distribution expressed in an ab coordinate system in which the horizontal axis a represents the particle diameter and the vertical axis b represents the number of particles. In the particle size distribution, when the volume is integrated from particles having a small value a, the value a at which the cumulative volume becomes x% of the total is the particle diameter D. _x It becomes.
[0065]
For the granulated material, the spacing d of the (002) plane ₀₀₂ By performing sufficient graphitization to reach 3.362 ° or less, high-capacity graphite particles having a reversible capacity exceeding 340 mAh / g can be obtained.
[0066]
The average particle circularity and particle diameter D of the graphite particles A are adjusted by adjusting the particle size of the base material, the mixing ratio of the binder (pitch and / or thermosetting resin), kneading and granulation conditions, and the like. ₅₀ And ratio: D ₁₀ / D ₉₀ Is adjusted to the above range, it is possible to provide a negative electrode mixture which is excellent in handleability of kneading and coating and excellent in electrolyte permeability (impregnation) when a high-density electrode is formed.
[0067]
When the particle image is projected on a plane, the particle circularity is a ratio of a peripheral length 1 of an equivalent circle having the same area as the particle projected image to a peripheral length L of the particle projected image: 1 / L. Given.
[0068]
From the viewpoint of producing a high density negative electrode without damaging the particles by forming a mixture coating film at high density on a copper core material and rolling it with a weak press pressure, artificial graphite particles A Is preferably large. Specifically, the tap density is 1 g / cm ³ The above is used.
[0069]
Here, the tap density is a value obtained when 900 tappings are performed. The tap density changes depending on measurement conditions such as the number of tappings. For example, tapping about 100 times is not enough, but tapping 300 to 500 times reaches a substantially constant value. Therefore, tapping 900 times is enough for the tap density to reach a certain value.
[0070]
Further, in the present invention, the specific surface area, the initial irreversible capacity, and the correlation with the thermal stability of the negative electrode (such as the heat resistance of the charged negative electrode) are measured using the BET method in the present invention. The specific surface area of the artificial graphite particles A is 1 m ² / G is controlled at a very low level.
[0071]
Here, the graphite structure is sufficiently developed, the graphite crystals are oriented in random directions, the average particle diameter and the particle diameter are in the above ranges, and the BET specific surface area is 1 m. ² It is considered that the artificial graphite particles suppressed to not more than / g at present can only be obtained by the above production method. That is, as far as the present inventors have studied, the base material prepared by pulverizing the bulk mesophase pitch, and the pitch and / or thermosetting resin in a softened state were kneaded and granulated, and were obtained. It is noteworthy that only particles obtained by carbonizing the granulated material at 700 to 1500 ° C. and further graphitizing at 2500 to 3000 ° C. satisfy all of the above physical properties.
[0072]
When such graphite particles are used as the main active material, the reversible capacity is large, the initial irreversible capacity is small, the charge / discharge rate characteristics are excellent, the expansion / contraction accompanying charge / discharge is small, the cycle life characteristics are excellent, and the high An almost ideal negative electrode that also has safety can be obtained.
[0073]
Next, the physical properties of the spherical graphite particles B are as follows:
(1) an average particle circularity of 0.88 to 1,
(2) Particle diameter D at a volume fraction of 50% measured using a laser diffraction type particle size distribution analyzer ₅₀ Is 5 to 15 μm,
(3) Plane spacing d of (002) plane obtained by powder X-ray diffraction method ₀₀₂ Is less than 3.357Å
(4) The specific surface area measured using the BET method is 8 m ² / G or less.
[0074]
Such graphite particles can be produced, for example, by classifying certain types of agglomerated natural graphite particles or artificial graphite particles and removing coarse powder. As the spherical graphite particles B, the spacing d of the (002) plane ₀₀₂ If the graphitized structure of which is not more than 3.357 ° is very developed, a large reversible capacity of graphite can be expected. At the same time, electrostatic repulsion (interaction of π electrons) between particles, which is unique to graphite, can also be drawn out. Therefore, if spherical graphite particles B are combined with artificial graphite particles A to form a negative electrode active material, the density of the negative electrode mixture increases. Roll forming can be facilitated.
[0075]
In the present invention, the spherical graphite particles B are not scaly particles, but have a very high average particle circularity of 0.88 to 1 and a particle diameter D. ₅₀ Is characterized in that a material having a range of 5 to 15 μm is used. By using such particles having a high circularity, the orientation of the spherical graphite particles B in the vicinity of the surface of the mixture during rolling to a high density is suppressed, and the electrolyte is sufficiently penetrated (impregnated) into the inside of the mixture. )
[0076]
The spherical graphite particles B having the above physical properties are preferably mixed with the artificial graphite particles A so that the mixing ratio of the spherical graphite particles B to the entire active material is in the range of 5 to 45% by weight. Within such a range, it is possible to fill the voids between the graphite particles A as the main material with the spherical graphite particles B when forming the mixture layer (coating the mixture on the copper core material). It becomes. Therefore, it is possible to densely fill the spherical graphite particles B, and it is possible to obtain a mixture layer which is particularly rolled and formed at a high density.
[0077]
The lower the BET specific surface area of the spherical graphite particles B, the better. When the average particle diameter is reduced to the above range, the BET specific surface area has a somewhat large value. However, in order to obtain the effect of the present invention, the upper limit of the BET specific surface area of the spherical graphite particles B is set to 8 m. ² / G.
[0078]
The spherical graphite particles B are preferably natural graphite particles and / or surface-modified natural graphite particles. Particularly when the spherical graphite particles B are those derived from natural graphite, the largest reversible capacity can be expected from the graphite particles B. Also, depending on the place of origin and the degree of the impurity grade, natural graphite is generally less expensive than artificial graphite which requires a graphitization step. Therefore, it is advantageous in terms of cost to use spherical graphite particles B derived from natural graphite.
[0079]
From the viewpoint of the safety (heat resistance) of the graphite negative electrode, as described above, the BET specific surface area of the spherical graphite particles B is preferably as low as possible. Therefore, it is most preferable to use natural graphite particles that have been subjected to a modification treatment (coating treatment or the like) for smoothing the particle surface. Examples of a method for smoothing the particle surface include a method in which the particle surface is coated with an organic substance such as tar or pitch and carbonized or graphitized. Further, a method of applying a strong mechanical impact / shear force or the like to the graphite particles and making the surface amorphous by a mechanochemical reaction to smooth the surface is also included.
[0080]
In the negative electrode mixture (or the aqueous paste), the optimal addition amount of the rubbery binder containing a butadiene unit is 3% by weight or less based on the weight of the active material. A rubbery binder containing a butadiene unit is often added to a paste as an aqueous dispersion of rubber fine particles. The amount of the binder to be added is usually determined in consideration of the binding strength between the mixture layer and the copper core. However, since the rubber fine particles are basically an insulator, if added in excess, the surface layer of the graphite active material particles will be covered with the insulator particles, and the charge / discharge characteristics of the negative electrode will be impaired. In consideration of the average particle size, particle size, and BET specific surface area of the graphite active material particles to be used, in the present invention, it is desired to suppress the addition amount of the rubbery binder to 3% by weight or less.
[0081]
The optimal mode of the negative electrode mixture layer formed on the copper core material is that the density of the negative electrode mixture layer is 1.6 to 1.8 g / cm. ³ And the thickness of the negative electrode mixture layer is 40 to 100 μm. The density of the negative electrode mixture layer is 1.6 to 1.8 g / cm. ³ By setting as high as possible, it is possible to design a lithium secondary battery with a high energy density that greatly exceeds 350 Wh / L. Here, the reason why the thickness of the negative electrode mixture layer is set to 40 to 100 μm is that when the thickness of the mixture layer exceeds 100 μm, diffusion of Li ions into the active material particles of the inner layer of the mixture becomes difficult, and the charge / discharge rate is increased. This is because the characteristics are deteriorated. Conversely, if the thickness of the mixture layer is reduced to less than 40 μm, the D of the graphite active material particles A ₉₀ The value needs to be reduced to about 30 μm or less. In the artificial graphite particles A used through the granulation process and used as the main active material in the present invention, it is practically difficult to adjust the particle size so far.
[0082]
【Example】
First, a method for measuring the physical properties of the graphite active material particles (powder) used in this example will be described.
(1) Spacing d of (002) plane ₀₀₂ Measurement
A powder X-ray diffractometer “RINT2000 / PC” manufactured by Rigaku Corporation was used. The carbon powder to which high-purity silicon was added as an internal standard was irradiated with monochromatic X-rays, and the peak corresponding to the (002) plane of graphite was measured. Then, by correcting the peak position based on the silicon peak of the internal standard, d corresponding to the graphite layer interval is obtained. ₀₀₂ Was calculated. The specific evaluation method was based on the method prescribed by the 117th Committee of the Japan Society for the Promotion of Science.
[0083]
(2) Peak intensity ratio I ₀₀₂ / I ₁₁₀ Measurement
The graphite powder is placed in a specific holder, and the density is 1.6 g / cm using a flat pressure press. ³ Into pellets. The pellet was irradiated with X-rays using the same X-ray diffractometer as in (1) above, and the diffraction pattern was measured. Then, the ratio of the peak intensity corresponding to the (002) plane to the peak intensity corresponding to the (110) plane: I ₀₀₂ / I ₁₁₀ Was calculated. Here, the peak height was used as the peak intensity.
[0084]
(3) Measurement of particle circularity
Using a scanning electron microscope “S-2500” manufactured by Hitachi, Ltd., an image of the graphite particles (powder) at a magnification of 1000 was obtained. Then, the perimeter l of the equivalent circle having the same area as the projected image of the observed particle was obtained. The ratio of the perimeter l to the perimeter L of the projected particle image: l / L was determined for 50 particles, and the average value was defined as the average particle circularity. In addition, such a measurement can also be implemented using a flow-type particle image analyzer. For example, it was experimentally confirmed that even when the particle circularity was measured using a powder measuring device (FPIA-1000) sold by Hosokawa Micron Corporation, almost the same value was obtained.
[0085]
(4) Particle size at 50% volume fraction (D ₅₀ ), Particle size at a volume fraction of 10% (D ₁₀ ) And the particle size at 90% volume fraction (D ₉₀ ) Measurement
About 1 cc of a 2% by volume aqueous solution of polyoxyethylene sorbitan monoureate was prepared as a surfactant. This surfactant was previously mixed with graphite particles (powder). Then, using ion-exchanged water as a dispersion medium, and using a laser diffraction type particle size distribution analyzer “LA-700” manufactured by Horiba, Ltd., the particle diameter (that is, the average particle diameter) at a volume fraction of 50% D ₅₀ (Median), particle diameter D at a volume fraction of 10% ₁₀ And the particle diameter D at a volume fraction of 90% ₉₀ Got.
[0086]
(5) Measurement of tap density
The tap density of the graphite powder was basically measured according to the following procedure according to JIS-K5101.
Using "Powder Tester PT-R" manufactured by Hosokawa Micron Co., Ltd., a sieve having openings of 200 μm was used as a sieve through which the sample passed. The graphite powder was dropped into a 20 cc tapping cell, and after the cell was completely filled, tapping with a stroke length of 18 mm was performed 900 times once / second. And the tap density at that time was measured.
[0087]
(6) Measurement of BET specific surface area
"AMS-8000" manufactured by Okura Riken Co., Ltd. was used. As preliminary drying, the graphite powder was heated to 350 ° C. and exposed to a nitrogen gas flow for 15 minutes. Then, the specific surface area was measured by the BET one-point method at a relative pressure of 0.3 by nitrogen gas adsorption.
[0088]
(Production of negative graphite particles)
In this example, the negative graphite particles obtained by the following procedure were examined.
1. Artificial graphite particles A1
Coal tar was placed in a vacuum distillation apparatus, the pressure was reduced, and the mixture was heated and stirred at 350 ° C. in the presence of nitric acid to promote the increase in tar molecular weight. Thereafter, this was heated at 500 ° C. to form a mesophase, thereby obtaining a bulk mesophase pitch having a small volatile content.
The bulk mesophase pitch was taken out of the apparatus after cooling, and pulverized by a rotary impact type pulverizer (fine mill) so as to have an average particle diameter of 13 μm to obtain a substrate.
[0089]
Next, 15 parts by weight of coal tar pitch (softening point: 80 ° C.) as a binder was mixed with 85 parts by weight of the base material, and kneaded at 200 ° C. for 120 minutes in a Z-type kneader. In this process, the mixture gradually increased in viscosity and became particulate.
[0090]
The obtained granules are taken out of the kneader after cooling, crushed, and then put into a graphite crucible, and carbonized (fired) in a lead hammer type continuous firing furnace at 800 ° C. in a nitrogen atmosphere. ).
[0091]
Further, the carbonized particles were placed in a graphite crucible and were graphitized in an Acheson type graphitization furnace at 2950 ° C. in a nitrogen atmosphere. Thereafter, crushing and classification are performed to obtain a particle diameter D at a volume fraction of 50% ₅₀ Was 23 μm.
[0092]
2. Artificial graphite particles A2
Except for using a phenol resin instead of coal tar pitch as a binder, the particle diameter D at a volume fraction of 50% was obtained under the same process and conditions as in the case of the artificial graphite particles A1. ₅₀ Was 23 μm.
[0093]
3. Spherical graphite particles B1
Chinese scale-like natural graphite is crushed by a counter-type jet mill to obtain an average particle diameter D. ₅₀ Was 20 μm scaly natural graphite. This was introduced into another counter-type jet mill, the operating conditions were adjusted, and the particles were collided in a high-speed air flow to control the shape (sphericalization) of the graphite particles. Then, in order to remove impurities (ash), after washing with a hydrofluoric acid aqueous solution, the graphite particles are dried, and further subjected to strong air classification to remove coarse powder. ₅₀ Was obtained about 10 μm in spherical graphite particles B1.
[0094]
4. Spherical graphite particles B2
The acicular coke (anisotropic coke) powder whose average particle diameter was adjusted to 20 μm was put in a graphite crucible and was graphitized at 2950 ° C. in an Acheson type graphitization furnace. Thereafter, the graphitized particles are introduced into the same counter-type jet mill as in the case of the spherical graphite particles B1, and the operating conditions are adjusted. Shape control (sphericalization) was performed. Then, strong air classification is performed on this to remove coarse powder, and D ₅₀ Was obtained about 10 μm in spherical graphite particles B2.
[0095]
5. Comparative artificial graphite particles C1 (artificial graphite with randomly oriented crystals)
20 parts by weight of tar pitch and 15 parts by weight of coal tar as a binder were mixed with 50 parts by weight of a base material of acicular coke (anisotropic coke) powder whose average particle diameter was adjusted to 8 μm, and heated and kneaded in a mixer. And granulated.
[0096]
The obtained granules were taken out of the mixer after cooling, crushed, made into blocks by isotropic pressure molding, and carbonized (fired) at a temperature of 800 ° C. Further, it is graphitized in a graphitization furnace at 2950 ° C., then pulverized by a mill and classified to obtain a particle diameter D at a volume fraction of 50%. ₅₀ Was 21 μm.
[0097]
6. Comparative artificial graphite particles C2 (increased degree of graphitization of artificial graphite in which crystals are randomly oriented)
20 parts by weight of tar pitch as a binder, 15 parts by weight of coal tar, and carbonization as a catalyst for promoting graphitization were added to 50 parts by weight of a base material of acicular coke (anisotropic coke) powder whose average particle diameter was adjusted to 8 μm. Boron (B ₄ C) was mixed in an amount of 5 parts by weight, heated and kneaded in a mixer, and granulated.
[0098]
The obtained granules were taken out of the mixer after cooling, crushed, made into blocks by isotropic pressure molding, and carbonized (fired) at a temperature of 800 ° C. Further, this was graphitized at 2800 ° C. in a graphitization furnace in an argon atmosphere, then pulverized by a pin mill and classified to obtain a particle diameter D at a volume fraction of 50%. ₅₀ Was 21 μm.
[0099]
7. Comparative artificial graphite particles D (graphitized milled MCF)
A bulk mesophase pitch (derived from petroleum pitch) in a molten state under a 360 ° C. atmosphere was spun by a melt blow method, and infusibilized by surface oxidation. Next, carbonization (firing) was performed at 800 ° C. to obtain a mat-like carbide. This was cut and pulverized by a high-speed rotating mill, and coarse powder was removed by a vibrating sieve to obtain a carbon fiber mill having an average particle diameter of 18 μm (milled MCF: MCF stands for mesocarbon pitch-based fiber). Then, as a catalyst for promoting graphitization, boron carbide (B ₄ C) 5 parts by weight were added and graphitized at 2800 ° C. in a graphitization furnace in an argon atmosphere to obtain comparative artificial graphite particles D.
[0100]
8. Comparative artificial graphite particles E (graphitized MCMB)
The coal tar was heated at 350 ° C. to produce mesophase spherules. After adding a solvent thereto, small spheres were separated and extracted using a filter press. Next, the small spheres were carbonized (fired) at 800 ° C., and then classified by a vibrating sieve to obtain mesocarbon microbeads (MCMB) having an average particle diameter of 26 μm. Then, MCMB was graphitized at 2950 ° C. in an Acheson type graphitization furnace, and then crushed and classified to obtain comparative artificial graphite particles E.
[0101]
9. Comparative artificial graphite particles F (graphitized bulk mesophase)
A bulk mesophase pitch is produced by a process similar to that described in the first half of the case of the artificial graphite particles A1, and this is crushed by a mill to obtain a particle diameter D at a volume fraction of 50%. ₅₀ Was adjusted to 20 μm. Thereafter, the particles were carbonized (fired) at 800 ° C. This was placed in a graphite crucible and graphitized in an Acheson type graphitization furnace at 2950 ° C., and then crushed and classified to obtain comparative artificial graphite particles F.
[0102]
10. Agglomerated natural graphite particles G
The flake natural graphite from Sri Lanka is compacted and then pulverized by a mill to obtain a particle diameter D at a volume fraction of 50%. ₅₀ Was 21 μm scaly natural graphite. The scaly natural graphite was washed with hydrofluoric acid to remove impurities (ash), and then dried. Next, the flake-like natural graphite particles were subjected to spheroidization and air classification by a hybridization system to obtain agglomerated natural graphite G. Here, the term “hybridization system” refers to a method in which particles are placed in a chamber and the shape is adjusted by applying an impact / shear force by high-speed rotation.
[0103]
11. Surface-coated natural graphite particles H
5 parts by weight of petroleum pitch was mixed with 100 parts by weight of the agglomerated natural graphite G and kneaded in a heated mixer to attach the pitch to the surface of the graphite G. This was heat-treated (annealed) in a firing furnace at 1300 ° C. to carbonize the pitch attached to the surface of graphite G. Subsequently, crushing and classification were performed to obtain surface-coated natural graphite H.
Table 1 summarizes powder physical property data measured for these 11 types of graphite particles.
[0104]
[Table 1]

[0105]
When a negative electrode of a lithium secondary battery is manufactured using the graphite particles shown here, any one of them can be used alone as a negative electrode active material, or two or more thereof can be mixed at a predetermined ratio to prepare a negative electrode active material. It can also be used for substances.
[0106]
The present inventors have studied various graphite particles so far. According to the empirical rule, the graphite mixture layer applied on the copper core material is rolled by a roll press or the like, and the mixture density is 1.6 g / cm. ³ In the case of producing a high-density negative electrode exceeding the above, when the special artificial graphite particles derived from the mesophase carbon of D, E and F are used alone as an active material, the density cannot be increased to a predetermined density. There are many. This is considered to be because the surface layer of the mesophase particles was substantially infusible (loose oxidation treatment) at a stage prior to the carbonization / graphitization step in the manufacturing process for each of these materials. That is, the particle surface layer is in a state close to amorphous where graphitization has not progressed much. Amorphous carbon that does not have a layered structure is poor in slipperiness because there is little electrostatic repulsion (interaction of π electrons) between particles peculiar to the graphite layered structure.
[0107]
In a preliminary study of the graphite particles A1 and A2 used in the present invention, a compression test was performed on a coating film simply formed on a copper core material. As a result, although not as high as D, E, and F, data was obtained that it was relatively difficult to increase the density. Therefore, when the graphite particles A1, A2, D, E and F are used as an active material, it is considered essential to add spherical graphite particles B1 or B2 having a large circularity to these. Therefore, the negative electrode active materials a to v (total 22 types) having the mixing ratio (weight ratio) shown in Table 2 were examined.
[0108]
When only the graphite particles A1 or A2 are used alone, even if a high-density negative electrode can be produced, the particles are relatively likely to fall off the electrode plate due to the repetition of the charge / discharge cycle, and the cycle characteristics are poor. It is considered that it easily deteriorates. This is because it is expected that since the particles are relatively hard, stress cannot be dispersed when the negative electrode mixture layer expands and contracts due to charge and discharge.
On the other hand, when the spherical graphite particles B1 or B2 are used alone, it is considered that a capacity deterioration due to repetition of charge / discharge cycles and a safety problem due to a large surface area occur.
[0109]
[Table 2]

[0110]
(Preparation of negative electrode)
To 100 parts by weight of the negative electrode active material a, 100 parts by weight of a 1% by weight aqueous solution of carboxymethyl cellulose (CMC) and an aqueous dispersion of styrene butadiene rubber (SBR) as a binder are added, and the mixture is sufficiently kneaded. An agent slurry was prepared. Here, the addition amount of SBR was adjusted such that the ratio of the solid content (rubber component) to 100 parts by weight of the negative electrode active material a was 2 parts by weight.
[0111]
The slurry thus prepared was applied to both surfaces of a copper foil (thickness: 10 μm) to a constant thickness using a coating machine, dried with hot air at 100 ° C., and then rolled using a roll press. Here, the density of the mixture layer (value including the weight of CMC and SBR) is 1.7 g / cm. ³ Then, the thickness was adjusted to be 70 μm (the thickness of the electrode was about 150 μm). Then, this was cut into a predetermined size, and a nickel lead for current collection was attached to obtain a negative electrode a. Also for the negative electrode active materials b to v, negative electrodes b to v corresponding to the signs of the respective negative electrode active materials were produced under the same conditions as above.
[0112]
(Preparation of positive electrode)
In this study, Co was used as the positive electrode active material. ₃ O ₄ And Li ₂ CO ₃ LiCoO produced by baking the mixture at 950 ° C. in the air atmosphere and then pulverizing and adjusting the particle size ₂ It was used. In preparing a positive electrode plate, 3 parts by weight of acetylene black (AB) as a conductive material is added to 100 parts by weight of a positive electrode active material, and the mixture is sufficiently mixed and dispersed in a dry mixer, and then polyvinylidene fluoride as a binder is added. (PVDF) was added in an amount of 5 parts by weight, and kneaded while appropriately adding N-methyl-2-pyrrolidone (NMP) as a solvent to prepare a mixture slurry.
[0113]
The slurry thus prepared was applied to both sides of an aluminum foil (thickness: 20 μm) to a constant thickness using a coating machine, dried with dry air at 100 ° C., and rolled using a roll press. Here, the density of the mixture layer (value including the weight of AB and PVDF) is 3.7 g / cm. ³ The thickness was adjusted so as to be 70 μm (the thickness of the electrode was about 160 μm). Then, this was cut into a predetermined size, and an aluminum lead for current collection was attached to obtain a positive electrode.
[0114]
(Production of lithium secondary battery)
Negative electrode a, LiCoO prepared by the above procedure ₂ Vacuum drying of the positive electrode and a polyethylene porous membrane separator (thickness: 25 μm) for physically separating the two was performed for the purpose of removing excess water. The negative electrode and the positive electrode were vacuum-dried at 100 ° C. for 8 hours, and the separator was vacuum-dried at 50 ° C. for 12 hours.
[0115]
Subsequently, the negative electrode a and the positive electrode were wound with a separator interposed therebetween, to form an electrode group 1 having a substantially quadrangular prism shape (having a substantially rectangular cross section) as shown in FIG. The electrode group 1 having a substantially square pole shape was inserted into a battery case 4 made of a rectangular aluminum alloy having a size of 533048 (thickness 5.3 mm × width 30 mm × height 48 mm). Then, the positive electrode lead 2 was welded to the upper sealing plate 5, and the negative electrode lead 3 was welded to the negative electrode terminal 6 electrically separated from the sealing plate by an insulating gasket. Thereafter, the sealing plate 5 was joined to the battery case 4 by laser welding. Subsequently, a non-aqueous electrolyte was injected from an injection port provided in the sealing plate 5, and the electrode group 1 was impregnated in vacuum.
[0116]
Then, partial charging was performed for the first time while the injection port was kept open. In the initial stage of the first charge, decomposition of the electrolytic solution or the like occurs with the formation of the film on the negative electrode, and gas is generated. This gas was sufficiently diffused and removed. Thereafter, the injection port was closed with a plug 7 made of an aluminum alloy, which was melted with a laser and then solidified to seal the injection port. Thus, a lithium secondary battery a (design capacity: 800 mAh) using the negative electrode a was obtained.
[0117]
Also, lithium secondary batteries b to v corresponding to the respective negative electrodes were manufactured under the same conditions as above except that the negative electrodes a to b were used instead of the negative electrodes a. Here, the dew point of each process of the configuration of the electrode plate group, welding of the positive and negative electrode leads, joining of the sealing plate to the case, injection and impregnation of electrolyte, initial partial charging, and sealing by plugging The test was performed in a dry air atmosphere at -40 ° C or lower.
[0118]
The non-aqueous electrolytic solution contains 1.0 M (M: mol) in a solvent obtained by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) at a volume ratio of 1: 2: 1. / L) so that the concentration of LiPF ₆ Was used. After the injection of the electrolytic solution, the first partial charging of the battery was performed in a 20 ° C. atmosphere for 2 hours at a charging rate of 0.1 C (here, 80 mA assuming that 1 C = 800 mA).
[0119]
(Evaluation of battery characteristics)
The following battery characteristics were evaluated for the lithium secondary battery prepared above. (1) Measurement of irreversible capacity
Three charge / discharge cycles were performed on the 22 types of lithium secondary batteries prepared above in the following pattern.
・ Charging: Constant current method 0.2C (160mA), final voltage 4.1V
-Discharge: constant current 0.2C (160mA), discharge cut voltage 3.0V
・ Ambient temperature: 20 ℃
Then, the initial irreversible capacity of each battery was calculated by the following calculation.
Initial irreversible capacity = {(initial partial charge capacity at the time of battery production: 160 mAh) + (total charge capacity of the above three cycles) − (total discharge capacity of the above three cycles)} / 3
[0120]
(2) High rate discharge characteristics
The 22 types of lithium secondary batteries for which the measurement of the irreversible capacity was completed were subjected to the following charge / discharge test, and the discharge capacity ratio (2C discharge capacity C ₂ And 0.2C discharge capacity C _0.2 Ratio with: C ₂ / C _0.2 ) Was calculated to evaluate the high rate discharge characteristics of each battery. Here, the ambient temperature at the time of the test was 20 ° C.
[0121]
1st cycle (0.2C discharge)
・ Charging: Constant current constant voltage method 0.7C (560mA), charge control voltage 4.2V, total charge time 2.5 hours
-Discharge: constant current 0.2C (160mA), discharge cut voltage 3.0V
2nd cycle (2C discharge)
・ Charging: Constant current constant voltage method 0.7C (560mA), charge control voltage 4.2V, total charge time 2.5 hours
-Discharge: constant current 2C (1600mA), discharge cut voltage 3.0V
[0122]
(3) Low temperature discharge characteristics
For the 22 types of lithium secondary batteries for which the measurement of the irreversible capacity was completed, the following charge / discharge test different from (2) was performed, and the ratio of the discharge capacity (discharge capacity C at 1 C at -10 ° C.) _-10 And discharge capacity C at 1 C under 20 ° C. ₂₀ Ratio with: C _-10 / C ₂₀ ) Was calculated to evaluate low-temperature discharge characteristics.
[0123]
1st cycle (20 ° C)
・ Charging: Constant current constant voltage method 0.7 C (560 mA), charge control voltage 4.2 V, total charge time 2.5 hours, ambient temperature 20 ° C.
-Discharge: constant current 1C (800mA), discharge cut voltage 2.5V
(The capacity is calculated based on the amount of discharge up to 3.0 V)
Atmospheric temperature 20 ° C
[0124]
2nd cycle (-10 ° C)
・ Charging: Constant current constant voltage method 0.7 C (560 mA), charge control voltage 4.2 V, total charge time 2.5 hours, ambient temperature 20 ° C.
-Discharge: constant current 1C (800mA), discharge cut voltage 2.5V
(The capacity is calculated based on the amount of discharge up to 3.0 V)
Ambient temperature -10 ° C
[0125]
(4) Cycle life characteristics
The following charge / discharge was repeated for 500 cycles for the 22 lithium secondary batteries for which the measurement of the irreversible capacity was completed. And the capacity C at the time of 500 cycles ₅₀₀ And capacity C of the first cycle _ini And the capacity maintenance rate (C ₅₀₀ / C _ini ). In addition, the swelling (expansion) of the battery case in the thickness direction due to the cycle, which appears as a phenomenon peculiar to the rectangular lithium secondary battery as described above, was also measured as the swelling amount (mm) of the case from the beginning.
[0126]
・ Charging: Constant current constant voltage method 0.7C (560mA), charge control voltage 4.2V, total charge time 2.5 hours
・ Pause after charging: 30 minutes
-Discharge: constant current 0.7C (560mA), discharge cut voltage 3.0V
・ Pause after discharge: 30 minutes
・ Evaluation atmosphere temperature: 20 ℃
[0127]
Table 3 summarizes the results of the above battery evaluation. In Table 3, the value of the evaluation result of the battery a was standardized as 100, and the performances of the batteries b to v were relatively compared. In Table 3, the batteries a to d according to the present invention are clearly superior in all characteristics as compared with the batteries q to v using the negative electrode mainly composed of natural graphite. In addition, there is a disadvantage that the irreversible capacity is slightly larger than batteries ep using an anode mainly composed of artificial graphite, but other characteristics such as a discharge rate ratio, a low-temperature discharge characteristic, and a capacity retention rate at 500 cycles. In the above, the batteries a to d are excellent, and the battery swelling is sufficiently suppressed.
[0128]
[Table 3]

[0129]
As described above, it is considered that the reason that the batteries a to d of the present invention tend to be superior to the others is that a portion based on the powder properties of the artificial graphite particles A as the main active material of the negative electrode is large.
First, regarding the high-rate discharge characteristics, the artificial graphite particles A1 and A2 are particles in which fully developed graphite crystals are oriented in random directions depending on the manufacturing process (powder physical properties). Value: I ₀₀₂ / I ₁₁₀ Value is small enough). That is, as in this example, the mixture density (value including the weight of CMC and SBR) is 1.70 g / cm. ³ It is considered that some of the graphite particles are oriented in the plane direction of the copper core material in the high-density negative electrode having a temperature of up to. However, it is presumed that graphite crystallites present randomly in the particles are not affected by the orientation, and that the occlusion and release of Li ions can smoothly proceed between the graphite particles and the electrolyte.
[0130]
Regarding the low-temperature discharge characteristics, it is presumed that the electron conductivity (electrode plate resistance) of the negative electrode mixture has a great influence.
The bulk mesophase pitch, which is the base material of the artificial graphite particles A1 and A2 used in the batteries a to d of the present invention, is more graphitizable than acicular coke, and the graphite crystal structure is sufficiently developed by graphitization. I have. Therefore, compared with the graphite particles C1 and C2 (batteries e to j) using acicular coke as a base material, the particles themselves have higher electron conductivity.
[0131]
In addition, it is compared with other artificial graphite particles D (graphitized milled MCF: elongated columnar shape) derived from mesophase carbon used in batteries k and l, and graphite particles E (graphitized MCMB: true spherical shape) used in batteries m and n. And, the particles A1 and A2 are a lump having an appropriate shape. Therefore, the particles A1 and A2 can secure many contact points with the graphite particles B1 and B2, and the electron conductivity of the whole negative electrode mixture increases. Therefore, it is considered that the degree of reduction in discharge voltage during low-temperature discharge was reduced, and excellent low-temperature discharge characteristics were secured. Regarding the difference in low-temperature discharge characteristics between batteries o and p using artificial graphite particles F (graphitized bulk mesophase), as described above, graphite crystals are oriented in random directions in anode graphite particles A1 and A2. It is considered that the existing point has influenced.
[0132]
Incidentally, as is clear from Table 3, for example, the battery e using only the comparative artificial graphite particles C1 alone and the batteries f and g using the mixture of the comparative artificial graphite particles C1 and the spherical graphite particles B1 or B2 were used. In comparison, there is no significant difference in performance. This indicates that the same effect as in the present invention cannot be obtained even when the comparative artificial graphite particles C1 are used instead of the artificial graphite particles A1 or A2. In other words, it can be said that a combination of the artificial graphite particles A1 or A2 and the spherical graphite particles B1 or B2 makes it possible to obtain a negative electrode or a battery having particularly excellent characteristics.
[0133]
In the negative electrode of the battery e using only the comparative artificial graphite particles C1 alone, during the rolling, fine powder of artificial graphite is generated, which is considered to have the same effect as the spherical graphite particles B1 or B2. . Therefore, it is assumed that there is no difference in characteristics between the battery e and the batteries f and g. It is considered that fine powder of artificial graphite is generated during the rolling of the negative electrode e because the manufacturing process of the comparative artificial graphite particles C1 includes a pulverizing process of graphite, so that the bonding of the primary particles is considered to be weak. It is.
[0134]
Regarding the batteries a to d of the present invention, the following are considered to be the main factors in that the charge / discharge cycle characteristics are superior to others and the degree of battery swelling due to charge / discharge is small.
(1) In the negative electrode active material particles a to d used in the battery of the present invention, the spherical graphite particles B1 or B2 are optimally arranged so as to fill the voids of the artificial graphite particles A1 or A2. Therefore, the mixture density is 1.70 g / cm ³ , The graphite particles in the vicinity of the surface of the mixture layer can be prevented from crushing or collapsing and orienting in the plane direction of the copper core material. In addition, the permeability (impregnation) of the electrolytic solution into the mixture is not hindered. In other words, high permeability (impregnation) of the electrolyte is ensured even in the mixture layer, so that even if the decomposition or reduction of the electrolyte partially occurs in a long-term cycle, a smooth charge / discharge reaction is ensured. Is done.
[0135]
(2) Since the graphite crystallites of the artificial graphite particles A1 and A2, which are the main active materials, are oriented in random directions, the particles expand and contract with repeated charge / discharge cycles (insertion / desorption of Li ions). And the degree of increase (swelling) in the thickness of the negative electrode is small.
[0136]
(3) Although it is related to the above (2), the artificial graphite particles A1 and A2 have a small degree of expansion and contraction of the particles due to repetition of the charge / discharge cycle, so that the graphite active material particles crack as the cycle progresses. Is less likely to occur. Therefore, the decomposition and consumption reaction of the electrolytic solution accompanied by gas generation caused by the cracking of the graphite active material particles (exposure of the new graphite edge surface) is suppressed.
[0137]
(4) In general, artificial graphite particles derived from mesophase carbon tend to have low Li ion acceptability during high-rate charging. However, the wettability of the surface of the artificial graphite particles A1 or A2 (depending on the type and concentration of the surface functional group) changes due to the granulation and graphitization of the bulk mesophase pitch pulverized particles and the pitch or thermosetting resin. And improved to relatively high levels. Therefore, the phenomenon of deposition of metallic lithium on the negative electrode surface accompanying the progress of the charge / discharge cycle is suppressed.
In particular, the batteries a to d of the present invention are more excellent in charge / discharge cycle characteristics than the batteries e to j using the artificial graphite particles C1 and C2 whose production methods are similar to the artificial graphite particles A1 and A2. Although the detailed mechanism cannot be elucidated for the reason, the following points can be considered.
[0138]
(5) Since the artificial graphite particles A1 and A2 do not undergo a pulverizing step after carbonization and graphitization, they have high circularity and high tap density. Therefore, the degree of crushing (disintegration) of the particles in the high-density negative electrode produced using the same is smaller than that of the high-density negative electrodes e to j produced using the artificial graphite particles C1 and C2.
[0139]
(6) The degree of progress of the cracking of the graphite particles due to the charge / discharge cycle is affected by the difference in the base carbon source used in producing the artificial graphite particles, that is, the difference between the bulk mesophase pitch ground particles and the acicular coke. The artificial graphite particles A1 and A2 are smaller than the artificial graphite particles C1 and C2. Here, the battery (discharged state) repeatedly charged and discharged in the initial 10 cycles and the battery (discharged state) after 500 cycles are disassembled, the negative electrode mixture is extracted and washed, and the active material particles are subjected to the BET method. Was measured. As a result, the batteries a to d (negative electrodes a to d) have a smaller specific surface area of the particles from the beginning than the batteries e to j (negative electrodes e to j), and the specific surface area of the particles increases with the cycle. It was actually confirmed that the degree was small.
[0140]
(Safety test)
It is generally considered that the graphite negative electrode of a lithium secondary battery has a strong correlation with the thermal stability of the battery. Here, there are various standards and guidelines on the evaluation method and evaluation criteria of the thermal stability (heat exposure) of the lithium secondary battery, but they are not unified. Therefore, in this study, the following conditions were adopted as conditions that are relatively strict and that the difference in the type of the negative electrode is reflected as clearly as possible, and a heat resistance test of the battery was performed.
[0141]
First, 22 kinds of lithium secondary batteries corresponding to the above negative electrodes a to v were charged to 4.3 V in a 20 ° C. atmosphere at a constant current of 0.1 C (80 mA) and a constant voltage of 2 hours. . Then, a thermocouple was attached to the battery so that the surface temperature of the battery could be monitored, and the battery was suspended in a thermostat at 20 ° C. Then, the temperature of the thermostat was raised to 165 ° C. at a rate of 5 ° C./min, and then maintained at 165 ° C.
[0142]
In this test, even when the temperature of the thermostat was maintained at 165 ° C., a part of the charged negative electrode graphite active material particles reacted with the electrolyte solution or the binder, or the coating on the graphite surface was decomposed. As a result, heat of reaction is generated. Therefore, the battery surface temperature reaches a temperature of 165 ° C. or higher. If the maximum temperature at this time is extremely high, an internal short circuit due to a chain of exothermic reactions (thermal runaway) of the positive electrode (or the negative electrode) inside the battery or rapid separator shrinkage is caused. The lower the maximum temperature of the battery, the higher the safety of the battery. Table 4 summarizes the results.
[0143]
[Table 4]

[0144]
From these results, it is understood that the superiority in the heat resistance test has a very high correlation with the BET specific surface area of the graphite particles forming the negative electrode. Regarding the negative electrodes used in the batteries a to d of the present invention, the BET specific surface area of the main graphite active material particles A1 and A2 was 0.4 to 0.5 m. ² / G and 1m each ² / G or less. It turns out that this is very advantageous from the viewpoint of heat resistance. The BET specific surface areas of the spherical graphite particles B1 and B2 of the negative electrode used in the batteries a to d were 6.9 to 7.2 m. ² / G, which is a considerably large value. In this regard, it can be said that it is more preferable to use a material having a reduced BET specific surface area as much as possible.
[0145]
From the above examination results, it can be confirmed that the batteries a to d of the present invention have high discharge characteristics and excellent cycle life characteristics, and also ensure high safety.
Here, as to the form of the battery, in the examples, a form in which a substantially quadrangular prismatic electrode group is inserted into a battery case made of a square aluminum alloy (FIG. 1) was used, but the present invention is not limited to this. . For example, the same applies to a lithium secondary battery in a form in which an electrode group formed by winding into a substantially square column or an elliptic column is enclosed in a case made of a laminate sheet of an aluminum foil and a resin film, and a non-aqueous electrolyte is injected. In addition, a thin and lightweight lithium secondary battery having excellent discharge characteristics, cycle life characteristics, and safety, high energy density, and low weight can be obtained.
[0146]
Similarly, the density of the negative electrode mixture layer is set to 1.6 to 1.8 g / cm. ³ A negative electrode set at a high level, a positive electrode filled at a high density, and a separator are combined to form a columnar (spiral) electrode group, which is housed in a cylindrical case made of nickel-plated steel sheet, Even when the liquid is injected, a 400 Wh / L high-capacity lithium secondary battery having excellent discharge characteristics, cycle life characteristics and safety can be obtained.
[0147]
Further, in producing the artificial graphite particles A1 or A2, pulverized bulk mesophase pitch particles having an average particle diameter of 13 μm were used as a base material, but the invention is not limited thereto. If the average particle diameter is about 7 to 20 μm, similar artificial graphite particles can be produced.
[0148]
In addition, coal tar pitch (coal pitch) and phenol resin were used as binders, but pitches such as petroleum pitch and naphthalene pitch, and thermosetting resins such as polyimide resin, polyvinyl chloride resin, cellulose resin, and furfuryl alcohol resin. Can also be used.
[0149]
In the kneading and granulation, the mixing ratio of the base material and the binder was 85:15 (weight ratio), the temperature was 200 ° C., the time was 120 minutes, and the kneading apparatus was a Z-type kneader. It is not limited. The circularity of the obtained granules is 0.85 to 0.95, and the average particle diameter D ₅₀ Is 15 to 30 μm, D ₁₀ / D ₉₀ May be adjusted so that the value of is 0.2 to 0.5.
[0150]
In addition, the temperature of carbonization (firing) of the granulated material was set to 800 ° C, but it may be a non-oxidizing atmosphere of 700 to 1500 ° C. Although the graphitization temperature was set at 2950 ° C., similar artificial graphite particles can be obtained by heating in a non-oxidizing atmosphere at 2500 to 3000 ° C. to sufficiently advance graphitization.
[0151]
In addition, as spherical graphite particles, particles B1 obtained by subjecting crushed flaky natural graphite to impact in a counter-type jet mill to form spheroidized and classified particles B1 or acicular coke powder are graphitized, and then similar spheroidized and classified. Was used, but the present invention is not limited to this. In particular, when natural graphite is used as a raw material, the largest reversible capacity can be obtained from graphite particles, and the graphitization step can be omitted, so that inexpensive particles can be obtained.
[0152]
This sphering means is not limited to the impact method in a counter-type jet mill, but can also be obtained by adding various improvements to the crushing equipment and conditions from the raw ore. ₅₀ Is 5 to 15 μm, and spherical graphite particles having a large circularity of 0.88 to 1 can be obtained.
[0153]
In addition, the spherical graphite particles preferably have a BET specific surface area as low as possible from the viewpoint of safety (heat resistance) of the graphite negative electrode. Most preferred.
[0154]
In addition, in preparing the negative electrode active material, the mixing ratio of the spherical graphite particles B1 or B2 to the entire active material was set to 25% by weight. However, the same effect can be obtained if it is within the range of 5 to 45% by weight.
[0155]
In preparing the negative electrode mixture, styrene-butadiene rubber (SBR) was used as the rubbery binder. Butadiene, an aromatic vinyl monomer, and ethylenic rubber were used as rubbery binders composed of similar butadiene derivatives. A similar negative electrode can be prepared by using a rubbery polymer made of a copolymer with an unsaturated carboxylic acid ester monomer as a binder. Here, styrene, α-methylstyrene, etc. can be used as the aromatic vinyl monomer, and acrylates (eg, methyl acrylate, ethyl acrylate, propyl acrylate, etc.) are used as the ethylenically unsaturated carboxylic acid ester monomers. ) And methacrylates (eg, methyl methacrylate, ethyl methacrylate, propyl methacrylate).
[0156]
In addition, the addition amount of the rubber-like binder is set to 2% by weight based on the weight of the negative electrode active material. However, if the addition amount is, for example, 3% by weight or less, the battery can be manufactured without impairing the negative electrode characteristics.
[0157]
Further, the density of the mixture layer is 1.7 g / cm. ³ The negative electrode was produced by adjusting the rolling so that the thickness became 70 μm, but the mixture density was 1.6 to 1.8 g / cm. ³ When the thickness of the mixture is, for example, 40 to 100 μm, a lithium secondary battery having the same excellent characteristics can be manufactured.
[0158]
Further, the non-aqueous electrolytic solution has a concentration of 1.0 M in a solvent in which ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed at a volume ratio of 1: 2: 1. LiPF ₆ Was used, but the present invention is not limited to this.
[0159]
【The invention's effect】
As described above, according to the present invention, the charge-discharge cycle characteristics of a high energy density lithium secondary battery can be significantly improved, and at the same time, the discharge rate characteristics, low-temperature discharge characteristics, and safety (heat resistance) can be improved. An excellent battery can be provided. Therefore, the industrial value is very large.
[Brief description of the drawings]
FIG. 1 is a partially cutaway perspective view of a lithium secondary battery manufactured in an example.
[Explanation of symbols]
1 Electrode group
2 Positive electrode lead
3 Negative electrode lead
4 Battery case
5 sealing plate
6 Negative electrode terminal
7 Sealing

Claims (7)

A lithium secondary battery comprising a positive electrode, a negative electrode and a non-aqueous electrolyte,
The negative electrode comprises a copper core material and a negative electrode mixture layer fixed on the core material,
The negative electrode mixture layer includes an active material formed of a mixture of artificial graphite particles A and spherical graphite particles B,
The artificial graphite particles A are isotropic artificial graphite particles in which the graphite structure is randomly oriented in the particles,
(1) The plane distance d ₀₀₂ of the (002) plane determined by the powder X-ray diffraction method is 3.362 ° or less.
(2) The peak intensity I ₀₀₂ attributed to the (002) plane and the peak intensity attributed to the (110) plane in the diffraction pattern when a pellet is formed to a density of 1.6 g / cm ³ and subjected to X-ray diffraction measurement. Ratio to I ₁₁₀ : I ₀₀₂ / I ₁₁₀ is 1000 or less,
(3) an average particle circularity of 0.85 to 0.95,
(4) a laser diffraction particle size distribution analyzer particle size _{D 50} when the volume fraction of 50% was measured using the in 15 to 30 [mu] m, the particle diameter _{D 10} when the volume fraction of 10% and the volume fraction of the time 90% the ratio of the particle diameter _{D 90: D} 10 _{/ D 90} is 0.2 to 0.5,
(5) Tap density is 1 g / cm ³ or more,
(6) The specific surface area measured by using the BET method is 1 m ² / g or less,
The spherical graphite particles B are:
(1) an average particle circularity of 0.88 to 1,
(2) a particle diameter D50 at a volume fraction of 50% measured using a laser diffraction type particle size distribution analyzer of ₅ to 15 μm,
(3) The plane distance d ₀₀₂ of the (002) plane determined by the powder X-ray diffraction method is 3.357 ° or less.
(4) A lithium secondary battery having a specific surface area of not more than 8 m ² / g measured using the BET method. The artificial graphite particles A are obtained by kneading and granulating a base material produced by pulverizing a bulk mesophase pitch and a pitch and / or thermosetting resin in a softened state. The lithium secondary battery according to claim 1, wherein the particles are obtained by carbonizing at 1500 ° C and further graphitizing at 2500 to 3000 ° C. The lithium secondary battery according to claim 1, wherein a mixing ratio of the spherical graphite particles B to the entire active material is 5 to 45% by weight. The positive electrode and the negative electrode are wound around a separator to form an electrode group, and are enclosed in a square metal case or a case made of a laminate sheet of an aluminum foil and a resin film. The lithium secondary battery according to any one of claims 1 to 3. The lithium secondary battery according to any one of claims 1 to 4, wherein the negative electrode mixture layer further includes a rubbery binder containing a butadiene unit and a cellulosic thickener. The lithium secondary battery according to any one of claims 1 to 5, wherein the spherical graphite particles B are natural graphite particles and / or natural graphite particles that have been modified to partially amorphize only the surface. . The amount of the rubbery binder is 3 parts by weight or less based on 100 parts by weight of the active material, the density of the negative electrode mixture layer is 1.6 to 1.8 g / cm ³ , The lithium secondary battery according to claim 5, wherein the thickness of the negative electrode mixture layer is 40 to 100 m.

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