JP2004303674A

JP2004303674A - Non-aqueous electrolyte secondary battery and negative electrode material

Info

Publication number: JP2004303674A
Application number: JP2003097558A
Authority: JP
Inventors: Kazushige Kono; 一重河野; Juichi Arai; 寿一新井
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 2003-04-01
Filing date: 2003-04-01
Publication date: 2004-10-28
Anticipated expiration: 2023-04-01
Also published as: JP4069784B2

Abstract

<P>PROBLEM TO BE SOLVED: To overcome the conventional problem in which a natural graphite with highly grown graphite crystal structure and an artificial graphite made of coke tend to suffer from large voltage variance due to polarization of negative electrode side in the last stage of discharge, causing it difficult to fully utilize the capacity existing in the negative electrode by setting a higher terminal voltage for suppressing the over-discharge. <P>SOLUTION: A mixing material of a graphite material and a non-graphite material having higher discharge capacity than the graphite material is used as a negative electrode active material, so that the voltage drop between the electrodes during the last stage of discharge is moderated and the capacity for discharge is increased. The content of the non-graphite material is set to 10-90 vol.%. A material having an initial charge/discharge efficiency of 85% or higher when the material is charged to 0 V at Li reference potential then is discharged to 1.5 V is used as the non-graphite material. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、ポータブル電子機器等に用いるのに好適な新規な非水系電解液二次電池とその負極材料に関する。
【０００２】
【従来の技術】
従来、非水系電解液二次電池の負極には黒鉛質材料を用いられている（例えば、天然黒鉛材料，コークス等を黒鉛化した人造黒鉛等）。しかしながら、黒鉛結晶が発達している天然黒鉛及びコークスを黒鉛化した人造黒鉛は、放電末期において負極側の分極による電圧変化が大きく、過放電を抑制する為に終止電圧を高く設定し、負極材料が有する容量を充分に活用することが困難であった。例えば、電池の放電終止電圧を２．７Ｖと設定した場合、放電終止時の正極の電位を約３．０Ｖと推定すると、負極の電位としては約０．３Ｖに達するまでの放電容量が、実質的な電池容量となる。そこで、黒鉛質材料よりリチウムイオンの拡散が早い、非黒鉛質材料との混合負極を用い、放電末期の分極を抑制する手法が提案されている。
【０００３】
特許文献１では黒鉛質材料とＸ線回折で求めた００２面の間隔が０．３４３〜０．３９ｎｍ、結晶子の大きさを示すＬｃが０．９〜９．９ｎｍを示す炭素質材料を混合する手法が提案されている。また、特許文献２では黒鉛質材料と黒鉛質材料の重量あたりの容量８０％以上を有する非黒鉛炭素材料と混合し、負極の分極を抑制する手法などが提案されている。
【０００４】
【特許文献１】
特開平６−３６７６０号公報
【特許文献２】
特開平７−１９２７２４号公報
【０００５】
【発明が解決しようとする課題】
しかし、特許文献１に記載の材料を電池に用いた際には、混合する炭素質材料が黒鉛質材料より、重量あたりの放電容量が低い物質であり、混合して形成された負極を用いた電池では、重量あたりの放電容量が低くなってしまう危険性があった。また、炭素質材料の混合量が３０体積％を超えると、分極が大きくなり電池の放電電位が低くなってしまう問題があった。
【０００６】
また、特許文献２に記載の材料を電池に用いた際に充電終止時の負極側の電位は、黒鉛質材料と比較して卑になる。しかし、混合する炭素質材料に黒鉛質材料と比較して、重量あたりの放電容量が８０％〜９０％程度の材料を用いた際に、重量あたりの放電容量が低い電池になってしまう危険性があった。
【０００７】
本発明の目的は、放電末期に両極間の電圧低下を緩やかにし、放電容量の優れた非水系電解液二次電池とその負極材料を提供することにある。
【０００８】
【課題を解決するための手段】
本発明は、負極活物質に黒鉛質材料と該黒鉛質材料より高い放電容量を有する非黒鉛質材料の混合材料を用いる（但しここで述べる放電容量は、対極を金属リチウムとして、ある電流密度で端子間電圧が０Ｖに成るまでリチウムをドープし、その後１．５Ｖまでリチウムを脱ドープした際の負極材料単位重量あたりの放電量を指す）。それにより、放電末期に両極間の電圧低下を緩やかにし、放電可能な電池容量が向上される。また、非黒鉛質材料の混合量は１０体積％より大きく９０体積％未満とする。また、非黒鉛質材料は参照極及び対極に金属リチウムを用いて作成したモデル電池評価において、Ｌｉ基準電位で０Ｖまで充電後、
１．５Ｖまで放電した際の初回充放電効率（但しここで述べる初回充放電効率は、対極を金属リチウムとして、一定電流密度で端子間電圧が０Ｖに成るまでリチウムをドープした負極材料単位重量あたりの充電量Ｃと、その後１．５Ｖまでリチウムを脱ドープした際の負極材料単位重量あたりの放電量Ｄとの百分率比＝（Ｄ／Ｃ）×１００を指す）が８５％より高い値を示す材料を用いる。
【０００９】
具体的用いる非黒鉛質材料物性としては、Ｘ線回折結果より算出される結晶子の厚みを示すＬｃ値が０．７０〜２．２０ｎｍ程度で、且つラマン分光スペクトルで測定される１３００〜１４００ｃｍ^−１範囲の非結晶質乱層構造に由来する振動モードを示すピークの高さ（Ｉ_Ｄ）と１５８０〜１６２０ｃｍ^−１の範囲の黒鉛結晶質構造に由来する振動モードを示すピークの高さ（Ｉ_Ｇ）の比から求められる黒鉛化度Ｒ値（Ｉ_Ｄ／Ｉ_Ｇ）が０．９０〜１．２０程度、窒素吸着比表面積が＜２ｍ^２／ｇであるものを用いることが望ましい。この、物性値を外れると、放電容量が低く成ったりする危険性がある。また、Ｌｉ基準電位で０Ｖまで充電後、１．５Ｖまで放電した際の初回充放電効率８５％以下である場合、不可逆容量が大きくなり、電池として過剰な活物質が必要となり重量あたりのエネルギー密度が低下したり、コストが高くなってしまう問題が生じる。
【００１０】
本発明により、放電末期に両極間の電圧低下を緩やかにすることが可能になり、０Ｖまで充電し、例えば０．３Ｖまで放電可能な容量が、黒鉛質材料単体及び非黒鉛質材料単体における評価容量から推算される、予想値以上に優れた特性を示す負極材料を見出した。
【００１１】
即ち、本発明は、リチウムの吸蔵放出が可能な正極活物質が集電体箔の両面に形成された正極と、リチウムの吸蔵放出が可能な負極活物質が集電体箔の両面に形成された負極と、リチウム塩を含む非水電解液とを有し、前記正極及び負極がセパレータを介して巻回または積層された非水系電解液二次電池において、前記負極活物質が黒鉛質材料と該黒鉛質材料より高い放電容量を有する非黒鉛質材料の混合材料からなることを特徴とする。
【００１２】
従来の黒鉛質材料と非黒鉛質材料の混合負極の評価では、結晶内にリチウムイオンを可逆的にドープ／脱ドープする為の指標として用いられていた００２面の面間隔ｄ（００２）及び結晶子サイズＬｃを適正化していた。それに対して本発明の非黒鉛質材料はＬｃ値を最適化し、且つ、リチウムイオンの可逆的にドープ／脱ドープするためのサイトとなる材料表面の黒鉛化度Ｒ値が結晶質な性質と炭素質な性質の両面を持ち合わせる適正値にし、更に非黒鉛質材料の比表面積及び充放電効率を適正化することにより、電解液の分解等の副反応を抑制し、活物質界面でのイオンのドープ／脱ドープに伴う電荷移動などに起因する反応分極を抑制し、従来黒鉛質材料と非黒鉛質材料の混合負極と比較して、放電末期に両極間の電圧低下を緩やかにすることが可能になり、０Ｖまで充電後、例えば負極電位で０．３Ｖまで放電可能な容量が、黒鉛質材料単体及び非黒鉛質材料単体における評価容量から推算される、予想値以上の優れた特性が得られるものである。
【００１３】
負極材料を集電体と密着させるために用いるバインダーは、ポリフッ化ビニリデン−６フッ化プロピレン共重合体，エチレン−プロピレン−ジエン共重合体等が好ましい。また非水溶媒としては、プロピレンカーボネート，ジメチルカーボネート１，２−ジメトキシエタン，テトラヒドロフラン等の単体または混合物が好ましい。
【００１４】
電解質は、過塩素酸リチウム，六フッ化リン酸リチウム，ホウフッ化リチウム，ビストリフルオロメチルスルホニイミドリチウムなどを用いることが好ましい。
【００１５】
本発明の負極材料と組み合わせて使用する正極材料としては、化学式がＬｉＣｏＯ_２，ＬｉＮｉＯ_２，ＬｉＭｎ_ｘＮｉ_１−ｘＯ_２，ＬｉＭｎ_２Ｏ_４，ＬｉＭｎＯ_２（但しｘは０．００１≦ｘ≦０．５の範囲）等が上げられる。
【００１６】
【発明の実施の形態】
本実施例で示すバインダー，非水系溶媒，集電体，電解質等は一例であり、本発明は本実施例に示すものには限定されない。
【００１７】
（実施例１）
本実施例の非水系電解液二次電池用負極材料の調製方法を以下に示す。実施形態１〜４において非黒鉛質材料としては、セルロース系の先駆体を焼成したものを用いた。焼成方法の一例を以下に示す。まず、前記先駆体を５℃／分の昇温速度で３００℃まで加熱し１時間保持後（大気中）、冷却後粉砕する。その後、分級することで７０ミクロン以上の粗粒を除去した。分級した粉末を１０^−２Ｔｏｒｒの真空下で７００℃まで昇温（５℃／分）して１時間保持後、更に１２００℃まで昇温（５℃／分）し２時間保持して調製した。このようにして調製した非黒鉛材料の物性値は、Ｘ線回折結果より算出される結晶子の厚みを示すＬｃ値が１ｎｍで、且つラマン分光スペクトルで測定される非結晶質乱層構造に由来するピークの高さ（Ｉ_Ｄ）と黒鉛結晶質構造に由来するピークの高さ（Ｉ_Ｇ）の比から求められる黒鉛化度Ｒ値（Ｉ_Ｄ／Ｉ_Ｇ）が０．９８、窒素吸着による比表面積は１ｍ^２／ｇであった。また、用いた黒鉛質材料としては、放電容量が３６０ｍＡｈ／ｇで初回の充放電効率が９０％である人造黒鉛を用いた。また比較例１〜４は非黒鉛材料として前述の実施形態１と同じ物を用い混合割合を変化させたものである。また、比較例５〜７は原料として前述の実施形態１と同じ物を用い、７００℃での保持をなくしＡｒ雰囲気下で焼成したもの、最高到達温度１５００℃で１０時間保持したもの、最高到達温度９００℃としたものをそれぞれ用いた。その他の焼成の詳細条件は実施形態１に準じた。このようにして調整した非黒鉛質材料の物性値，混合割合を表１に示す。
【００１８】
【表１】

【００１９】
以下にこれらの負極材料を用いて、電極を作成し負極材料特性を評価する手法を示す。まず電極の作成方法を以下に示す。任意の割合で秤量した負極材料にポリフッ化ビニリデン（クレハ製＃１１２０）１０重量部と成るように添加して、溶媒として１−メチル−２−ピロリドンを、固形分濃度が４５重量部となるように添加して混練する。そうして得たスラリーを集電体である銅箔上に塗布し、１２０℃で３時間恒温槽内において乾燥させた。その後、直径１５ｍｍに打ち抜き電極とした。
【００２０】
以下にテストセルを作製し、負極材料特性を評価する方法を示す。負極材料特性評価に用いたテストセルは、直径が１５ｍｍでセパレータとしてはポリプロピレン製多孔質膜を介して対極となるリチウム金属箔（１ｍｍ厚）と対向させて配置し、その間に、非水系電解液を充填させた。本実施例で用いた電解液はエチレンカーボネートとメチルエチルカーボネートとの混合溶媒（容量比１：１）に電解質として六フッ化リン酸リチウムを１Ｍ溶解させたものを用いた。このようにして調製したテストセルの充放電条件は、以下に示す条件で行った。充電は定電流充電０．５ｍＡ（０．２８４ｍＡ／ｃｍ^２），定電圧充電０Ｖ，０．０５ｍＡで実施し、放電は電流０．５ｍＡでカット電圧０．３Ｖとした。表１に示す放電容量及び初回充放電効率は、別途作成したテストセルで、前述の充放電条件と同一電流密度とし、放電カット電圧を１．５Ｖとし評価した値である。
【００２１】
上記の条件で充放電を行った後に、混合物負極材料単位量あたりの放電容量を算出した。その結果を図１に示す。図１より本発明である混合負極は、黒鉛質材料単独負極及び非黒鉛質材料単独負極の特性評価から推算される値より高い放電量を示し、優れた負極材料であることが確認された。
【００２２】
（実施例２）
本発明の非水系電解液二次電池用負極材料を用いたリチウム二次電池の性能評価方法を示す。本実施例で用いた電極及び電池の調製方法を以下に示す。正極は材料としてＬｉＭｎ_２Ｏ_４を用いた。正極活物質８７重量部に導電助剤となる人造黒鉛８．７重量部，１−メチル−２−ピロリドンに溶解させたポリフッ化ビニリデン（クレハ製＃１１２０）を固形分で４．３重量部と成るように混合し、ペーストを作成して集電体であるアルミ箔に両面塗布し、８０℃で３時間乾燥させた。その後、プレスして約２．７ｇ／ｃｍ^３程度の密度とし、１２０℃で３時間真空乾燥して正極を得た。
【００２３】
負極は実施形態１に示す非黒鉛材料に任意の割合で黒鉛材料を混合した材料にポリフッ化ビニリデン（クレハ製＃１１２０）１０重量部と成るように添加して、溶媒として１−メチル−２−ピロリドンを、固形分濃度が４５重量部となるように添加して混練する。そうして得たスラリーを集電体である銅箔に両面の塗布し、８０℃で３時間乾燥させた。その後、プレスして約１．０ｇ／ｃｍ^３程度の密度とし、１２０℃で３時間真空乾燥して得た。
【００２４】
実施形態５〜８の電池は実施形態１〜４に示す負極材料を用い、また、比較例９〜１４の電池は比較例１〜７の負極材料を用いて、前述と同様の手法で電極を作成した。得られた正極及び負極は、ポリエチレン製多孔質膜セパレータ（厚み：０．０２５ｍｍ）と組み合わせて捲回し、外寸法が直径１８ｍｍ，長さ６５ｍｍの電池缶に収納した。用いた電解液は、エチレンカーボネートとメチルエチルカーボネートとの混合溶媒（容量比１：１）に電解質として六フッ化リン酸リチウムを１Ｍ溶解させたものを用いた。図２に本試験に用いた電池の一例の一部断面正面図を示す。
【００２５】
図２に示す円筒型リチウム二次電池は、薄板状に加工された正極１と同様に加工された負極２がポリエチレン製多孔膜等のセパレータ３を介して重ね合わせたものを捲回し、これを金属製等の電池缶７に挿入し、密閉化されている。正極には正極タブ４を介して正極蓋６に接合され、負極２は負極タブ５を介して電池底部へ接合されている。正極蓋６はガスケット８にて電池缶７へ固定されている。
【００２６】
上記のようにして調製した電池の充放電条件は、以下に示す条件で行った。電流５００ｍＡ一定で、充電停止電圧４．２Ｖ，放電停止電圧２．７Ｖとして性能評価を実施した。表２に電池の充放電効率評価結果を示す。本実施例においても実施形態１〜４と同様の傾向を有し、黒鉛質材料単独負極を用いた際の電池の放電容量と、非黒鉛質材料単独負極を用いた際の電池の放電容量から推算される容量より、優れた放電容量を示す二次電池を製造できることが確認された。
【００２７】
【表２】

【００２８】
【発明の効果】
本発明の負極材料を用いることにより、放電容量特性が優れた非水系電解液二次電池を製造することが可能となり、ポータブル電子機器、等の小型化に寄与できる。
【図面の簡単な説明】
【図１】本発明の負極材料の単極容量特性を示す線図。
【図２】本発明に係る円筒型リチウム二次電池の断面図。
【符号の説明】
１…正極、２…負極、３…セパレータ、４…正極タブ、５…負極タブ、６…正極蓋、７…電池缶、８…ガスケット。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a novel non-aqueous electrolyte secondary battery suitable for use in portable electronic devices and the like, and a negative electrode material thereof.
[0002]
[Prior art]
Conventionally, a graphite material has been used for the negative electrode of a non-aqueous electrolyte secondary battery (for example, natural graphite material, artificial graphite obtained by graphitizing coke, etc.). However, natural graphite in which graphite crystals are developed and artificial graphite in which coke is graphitized have a large voltage change due to polarization on the negative electrode side at the end of discharge, and the final voltage is set high to suppress overdischarge. It has been difficult to make full use of the capacity of the above. For example, when the discharge end voltage of the battery is set to 2.7 V, assuming that the potential of the positive electrode at the end of discharge is about 3.0 V, the discharge capacity until the potential of the negative electrode reaches about 0.3 V is substantially Battery capacity. Therefore, a method has been proposed in which a negative electrode mixed with a non-graphitic material, in which lithium ions diffuse faster than a graphite material, is used to suppress polarization at the end of discharge.
[0003]
In Patent Literature 1, a graphite material is mixed with a carbonaceous material having an interval between 002 planes determined by X-ray diffraction of 0.343 to 0.39 nm and a crystallite size Lc of 0.9 to 9.9 nm. Have been proposed. Patent Document 2 proposes a method of suppressing the polarization of a negative electrode by mixing a graphite material and a non-graphite carbon material having a capacity of 80% or more per weight of the graphite material.
[0004]
[Patent Document 1]
JP-A-6-36760 [Patent Document 2]
JP-A-7-192724 [0005]
[Problems to be solved by the invention]
However, when the material described in Patent Document 1 is used for a battery, the carbonaceous material to be mixed is a substance having a lower discharge capacity per weight than the graphite material, and a negative electrode formed by mixing is used. In the battery, there is a risk that the discharge capacity per weight is reduced. Further, when the mixing amount of the carbonaceous material exceeds 30% by volume, there is a problem that the polarization increases and the discharge potential of the battery decreases.
[0006]
In addition, when the material described in Patent Document 2 is used for a battery, the potential of the negative electrode at the end of charging becomes lower than that of the graphite material. However, when a material having a discharge capacity per weight of about 80% to 90% is used as the carbonaceous material to be mixed as compared with the graphite material, there is a risk that the battery may have a low discharge capacity per weight. was there.
[0007]
An object of the present invention is to provide a non-aqueous electrolyte secondary battery having excellent discharge capacity, which moderates the voltage drop between both electrodes at the end of discharge, and a negative electrode material thereof.
[0008]
[Means for Solving the Problems]
In the present invention, a mixed material of a graphite material and a non-graphitic material having a higher discharge capacity than the graphite material is used as the negative electrode active material (however, the discharge capacity described here is such that the counter electrode is metallic lithium, and The amount of discharge per unit weight of the negative electrode material when lithium is doped until the inter-terminal voltage becomes 0 V and thereafter undoped with lithium to 1.5 V). As a result, the voltage drop between both electrodes is moderated at the end of discharge, and the dischargeable battery capacity is improved. Further, the mixing amount of the non-graphitic material is greater than 10% by volume and less than 90% by volume. In addition, the non-graphitic material was charged to 0 V at the Li reference potential in a model battery evaluation made using metallic lithium for the reference electrode and the counter electrode,
Initial charge / discharge efficiency when discharged to 1.5 V (However, the initial charge / discharge efficiency described here is based on the unit weight of the negative electrode material doped with lithium until the inter-terminal voltage becomes 0 V at a constant current density with the counter electrode being metallic lithium. Of the amount of charge C and the amount of discharge D per unit weight of the negative electrode material when lithium is dedoped up to 1.5 V = (D / C) × 100) shows a value higher than 85%. Use materials.
[0009]
As specific properties of the non-graphitic material used, the Lc value indicating the thickness of the crystallite calculated from the X-ray diffraction results is about 0.70 to 2.20 nm, and 1300 to 1400 cm ⁻ measured by Raman spectroscopy. The height (I _D ) of a peak indicating a vibration mode derived from a non-crystalline turbostratic structure in ^one range and the height (I _D ) of a vibration mode derived from a graphite crystalline structure in a range of 1580 to 1620 cm ⁻¹ degree of graphitization R value calculated from the ratio of _{_{_{G) (I D / I G}}} ) is about 0.90 to 1.20, it is desirable that the nitrogen adsorption specific surface area used as a ^<2m 2 / g. If the physical properties deviate, there is a risk that the discharge capacity may be reduced. When the initial charge / discharge efficiency is 85% or less when the battery is charged to 0 V at the Li reference potential and then discharged to 1.5 V, the irreversible capacity becomes large, an excessive active material is required for the battery, and the energy density per weight becomes large. And the cost increases.
[0010]
According to the present invention, it is possible to moderate the voltage drop between the two electrodes at the end of discharging, and the capacity capable of charging to 0 V and discharging to, for example, 0.3 V is evaluated by the graphite material alone and the non-graphite material alone. We have found a negative electrode material that has properties better than expected, estimated from capacity.
[0011]
That is, the present invention provides a positive electrode in which a positive electrode active material capable of inserting and extracting lithium is formed on both surfaces of a current collector foil, and a negative electrode active material capable of inserting and extracting lithium is formed on both surfaces of a current collector foil. A non-aqueous electrolyte secondary battery having a non-aqueous electrolyte containing a lithium salt, wherein the positive electrode and the negative electrode are wound or laminated via a separator, wherein the negative electrode active material is a graphite material. It is characterized by being made of a mixture of non-graphitic materials having a higher discharge capacity than the graphite material.
[0012]
In a conventional evaluation of a mixed negative electrode of a graphitic material and a non-graphitic material, in the evaluation of the reciprocal doping / dedoping of lithium ions in the crystal, the 002 plane spacing d (002) and the crystal were used as indices. The child size Lc was optimized. On the other hand, the non-graphitic material of the present invention optimizes the Lc value, and shows that the graphitization degree R value of the material surface serving as a site for reversible doping / dedoping of lithium ions has a crystalline property and carbon property. By adjusting the specific surface area and charge / discharge efficiency of non-graphitic materials to appropriate values that have both sides of high quality properties, side reactions such as decomposition of electrolyte solution are suppressed, and doping of ions at the active material interface / Suppresses reaction polarization caused by charge transfer due to undoping and makes it possible to moderate the voltage drop between the two electrodes at the end of discharge compared to conventional mixed anodes of graphite and non-graphite materials In other words, a capacity that can be discharged to 0.3 V at the negative electrode potential after being charged to 0 V, for example, can be estimated from the evaluation capacities of the graphite material alone and the non-graphite material alone. It is.
[0013]
As the binder used for bringing the negative electrode material into close contact with the current collector, a polyvinylidene fluoride-6-propylene copolymer, an ethylene-propylene-diene copolymer, or the like is preferable. As the non-aqueous solvent, a simple substance or a mixture of propylene carbonate, dimethyl carbonate 1,2-dimethoxyethane, tetrahydrofuran and the like is preferable.
[0014]
As the electrolyte, it is preferable to use lithium perchlorate, lithium hexafluorophosphate, lithium borofluoride, lithium bistrifluoromethylsulfonimide, or the like.
[0015]
As the positive electrode material used in combination with the negative electrode material of the present invention, the chemical formula is LiCoO ₂ , LiNiO _2, LiMn _x Ni _1-x O ₂ , LiMn ₂ O ₄ , LiMnO ₂ (where x is 0.001 ≦ x ≦ 0). .5 range).
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
The binder, the non-aqueous solvent, the current collector, the electrolyte, and the like shown in this embodiment are merely examples, and the present invention is not limited to those shown in this embodiment.
[0017]
(Example 1)
The method for preparing the negative electrode material for a non-aqueous electrolyte secondary battery of this example is described below. In the first to fourth embodiments, a non-graphitic material obtained by firing a cellulose precursor was used. An example of the firing method is described below. First, the precursor is heated to 300 ° C. at a heating rate of 5 ° C./min, held for 1 hour (in the atmosphere), cooled, and pulverized. After that, coarse particles of 70 μm or more were removed by classification. The classified powder was heated to 700 ° C. (5 ° C./min) under a vacuum of 10 ⁻² Torr and held for 1 hour, and then heated to 1200 ° C. (5 ° C./min) and held for 2 hours. . The physical property value of the non-graphite material prepared in this way is derived from the non-crystalline turbostratic structure whose Lc value indicating the thickness of the crystallite calculated from the X-ray diffraction result is 1 nm and measured by Raman spectroscopy. heights of the peaks _{(I D)} and the height of the peak derived from a graphite crystalline structure relative degree of graphitization R value obtained from the _{_{(I G) (I D /}} I G) of 0.98, by nitrogen adsorption The specific surface area was 1 m ² / g. As the graphite material used, artificial graphite having a discharge capacity of 360 mAh / g and an initial charge / discharge efficiency of 90% was used. In Comparative Examples 1 to 4, the same non-graphite material as that of Embodiment 1 was used, and the mixing ratio was changed. In Comparative Examples 5 to 7, the same materials as those in Embodiment 1 described above were used as the raw materials, and were fired in an Ar atmosphere without holding at 700 ° C., held at a maximum temperature of 1500 ° C. for 10 hours, Each having a temperature of 900 ° C. was used. Other detailed firing conditions were the same as in the first embodiment. Table 1 shows the physical property values and mixing ratios of the non-graphitic materials thus adjusted.
[0018]
[Table 1]

[0019]
A method for preparing an electrode using these negative electrode materials and evaluating characteristics of the negative electrode material will be described below. First, a method for forming an electrode is described below. To the negative electrode material weighed at an arbitrary ratio, polyvinylidene fluoride (# 1120 manufactured by Kureha) was added in an amount of 10 parts by weight, 1-methyl-2-pyrrolidone was used as a solvent, and the solid content concentration was 45 parts by weight. And knead it. The slurry thus obtained was applied on a copper foil as a current collector, and dried in a thermostat at 120 ° C. for 3 hours. Thereafter, a punched electrode having a diameter of 15 mm was used.
[0020]
A method for preparing a test cell and evaluating the characteristics of the negative electrode material will be described below. The test cell used for the negative electrode material property evaluation has a diameter of 15 mm, and is disposed opposite a lithium metal foil (1 mm thick) serving as a counter electrode via a polypropylene porous membrane as a separator, and a non-aqueous electrolytic solution is interposed therebetween. Was filled. The electrolytic solution used in this example was prepared by dissolving 1 M of lithium hexafluorophosphate as an electrolyte in a mixed solvent of ethylene carbonate and methyl ethyl carbonate (volume ratio 1: 1). The test cell thus prepared was charged and discharged under the following conditions. The charging was performed at a constant current charge of 0.5 mA (0.284 mA / cm ² ), a constant voltage charge of 0 V and 0.05 mA, and a discharge was performed at a current of 0.5 mA and a cut voltage of 0.3 V. The discharge capacity and initial charge / discharge efficiency shown in Table 1 are values evaluated using a separately prepared test cell with the same current density as the charge / discharge conditions described above and a discharge cut voltage of 1.5 V.
[0021]
After charging and discharging under the above conditions, the discharge capacity per unit amount of the mixture negative electrode material was calculated. The result is shown in FIG. From FIG. 1, the mixed negative electrode of the present invention showed a higher discharge amount than the value estimated from the characteristic evaluation of the graphite material-only negative electrode and the non-graphite material-only negative electrode, and was confirmed to be an excellent negative electrode material.
[0022]
(Example 2)
The performance evaluation method of a lithium secondary battery using the negative electrode material for a non-aqueous electrolyte secondary battery of the present invention will be described. The method for preparing the electrode and the battery used in this example is described below. For the positive electrode, LiMn ₂ O ₄ was used as a material. To 87 parts by weight of the positive electrode active material, 8.7 parts by weight of artificial graphite serving as a conductive additive, and 4.3 parts by weight of polyvinylidene fluoride (# 1120 manufactured by Kureha) dissolved in 1-methyl-2-pyrrolidone as a solid content. The mixture was mixed to form a paste, applied to both sides of an aluminum foil as a current collector, and dried at 80 ° C. for 3 hours. Then, it was pressed to a density of about 2.7 g / cm ³ and vacuum dried at 120 ° C. for 3 hours to obtain a positive electrode.
[0023]
The negative electrode was added to a material obtained by mixing a graphite material in an arbitrary ratio with the non-graphite material described in Embodiment 1 so as to be 10 parts by weight of polyvinylidene fluoride (# 1120 manufactured by Kureha), and 1-methyl-2-methyl-2-methyl-2- as a solvent was used. Pyrrolidone is added and kneaded so that the solid concentration becomes 45 parts by weight. The slurry thus obtained was applied to both sides of a copper foil as a current collector, and dried at 80 ° C. for 3 hours. Then, it was pressed to a density of about 1.0 g / cm ³ and vacuum-dried at 120 ° C. for 3 hours.
[0024]
The batteries of Embodiments 5 to 8 use the negative electrode materials shown in Embodiments 1 to 4, and the batteries of Comparative Examples 9 to 14 use the negative electrode materials of Comparative Examples 1 to 7 to form electrodes in the same manner as described above. Created. The obtained positive electrode and negative electrode were wound in combination with a polyethylene porous membrane separator (thickness: 0.025 mm), and stored in a battery can having an outer diameter of 18 mm and a length of 65 mm. As an electrolytic solution, a solution obtained by dissolving 1 M of lithium hexafluorophosphate as an electrolyte in a mixed solvent of ethylene carbonate and methyl ethyl carbonate (volume ratio 1: 1) was used. FIG. 2 shows a partial cross-sectional front view of an example of the battery used in this test.
[0025]
The cylindrical lithium secondary battery shown in FIG. 2 is formed by winding a laminate of a negative electrode 2 processed in the same manner as a positive electrode 1 processed into a thin plate via a separator 3 such as a polyethylene porous film. It is inserted into a battery can 7 made of metal or the like and hermetically sealed. The positive electrode is connected to a positive electrode lid 6 via a positive electrode tab 4, and the negative electrode 2 is connected to the bottom of the battery via a negative electrode tab 5. The positive electrode lid 6 is fixed to the battery can 7 with a gasket 8.
[0026]
The battery prepared as described above was charged and discharged under the following conditions. The performance was evaluated at a constant current of 500 mA at a charge stop voltage of 4.2 V and a discharge stop voltage of 2.7 V. Table 2 shows the charging / discharging efficiency evaluation results of the battery. In this example, the same tendency as in Embodiments 1 to 4 is obtained, and the discharge capacity of the battery when the graphite material alone negative electrode is used and the discharge capacity of the battery when the non-graphite material only negative electrode is used are It was confirmed that a secondary battery exhibiting an excellent discharge capacity could be manufactured from the estimated capacity.
[0027]
[Table 2]

[0028]
【The invention's effect】
By using the negative electrode material of the present invention, a non-aqueous electrolyte secondary battery having excellent discharge capacity characteristics can be manufactured, which can contribute to miniaturization of portable electronic devices and the like.
[Brief description of the drawings]
FIG. 1 is a diagram showing single electrode capacity characteristics of a negative electrode material of the present invention.
FIG. 2 is a cross-sectional view of a cylindrical lithium secondary battery according to the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Positive electrode, 2 ... Negative electrode, 3 ... Separator, 4 ... Positive electrode tab, 5 ... Negative electrode tab, 6 ... Positive electrode cover, 7 ... Battery can, 8 ... Gasket.

Claims

黒鉛質材料と該黒鉛質材料より高い放電容量を有する非黒鉛質材料との混合物からなる負極材料において、前記非黒鉛質材料の混合量が１０体積％より大きく９０体積％未満で、且つ、前記非黒鉛質材料は参照極及び対極に金属リチウムを用いて作成したモデル電池評価において、Ｌｉ基準電位で０Ｖまで充電後、１．５Ｖまで放電した際の初回充放電効率が８５％より高い値を示す材料であることを特徴とする非水系電解液二次電池用負極材料。In a negative electrode material comprising a mixture of a graphite material and a non-graphite material having a higher discharge capacity than the graphite material, the amount of the non-graphite material is greater than 10% by volume and less than 90% by volume, and In a non-graphitic material, the initial charge / discharge efficiency was higher than 85% when the battery was charged to 0 V at the Li reference potential and then discharged to 1.5 V in a model battery evaluation prepared using metallic lithium for the reference electrode and the counter electrode. A negative electrode material for a non-aqueous electrolyte secondary battery, which is a material shown below.

リチウムの吸蔵放出が可能な正極活物質が集電体箔の両面に形成された正極と、リチウムの吸蔵放出が可能な負極活物質が集電体箔の両面に形成された負極と、リチウム塩を含む非水電解液とを有し、前記負極活物質が黒鉛質材料と該黒鉛質材料より高い放電容量を有する非黒鉛質材料の混合材料であり、前記正極及び負極がセパレータを介して巻回または積層された非水系電解液二次電池において、前記非黒鉛質材料の混合量が１０体積％より大きく９０体積％未満であり、また、前記非黒鉛質材料は０Ｖまで充電後、１．５Ｖまで放電した際の初回充放電効率が８５％より高い値を示す材料であることを特徴とする非水系電解液二次電池。A positive electrode in which a positive electrode active material capable of inserting and extracting lithium is formed on both sides of a current collector foil; a negative electrode in which a negative electrode active material capable of inserting and extracting lithium is formed on both sides of a current collector foil; A negative electrode active material is a mixed material of a graphite material and a non-graphite material having a higher discharge capacity than the graphite material, and the positive electrode and the negative electrode are wound via a separator. In a non-aqueous electrolyte secondary battery that has been repeatedly or laminated, the amount of the non-graphitic material mixed is greater than 10% by volume and less than 90% by volume, and the non-graphitic material is charged to 0V and then 1. A non-aqueous electrolyte secondary battery, characterized in that the material has an initial charge / discharge efficiency higher than 85% when discharged to 5V.