JP6319564B2 - Carbon nanotube dispersion and non-aqueous electrolyte secondary battery - Google Patents
Carbon nanotube dispersion and non-aqueous electrolyte secondary batteryInfo
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- JP6319564B2 JP6319564B2 JP2014073228A JP2014073228A JP6319564B2 JP 6319564 B2 JP6319564 B2 JP 6319564B2 JP 2014073228 A JP2014073228 A JP 2014073228A JP 2014073228 A JP2014073228 A JP 2014073228A JP 6319564 B2 JP6319564 B2 JP 6319564B2
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- carbon nanotube
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- nanotube dispersion
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- secondary battery
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- GELKBWJHTRAYNV-UHFFFAOYSA-K lithium iron phosphate Chemical compound [Li+].[Fe+2].[O-]P([O-])([O-])=O GELKBWJHTRAYNV-UHFFFAOYSA-K 0.000 description 1
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Classifications
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Description
本発明は、非水電解質二次電池電極活物質の導電助剤として添加した場合、サイクル特性を飛躍的に向上させる効果を持ったカーボンナノチューブ分散液を提供する。 The present invention provides a carbon nanotube dispersion having an effect of dramatically improving cycle characteristics when added as a conductive assistant for a nonaqueous electrolyte secondary battery electrode active material.
近年、ＡＶ機器やパソコン等の電子機器のポータブル化、コードレス化が急速に進んでおり、これらの駆動用電源として小型、軽量で高エネルギー密度を有する二次電池への要求が高くなっている。このような状況下において、充放電電圧が高く、充放電容量も大きいという長所を有するリチウムイオン二次電池が注目されている。 In recent years, electronic devices such as AV devices and personal computers are rapidly becoming portable and cordless, and there is an increasing demand for secondary batteries having a small size, light weight, and high energy density as power sources for driving these devices. Under such circumstances, a lithium ion secondary battery having advantages such as a high charge / discharge voltage and a large charge / discharge capacity has attracted attention.
このリチウムイオン二次電池において、近年では、高速・高容量化、高寿命化が加速している。これら特性の向上目的で、正極ならびに負極活物質に導電助剤を添加して電極を作製し、リチウムイオン二次電池を設計することが既に行われている。例えば、正極活物質の導電助剤としては、一般にはアセチレンブラック等が用いられることが多い。しかしながら、アセチレンブラック添加等では、サイクル特性を飛躍的に向上させるほどの効果は得られていないのが現状である。 In recent years, in this lithium ion secondary battery, high speed, high capacity, and long life have been accelerated. For the purpose of improving these characteristics, it has already been carried out to design a lithium ion secondary battery by preparing an electrode by adding a conductive additive to the positive electrode and the negative electrode active material. For example, in general, acetylene black or the like is often used as a conductive additive for the positive electrode active material. However, with acetylene black addition or the like, the present situation is that an effect that greatly improves the cycle characteristics is not obtained.
そこで、高い電気伝導性を示す炭素材料である炭素繊維やカーボンナノチューブが注目されている。特に径の細いカーボンナノチューブを導電助剤とすることにより、活物質間の間隙に多数の導電経路が形成され、充放電の際に起こる活物質の体積変化による容量劣化を防止する効果が期待される。しかしながら、炭素繊維やカーボンナノチューブの粉末は、径が細いほど電極合剤中での分散が困難となる。したがって、導電助剤として用いることができる炭素繊維やカーボンナノチューブの粉末は比較的径が太いものに限定されていた。 Therefore, carbon fibers and carbon nanotubes, which are carbon materials exhibiting high electrical conductivity, are attracting attention. In particular, by using carbon nanotubes with small diameters as conductive aids, many conductive paths are formed in the gaps between the active materials, and the effect of preventing capacity deterioration due to volume change of the active material that occurs during charging and discharging is expected. The However, the smaller the diameter of carbon fiber or carbon nanotube powder, the more difficult it is to disperse in the electrode mixture. Therefore, the carbon fiber and carbon nanotube powders that can be used as the conductive auxiliary agent are limited to those having a relatively large diameter.
一方、カーボンナノチューブは、従来から、例えば透明導電材料として期待され、応用されてきた。例えば、透明導電性膜の材料として、特許文献１〜５のようなカーボンナノチューブ分散液が知られている。 On the other hand, carbon nanotubes have been conventionally expected and applied as, for example, transparent conductive materials. For example, carbon nanotube dispersions as described in Patent Documents 1 to 5 are known as materials for transparent conductive films.
即ち、特許文献１、２は平均外径４ｎｍ以下、動的光散乱法による平均粒子径１００〜４０００ｎｍ、カーボンナノチューブの平均長さ０．５〜５μｍの水分散液の記載があるが、濃度が１％程度の低濃度であること、分散剤の添加量がカーボンナノチューブ１００重量部に対し、５０〜１０００重量部と非常に多量であるため、非水電解質二次電池電極活物質の導電助剤としては好ましくない。 That is, Patent Documents 1 and 2 describe an aqueous dispersion having an average outer diameter of 4 nm or less, an average particle diameter of 100 to 4000 nm by a dynamic light scattering method, and an average length of carbon nanotubes of 0.5 to 5 μm. Since the concentration is as low as 1% and the amount of the dispersant added is as large as 50 to 1000 parts by weight with respect to 100 parts by weight of the carbon nanotubes, the conductive assistant for the non-aqueous electrolyte secondary battery electrode active material It is not preferable.
また、特許文献３〜５には、カーボンナノチューブ水分散液の長期分安定性を実現するため、様々な分散剤あるいは添加剤の記載がある。これら分散剤・添加剤を使用した水分散液は透明導電膜作製を目的としており、カーボンナノチューブの濃度が高々１％であって、非水電解質二次電池電極活物質の導電助剤として使用するには希薄すぎるため好ましくない。 Patent Documents 3 to 5 describe various dispersants or additives in order to realize long-term stability of the carbon nanotube aqueous dispersion. Aqueous dispersions using these dispersants / additives are intended for the production of transparent conductive films, and the concentration of carbon nanotubes is at most 1%, and are used as conductive aids for non-aqueous electrolyte secondary battery electrode active materials. Is not preferable because it is too dilute.
そこで、本発明は、非水電解質二次電池電極活物質の導電助剤として添加することができ、サイクル特性を飛躍的に向上させる効果を持ったカーボンナノチューブ分散液を得ることを技術的課題とする。 Therefore, the present invention has a technical problem of obtaining a carbon nanotube dispersion liquid that can be added as a conductive aid for a nonaqueous electrolyte secondary battery electrode active material and has an effect of dramatically improving cycle characteristics. To do.
前記技術的課題は、次の通りの本発明によって達成できる。 The technical problem can be achieved by the present invention as follows.
即ち、本発明は、ＢＥＴ比表面積値が７０〜２５０ｍ２／ｇのカーボンナノチューブ粉末を２〜１５重量％含有した分散液であって、温度２５℃、ずり速度 ３８３ｓ−１における粘度値が２〜１１０ｍＰａ・ｓであることを特徴とするカーボンナノチューブ分散液である（本発明１）。 That is, the present invention is a dispersion containing 2 to 15% by weight of carbon nanotube powder having a BET specific surface area value of 70 to 250 m 2 / g, and has a viscosity value of 2 to 25 ° C. and a shear rate of 383 s −1 . A carbon nanotube dispersion characterized by 110 mPa · s (Invention 1).
また、本発明は、動的光散乱法によって測定した分散粒子径ｄ５０が１００〜６００ｎｍである本発明１記載のカーボンナノチューブ分散液である（本発明２）。 In addition, the present invention is the carbon nanotube dispersion liquid according to the present invention 1, wherein the dispersed particle diameter d50 measured by a dynamic light scattering method is 100 to 600 nm (the present invention 2).
また、本発明は、分散液中のカーボンナノチューブの長さを透過型電子顕微鏡画像から算出したカーボンナノチューブ長さｄ５０が、０．５〜２．０μｍである本発明１または本発明２記載のカーボンナノチューブ分散液である（本発明３）。 The present invention also provides the carbon according to the first or second aspect of the present invention, wherein the carbon nanotube length d50 calculated from the transmission electron microscope image of the length of the carbon nanotubes in the dispersion is 0.5 to 2.0 μm. It is a nanotube dispersion (Invention 3).
また、本発明は、含有するカーボンナノチューブ粉末のＣ純度が９０％以上である本発明１〜本発明３のいずれかに記載のカーボンナノチューブ分散液である（本発明４）。 Moreover, this invention is a carbon nanotube dispersion liquid in any one of this invention 1-this invention 3 whose C purity of the carbon nanotube powder to contain is 90% or more (this invention 4).
また、本発明は、分散媒が水であることを特徴とする本発明１〜本発明４のいずれかに記載のカーボンナノチューブ分散液である（本発明５）。 Further, the present invention is the carbon nanotube dispersion liquid according to any one of the present invention 1 to the present invention 4, wherein the dispersion medium is water (the present invention 5).
また、本発明は、分散媒が非水系分散媒であることを特徴とする本発明１〜本発明４のいずれかに記載のカーボンナノチューブ分散液である（本発明６）。 Further, the present invention is the carbon nanotube dispersion liquid according to any one of the present invention 1 to the present invention 4, wherein the dispersion medium is a non-aqueous dispersion medium (the present invention 6).
また、本発明は、本発明１〜本発明６のいずれかに記載のカーボンナノチューブ分散液を用いた電極合剤を使用したことを特徴とする非水電解質二次電池である（本発明７）。 Further, the present invention is a non-aqueous electrolyte secondary battery using the electrode mixture using the carbon nanotube dispersion liquid according to any one of the present invention 1 to the present invention 6 (the present invention 7). .
本発明に係るカーボンナノチューブ分散液を非水電解質二次電池電極活物質の導電助剤として添加した場合、サイクル特性を飛躍的に向上させることができる。 When the carbon nanotube dispersion liquid according to the present invention is added as a conductive additive for the non-aqueous electrolyte secondary battery electrode active material, the cycle characteristics can be dramatically improved.
本発明の構成をより詳しく説明すれば次の通りである。 The configuration of the present invention will be described in more detail as follows.
先ず、本発明に係るカーボンナノチューブ分散液について述べる。 First, the carbon nanotube dispersion according to the present invention will be described.
本発明に係るカーボンナノチューブ分散液は、ＢＥＴ比表面積値が７０〜２５０ｍ２／ｇのカーボンナノチューブ粉末を２〜１５重量％含有した分散液であって、温度２５℃、ずり速度 ３８３ｓ−１における粘度値が２〜８０ｍＰａ・ｓであることを特徴とするカーボンナノチューブ分散液である。 The carbon nanotube dispersion according to the present invention is a dispersion containing 2 to 15% by weight of carbon nanotube powder having a BET specific surface area value of 70 to 250 m 2 / g, and has a viscosity at a temperature of 25 ° C. and a shear rate of 383 s −1 . The carbon nanotube dispersion liquid has a value of 2 to 80 mPa · s.
本発明に係るカーボンナノチューブ分散液中に含有されるカーボンナノチューブ粉末のＢＥＴ比表面積値が７０ｍ２／ｇ未満の場合には、カーボンナノチューブのチューブ径が太いため、これを導電助剤として活物質に添加して電極を作製した場合、活物質間の間隙中に存在するチューブ本数が少ないため、十分な導電経路が確保されない。このため、電極活物質に対する導電性付与効果が十分に得られないので好ましくない。一方、カーボンナノチューブのＢＥＴ比表面積が２５０ｍ２／ｇを超える場合には、分散媒中にカーボンナノチューブを分散させることが困難になるため好ましくない。ＢＥＴ比表面積値のより好ましい範囲は、８０〜２４５ｍ２／ｇである。 When the BET specific surface area value of the carbon nanotube powder contained in the carbon nanotube dispersion liquid according to the present invention is less than 70 m 2 / g, the carbon nanotube has a large tube diameter. When the electrode is prepared by adding the electrode, a sufficient number of conductive paths cannot be ensured because the number of tubes existing in the gap between the active materials is small. For this reason, since the electroconductivity provision effect with respect to an electrode active material cannot fully be acquired, it is unpreferable. On the other hand, when the BET specific surface area of the carbon nanotube exceeds 250 m 2 / g, it is difficult to disperse the carbon nanotube in the dispersion medium, which is not preferable. A more preferable range of the BET specific surface area value is 80 to 245 m 2 / g.
本発明に係るカーボンナノチューブ分散液中に含有されるカーボンナノチューブ粉末の含有量が２重量％未満の場合、カーボンナノチューブ含有濃度が低すぎるため、電極活物質に対して必要十分な量のカーボンナノチューブが添加された電極合剤を調製することが困難となるため、好ましくない。一方、１５重量％を越える場合、分散液中のカーボンナノチューブの分散が困難となるため、好ましくない。カーボンナノチューブ粉末含有量のより好ましい範囲は、２〜１４重量％である。 When the content of the carbon nanotube powder contained in the carbon nanotube dispersion liquid according to the present invention is less than 2% by weight, the carbon nanotube content is too low. Since it becomes difficult to prepare the added electrode mixture, it is not preferable. On the other hand, if it exceeds 15% by weight, it is difficult to disperse the carbon nanotubes in the dispersion, which is not preferable. A more preferable range of the carbon nanotube powder content is 2 to 14% by weight.
本発明に係るカーボンナノチューブ分散液において、粘度値が２ｍＰａ・ｓを下回る分散液を調製することは困難である。一方、１１０ｍＰａ・ｓを越える場合、電極合剤の調製時にスラリーの粘度が高くなりすぎるため、電極作製が困難となり好ましくない。分散液粘度のより好ましい範囲は、２〜１０５ｍＰａ・ｓである。 In the carbon nanotube dispersion according to the present invention, it is difficult to prepare a dispersion having a viscosity value lower than 2 mPa · s. On the other hand, if it exceeds 110 mPa · s, the viscosity of the slurry becomes too high during preparation of the electrode mixture, which makes it difficult to produce the electrode. A more preferable range of the dispersion viscosity is 2 to 105 mPa · s.
本発明におけるカーボンナノチューブ分散液の動的光散乱法によって測定した分散粒子径ｄ５０は、１００〜６００ｎｍであることが好ましい。分散粒子径ｄ５０が１００ｎｍ未満の場合には、分散処理過程でカーボンナノチューブがダメージを受け、その繊維長が短くなりすぎていることに対応するため、好ましくない。６００ｎｍを越える場合には、分散液中のカーボンナノチューブが大きな凝集体を形成し、分散不良となっており好ましくない。分散粒子径ｄ５０のより好ましい範囲は、１００〜５８０ｎｍである。 The dispersed particle diameter d50 measured by the dynamic light scattering method of the carbon nanotube dispersion in the present invention is preferably 100 to 600 nm. When the dispersed particle diameter d50 is less than 100 nm, it is not preferable because the carbon nanotube is damaged during the dispersion treatment and the fiber length is too short. When the thickness exceeds 600 nm, the carbon nanotubes in the dispersion form a large aggregate, resulting in poor dispersion. A more preferable range of the dispersed particle diameter d50 is 100 to 580 nm.
本発明におけるカーボンナノチューブ分散液中のカーボンナノチューブ長さを透過型電子顕微鏡画像から算出したカーボンナノチューブ長さｄ５０は、０．５〜２．０μｍであることが好ましい。カーボンナノチューブ長さｄ５０が０．５μｍ未満の場合には、分散処理過程でカーボンナノチューブがダメージを受け、その繊維長が短くなりすぎていることに対応するため、好ましくない。２．０μｍを越える場合には、分散液中のカーボンナノチューブが大きな凝集体を形成し、分散不良となっており好ましくない。カーボンナノチューブ長さｄ５０のより好ましい範囲は、０．５５〜２．０μｍである。 The carbon nanotube length d50 calculated from the transmission electron microscope image of the carbon nanotube length in the carbon nanotube dispersion in the present invention is preferably 0.5 to 2.0 μm. When the carbon nanotube length d50 is less than 0.5 μm, it is not preferable because the carbon nanotube is damaged during the dispersion process and the fiber length becomes too short. In the case of exceeding 2.0 μm, the carbon nanotubes in the dispersion form a large aggregate, resulting in poor dispersion. A more preferable range of the carbon nanotube length d50 is 0.55 to 2.0 μm.
本発明におけるカーボンナノチューブ分散液中に含有されるカーボンナノチューブ粉末のＣ純度は、９０％以上であることが好ましい。Ｃ純度が９０％未満の場合、電極活物質に対する導電性付与効果が十分に得られないので好ましくない。Ｃ純度のより好ましい範囲は、９１％以上である。 The C purity of the carbon nanotube powder contained in the carbon nanotube dispersion in the present invention is preferably 90% or more. When the C purity is less than 90%, the effect of imparting conductivity to the electrode active material cannot be obtained sufficiently, which is not preferable. A more preferable range of C purity is 91% or more.
次に、本発明におけるカーボンナノチューブ分散液の製造法について述べる。 Next, a method for producing a carbon nanotube dispersion in the present invention will be described.
本発明に係るカーボンナノチューブ分散液は、カーボンナノチューブ粉末、分散剤、および分散媒を所定量添加混合し、適切な分散装置を用いて混合・分散することによって得ることができる。 The carbon nanotube dispersion liquid according to the present invention can be obtained by adding and mixing a predetermined amount of carbon nanotube powder, a dispersant, and a dispersion medium, and mixing and dispersing the mixture using an appropriate dispersing apparatus.
本発明において用いるカーボンナノチューブ粉末は、単層のカーボンナノチューブ、多層のカーボンナノチューブのいずれでもよく、分散液の用途に応じて選択することができる。また、カーボンナノチューブ粉末の製造方法も特に限定されるものではなく、熱ＣＶＤ・プラズマＣＶＤなどの熱分解法、アーク放電法、レーザー蒸発法等いずれを用いてもよい。 The carbon nanotube powder used in the present invention may be either a single-walled carbon nanotube or a multi-walled carbon nanotube, and can be selected according to the use of the dispersion. Also, the method for producing the carbon nanotube powder is not particularly limited, and any of pyrolysis methods such as thermal CVD and plasma CVD, arc discharge method, laser evaporation method and the like may be used.
本発明において用いる分散剤は、分散媒の種類に応じて適宜選択する。水系分散媒の場合は、多糖類高分子を用いることが好ましい。ここでいう多糖類とは、単糖分子が２個以上縮合した糖のことであり、その官能基を変換または修飾した誘導体も含まれる。具体的には、デンプン、グリコーゲン、アガロース、ペクチン、セルロースなどがあげられる。なかでも、修飾された多糖類の誘導体であるカルボキシルメチルセルロース類が好ましい。有機非水系分散媒の場合は、分散媒への溶解性が高く、カーボンナノチューブへの吸着能を有するものが好ましい。例えば、アクリレート類、アクリル酸、メタクリル酸などのアクリルモノマーの共重合体、ポリアリルアミン、ポリエチレンイミン、ポリビニルピロリドン、ポリフッ化ビニリデン、ポリビニルアセタール類、ポリエステル、セルロース誘導体等のポリマーまたはこれらの共重合体などが使用できる。なかでも、ポリビニルアセタール類が好ましく、その一例としてポリビニルブチラールが挙げられる。本発明において用いる分散剤の添加量は、カーボンナノチューブ１００重量部に対し、０．５〜３０重量部が好ましい。より好ましくは、カーボンナノチューブ１００重量部に対し、０．５〜２５重量部である。 The dispersant used in the present invention is appropriately selected according to the type of the dispersion medium. In the case of an aqueous dispersion medium, it is preferable to use a polysaccharide polymer. The term “polysaccharide” used herein refers to a saccharide obtained by condensing two or more monosaccharide molecules, and includes derivatives obtained by converting or modifying the functional group. Specific examples include starch, glycogen, agarose, pectin, and cellulose. Of these, carboxylmethylcelluloses which are derivatives of modified polysaccharides are preferred. In the case of an organic non-aqueous dispersion medium, those having high solubility in the dispersion medium and adsorbability to the carbon nanotubes are preferable. For example, copolymers of acrylic monomers such as acrylates, acrylic acid, methacrylic acid, polymers such as polyallylamine, polyethyleneimine, polyvinyl pyrrolidone, polyvinylidene fluoride, polyvinyl acetals, polyester, cellulose derivatives, etc. Can be used. Of these, polyvinyl acetals are preferable, and polyvinyl butyral is an example. The amount of the dispersant used in the present invention is preferably 0.5 to 30 parts by weight with respect to 100 parts by weight of the carbon nanotubes. More preferably, it is 0.5 to 25 parts by weight with respect to 100 parts by weight of the carbon nanotube.
本発明において用いる分散媒は、製造する分散液の用途に応じて適宜選択する。カーボンナノチューブ分散液を非水電解質二次電池負極活物質の導電助剤として添加する場合は、分散媒として水を用いることが多い。一方、カーボンナノチューブ分散液を非水電解質二次電池正極活物質の導電助剤として添加する場合は、通常、非水系分散媒が用いられる。非水系分散媒としては、Ｎ−メチル−２−ピロリドン、Ｎ，Ｎ−ジメチルホルムアミド、メチルエチルケトン、２−プロパノールなどが好ましく用いられる。 The dispersion medium used in the present invention is appropriately selected according to the use of the dispersion to be produced. When the carbon nanotube dispersion liquid is added as a conductive aid for the negative electrode active material of the nonaqueous electrolyte secondary battery, water is often used as the dispersion medium. On the other hand, when the carbon nanotube dispersion liquid is added as a conductive additive for the positive electrode active material of the nonaqueous electrolyte secondary battery, a nonaqueous dispersion medium is usually used. As the non-aqueous dispersion medium, N-methyl-2-pyrrolidone, N, N-dimethylformamide, methyl ethyl ketone, 2-propanol and the like are preferably used.
本発明に係るカーボンナノチューブ分散液を得るために使用可能な分散装置は、カーボンナノチューブ粉末、分散剤、および分散媒を含有する混合物を十分に混合・分散できる装置であれば特に限定されない。これのような装置としては、ナノマイザー、アルティマイザー、超音波分散機などの分散メディアを使用しない分散装置、ボールミル、ビーズミル、スパイクミル、ペイントシェイカー、高速撹拌装置等があげられる。 The dispersion apparatus that can be used for obtaining the carbon nanotube dispersion liquid according to the present invention is not particularly limited as long as it can sufficiently mix and disperse a mixture containing carbon nanotube powder, a dispersant, and a dispersion medium. Examples of such an apparatus include a dispersion apparatus that does not use dispersion media such as a nanomizer, an optimizer, and an ultrasonic dispersion machine, a ball mill, a bead mill, a spike mill, a paint shaker, and a high-speed stirring apparatus.
上記にあげた分散装置を使用し、発振周波数、メディア粒子径、処理時間、処理流量、装置回転数、装置内圧力、処理温度などの分散条件を適宜選択することによって、本発明に係るカーボンナノチューブ分散液を得ることができる。 The carbon nanotube according to the present invention is used by appropriately selecting the dispersion conditions such as oscillation frequency, media particle diameter, treatment time, treatment flow rate, device rotation speed, device internal pressure, treatment temperature, etc. A dispersion can be obtained.
次に、本発明に係るカーボンナノチューブ分散液を用いた非水電解質二次電池について述べる。 Next, a non-aqueous electrolyte secondary battery using the carbon nanotube dispersion according to the present invention will be described.
非水電解質二次電池は、負極、正極、セパレータおよび電解質から構成される。 The nonaqueous electrolyte secondary battery is composed of a negative electrode, a positive electrode, a separator, and an electrolyte.
二次電池用負極は、活物質として天然黒鉛等のグラファイト系材料と結着剤と分散媒を混合して塗料化した電極合剤を調製し、集電箔上に塗布・成形して得られる。なお、負極活物質の表面抵抗を低減する目的で、導電助剤を添加する場合がある。 A negative electrode for a secondary battery is obtained by preparing an electrode mixture obtained by mixing graphite material such as natural graphite, a binder and a dispersion medium as an active material, and applying and molding the mixture on a current collector foil. . In addition, a conductive support agent may be added for the purpose of reducing the surface resistance of the negative electrode active material.
二次電池用正極は、活物質として、リチウム含有酸化マンガン、マンガン酸リチウム、コバルト酸リチウム、ニッケル酸リチウム、リン酸鉄リチウム、五酸化バナジウム及びこれらの化合物の一部を他の元素で置換した化合物の一種または二種以上と結着剤と分散媒を混合して塗料化した電極合剤を調製し、集電箔上に塗布・成形して得られる。なお、正極活物質の表面抵抗を低減する目的で、導電助剤を添加する場合がある。 The positive electrode for a secondary battery has lithium-containing manganese oxide, lithium manganate, lithium cobaltate, lithium nickelate, lithium iron phosphate, vanadium pentoxide, and a part of these compounds substituted with other elements as an active material. An electrode mixture prepared by mixing one or more compounds, a binder and a dispersion medium into a paint is prepared, and is applied and molded on a current collector foil. Note that a conductive additive may be added for the purpose of reducing the surface resistance of the positive electrode active material.
本発明おける非水電解質二次電池は、本発明に係るカーボンナノチューブ分散液を用いた電極合剤を使用する。本発明に係るカーボンナノチューブ分散液を電極合剤に添加することで、カーボンナノチューブが負極活物質または正極活物質の導電助剤として作用する。 The nonaqueous electrolyte secondary battery in the present invention uses an electrode mixture using the carbon nanotube dispersion liquid according to the present invention. By adding the carbon nanotube dispersion liquid according to the present invention to the electrode mixture, the carbon nanotubes act as a conductive aid for the negative electrode active material or the positive electrode active material.
セパレータには、通常ポリプロピレン製マイクロポーラスフィルム等が用いられる。電解液には、炭酸プロピレン、炭酸エチレン、炭酸ジエチル、１，２−ジメトキシエタンなどの溶媒にＬｉＰＦ６、ＬｉＣｌＯ４、ＬｉＣＦ３ＳＯ３、ＬｉＮ（ＣＦ３ＳＯ２）２、ＬｉＢＦ４などのリチウム塩を溶解させたものが用いられる。 As the separator, a polypropylene microporous film or the like is usually used. The electrolyte includes a lithium salt such as LiPF 6 , LiClO 4 , LiCF 3 SO 3 , LiN (CF 3 SO 2 ) 2 , LiBF 4 in a solvent such as propylene carbonate, ethylene carbonate, diethyl carbonate, 1,2-dimethoxyethane. What dissolved is used.
本発明における非水電解質二次電池のサイクル特性は、８４％以上が好ましい。より好ましくは、８５％以上である。 The cycle characteristics of the nonaqueous electrolyte secondary battery in the present invention are preferably 84% or more. More preferably, it is 85% or more.
＜作用＞
本発明において最も重要な点は、本発明に係るカーボンナノチューブ分散液を非水電解質二次電池電極活物質の導電助剤として添加した場合、サイクル特性を飛躍的に向上させることができるという点である。
<Action>
The most important point in the present invention is that, when the carbon nanotube dispersion liquid according to the present invention is added as a conductive aid for a non-aqueous electrolyte secondary battery electrode active material, the cycle characteristics can be drastically improved. is there.
本発明においてサイクル特性が飛躍的に向上するのは、活物質表面へのカーボンナノチューブ被覆による表面抵抗の低減、および充放電にともなう活物質の体積変化による容量劣化防止（活物質間の導電経路の維持確保）の効果によるものと考えられる。 In the present invention, the cycle characteristics are dramatically improved because the surface resistance is reduced by coating the surface of the active material with carbon nanotubes, and the capacity deterioration due to the volume change of the active material due to charge / discharge is prevented (the conductive path between the active materials is reduced). This is thought to be due to the effect of maintenance).
本発明の代表的な実施の形態は、次の通りである。 A typical embodiment of the present invention is as follows.
本発明に用いるカーボンナノチューブ粉末は、内部に金属等の原子を有するものでも良く、カップ状のカーボンが積層したカップスタック型、バンブー型いずれでも良い。また、単層、二層、多層いずれのカーボンナノチューブでも良い。また、カーボンナノチューブ粉末の製造方法も特に限定されるものではなく、熱ＣＶＤ・プラズマＣＶＤなどの熱分解法、アーク放電法、レーザー蒸発法等いずれを用いてもよい。 The carbon nanotube powder used in the present invention may have a metal atom or the like inside, and may be a cup stack type or a bamboo type in which cup-shaped carbon is laminated. Further, single-walled, double-walled, or multi-walled carbon nanotubes may be used. Also, the method for producing the carbon nanotube powder is not particularly limited, and any of pyrolysis methods such as thermal CVD and plasma CVD, arc discharge method, laser evaporation method and the like may be used.
カーボンナノチューブ粉末の比表面積は、「モノソーブＭＳ−１１」（カンタクロム株式会社）を用いて、ＢＥＴ法により測定した値で示した。 The specific surface area of the carbon nanotube powder was shown as a value measured by BET method using “Monosorb MS-11” (Kantachrome Co., Ltd.).
カーボンナノチューブ粉末のＣ純度は、所定量のカーボンナノチューブ粉末を空気中、１０５０℃で３時間焼成し、残留した灰分重量から算出した。 The C purity of the carbon nanotube powder was calculated from the weight of the residual ash after firing a predetermined amount of the carbon nanotube powder in air at 1050 ° C. for 3 hours.
カーボンナノチューブ分散液の粘度は、Ｅ型粘度計を用い、温度２５℃、ずり速度３８１Ｓ−１の条件下で測定した。 The viscosity of the carbon nanotube dispersion was measured using an E-type viscometer under conditions of a temperature of 25 ° C. and a shear rate of 381S- 1 .
カーボンナノチューブ分散液中に含まれるカーボンナノチューブ粉末の含有量は、所定量のカーボンナノチューブ分散液を１２０℃で乾燥し、固形分重量から算出した。 The content of the carbon nanotube powder contained in the carbon nanotube dispersion was calculated from the solid content weight after drying a predetermined amount of the carbon nanotube dispersion at 120 ° C.
カーボンナノチューブ分散液中に分散したカーボンナノチューブの分散粒子径ｄ５０(ｎｍ)は、動的光散乱式粒度分布測定装置（大塚電子株式会社製 ＦＰＡＲ１０００）を用いて測定した。 The dispersed particle diameter d50 (nm) of the carbon nanotubes dispersed in the carbon nanotube dispersion was measured using a dynamic light scattering particle size distribution analyzer (FPAR1000 manufactured by Otsuka Electronics Co., Ltd.).
カーボンナノチューブ分散液中に分散したカーボンナノチューブの長さは、下記の通り、透過型電子顕微鏡を用いて測定する。まず、分散液をカーボン・マイクロ・グリッド上に捕集・乾燥し、透過型電子顕微鏡を用い、２０００倍で観察・撮影し、１２μｍ×１０μｍの視野内に存在すカーボンナノチューブの長さを繊維に沿って計測する。一視野当りのサンプリング数は、５０〜１００程度、最低１０視野以上について、サンプリングを行い、標本点数５００〜７００を確保して統計精度をあげる。得られたカーボンナノチューブ長さのデータについて、統計計算処理を実施し、カーボンナノチューブ長さｄ５０（μｍ）を算出する。 The length of the carbon nanotube dispersed in the carbon nanotube dispersion is measured using a transmission electron microscope as follows. First, the dispersion was collected and dried on a carbon microgrid, and observed and photographed at a magnification of 2000 using a transmission electron microscope, and the length of carbon nanotubes present in a 12 μm × 10 μm field of view was used as a fiber. Measure along. The number of samplings per field of view is about 50 to 100, and sampling is performed for at least 10 fields of view, and the number of sample points 500 to 700 is secured to improve the statistical accuracy. The obtained carbon nanotube length data is subjected to statistical calculation processing to calculate the carbon nanotube length d50 (μm).
非水電解質二次電池の電池特性は、下記製造法によって単層のラミネートセルを作製して評価した。 The battery characteristics of the nonaqueous electrolyte secondary battery were evaluated by producing a single-layer laminate cell by the following production method.
＜カーボンナノチューブ分散液を負極合剤に添加する場合＞
カーボンナノチューブ分散液を負極活物質の導電助剤として負極合剤に添加する場合の単層ラミネートセルは、以下のように作製した。
<When adding a carbon nanotube dispersion to the negative electrode mixture>
A single-layer laminate cell in the case where the carbon nanotube dispersion was added to the negative electrode mixture as a conductive aid for the negative electrode active material was prepared as follows.
（正極の作製）
ニッケル・コバルト・マンガン三元系リチウム複合酸化物（ＮＣＭ１１１：平均粒子径１０μｍ）とアセチレンブラックと結着剤のポリフッ化ビニリデンとを重量比で９３：４：３となるように精秤し、乳鉢で十分に混合してからＮ−メチル−２−ピロリドンに分散させて正極合剤スラリーを調製した。次に、このスラリーを集電体のアルミニウム箔に塗布し、１１０℃で真空乾燥してから所定の大きさに打ち抜き、一軸プレス機を用いてプレスし、正極板とした。
(Preparation of positive electrode)
Nickel / cobalt / manganese ternary lithium composite oxide (NCM111: average particle size 10 μm), acetylene black and binder polyvinylidene fluoride are precisely weighed to a weight ratio of 93: 4: 3, and a mortar And then thoroughly mixed with N-methyl-2-pyrrolidone to prepare a positive electrode mixture slurry. Next, this slurry was applied to an aluminum foil as a current collector, vacuum dried at 110 ° C., punched out to a predetermined size, and pressed using a uniaxial press to obtain a positive electrode plate.
（負極の作製）
活物質である表面改質天然黒鉛とカーボンナノチューブ分散液とカルボキシメチルセルロース（ＣＭＣ）と結着剤のスチレンブタジエンゴム(ＳＢＲ)とを活物質：カーボンナノチューブ：ＣＭＣ：ＳＢＲ＝９６：２：１：１．５（重量部数)となるように混合し、水に分散させて負極合剤スラリーを調製した。次に、このスラリーを集電体の銅箔に塗布し、８０℃で乾燥してから所定の大きさに打ち抜き、一軸プレス機を用いてプレスし、負極板とした。
(Preparation of negative electrode)
Surface-modified natural graphite, which is an active material, a carbon nanotube dispersion, carboxymethylcellulose (CMC), and a binder, styrene-butadiene rubber (SBR), are used as active materials: carbon nanotubes: CMC: SBR = 96: 2: 1: 1. The mixture was mixed so that the amount was 5 (parts by weight) and dispersed in water to prepare a negative electrode mixture slurry. Next, this slurry was applied to a copper foil as a current collector, dried at 80 ° C., punched out to a predetermined size, and pressed using a uniaxial press to obtain a negative electrode plate.
（電解液の調製）
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）との体積比３０：７０の混合溶液に電解質として六フッ化リン酸リチウム（ＬｉＰＦ６）を１モル／リットル混合して電解液とした。
(Preparation of electrolyte)
An electrolyte solution was prepared by mixing 1 mol / liter of lithium hexafluorophosphate (LiPF 6 ) as an electrolyte in a mixed solution of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70.
（単層ラミネートセルの組み立て）
下記のようにして単層ラミネートセルを作製した。なお、以下の操作は下露点−８０℃以下のアルゴン雰囲気中で実施した。セパレータを介して正極・負極を対向させ、積層体を作製する。該積層体をアルミラミネートで包み、三辺をヒートシールした。その後、これに電解液を注入し、真空シールして試験用ラミネートセルとした。
(Assembly of single layer laminate cell)
A single-layer laminate cell was produced as follows. In addition, the following operation was implemented in argon atmosphere with a dew point of -80 degrees C or less. A laminated body is produced by making the positive electrode and the negative electrode face each other with a separator interposed therebetween. The laminate was wrapped with aluminum laminate and heat sealed on three sides. Thereafter, an electrolytic solution was injected into this, and vacuum-sealed to obtain a test laminate cell.
＜カーボンナノチューブ分散液を正極合剤に添加する場合＞
カーボンナノチューブ分散液を正極活物質の導電助剤として正極合剤に添加する場合の単層ラミネートセルは、以下のように作製した。
<When adding a carbon nanotube dispersion to the positive electrode mixture>
A single-layer laminate cell in the case where the carbon nanotube dispersion was added to the positive electrode mixture as a conductive assistant for the positive electrode active material was prepared as follows.
（正極の作製）
活物質であるニッケル・コバルト・マンガン三元系リチウム複合酸化物（ＮＣＭ１１１：平均粒子径１０μｍ）とカーボンナノチューブ分散液とアセチレンブラック(ＡＢ)と結着剤のポリフッ化ビニリデン(ＰＶＤＦ)とを活物質：カーボンナノチューブ：ＡＢ：ＰＶＤＦ＝９３：２：１：４（重量部数）となるように混合し、Ｎ−メチル−２−ピロリドンに分散させて正極合剤スラリーを調製した。次に、このスラリーを集電体のアルミニウム箔に塗布し、１１０℃で真空乾燥してから所定の大きさに打ち抜き、一軸プレス機を用いてプレスし、正極板とした。
(Preparation of positive electrode)
The active material is nickel, cobalt, manganese ternary lithium composite oxide (NCM111: average particle size 10 μm), carbon nanotube dispersion, acetylene black (AB), and polyvinylidene fluoride (PVDF) binder. : Carbon nanotubes: AB: PVDF = 93: 2: 1: 4 (parts by weight) were mixed and dispersed in N-methyl-2-pyrrolidone to prepare a positive electrode mixture slurry. Next, this slurry was applied to an aluminum foil as a current collector, vacuum dried at 110 ° C., punched out to a predetermined size, and pressed using a uniaxial press to obtain a positive electrode plate.
（負極の作製）
活物質である表面改質天然黒鉛とアセチレンブラック(ＡＢ)とカルボキシメチルセルロース（ＣＭＣ）と結着剤のスチレンブタジエンゴム(ＳＢＲ)とを活物質：ＡＢ：ＣＭＣ：ＳＢＲ＝９６：２：１：１．５（重量部数)の比率で混合し、水に分散させて負極合剤スラリーを調製した。次に、このスラリーを集電体の銅箔に塗布し、８０℃で乾燥してから所定の大きさに打ち抜き、一軸プレス機を用いてプレスし、負極板とした。
(Preparation of negative electrode)
Surface-modified natural graphite, which is an active material, acetylene black (AB), carboxymethyl cellulose (CMC), and a binder, styrene-butadiene rubber (SBR), are used as an active material: AB: CMC: SBR = 96: 2: 1: 1. The mixture was mixed at a ratio of 0.5 (parts by weight) and dispersed in water to prepare a negative electrode mixture slurry. Next, this slurry was applied to a copper foil as a current collector, dried at 80 ° C., punched out to a predetermined size, and pressed using a uniaxial press to obtain a negative electrode plate.
（電解液の調製）
エチレンカーボネート(ＥＣ)とジメチルカーボネート（ＤＭＣ）との体積比１０：２０の混合溶液に電解質として六フッ化リン酸リチウム（ＬｉＰＦ６）を１モル／リットル混合して電解液とした。
(Preparation of electrolyte)
A mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 10:20 was mixed with 1 mol / liter of lithium hexafluorophosphate (LiPF 6 ) as an electrolyte to obtain an electrolytic solution.
（単層ラミネートセルの組み立て）
下記のようにして単層ラミネートセルを作製した。なお、以下の操作は下露点−８０℃以下のアルゴン雰囲気中で実施した。セパレータを介して正極・負極を対向させ、積層体を作製する。該積層体をアルミラミネートで包み、三辺をヒートシールした。その後、これに電解液を注入し、真空シールして試験用ラミネートセルとした。
(Assembly of single layer laminate cell)
A single-layer laminate cell was produced as follows. In addition, the following operation was implemented in argon atmosphere with a dew point of -80 degrees C or less. A laminated body is produced by making the positive electrode and the negative electrode face each other with a separator interposed therebetween. The laminate was wrapped with aluminum laminate and heat sealed on three sides. Thereafter, an electrolytic solution was injected into this, and vacuum-sealed to obtain a test laminate cell.
＜電池評価＞
前記単層ラミネートセルを用いて、二次電池の充放電試験を行った。測定条件としては、正極に対する電流密度を０．２ｍＡ／ｃｍ２とし、カットオフ電圧が１．１Ｖから３．０Ｖの間で充放電を繰り返した。初期放電容量は、０．１Ｃ率での測定値を使用した。サイクル特性は、１Ｃ率での充放電サイクルを６０℃の恒温条件下で繰り返し、３００サイクル目の放電容量値に対する初回放電容量値の割合（百分率）を放電容量維持率として表した。
<Battery evaluation>
The charge / discharge test of the secondary battery was done using the single layer laminate cell. As measurement conditions, the current density with respect to the positive electrode was 0.2 mA / cm 2, and charge / discharge was repeated while the cut-off voltage was between 1.1 V and 3.0 V. The initial discharge capacity was measured at a rate of 0.1C. For the cycle characteristics, the charge / discharge cycle at a 1C rate was repeated under a constant temperature condition of 60 ° C., and the ratio (percentage) of the initial discharge capacity value to the discharge capacity value at the 300th cycle was expressed as the discharge capacity maintenance ratio.
実施例１
ＢＥＴ比表面積１６４ｍ２／ｇ、Ｃ純度９４．５％のカーボンナノチューブ粉末４．５ｇ、カルボキシメチルセルロースナトリウム（ダイセルファインケム株式会社製）の３％水溶液３０．０ｇ、イオン交換水５５．５ｇを１００ｍｌビーカーに測り取り、スパチュラ等を用いてダマがなくなるまで十分に混合・撹拌する。得られた混合スラリーを超音波分散機（米国ブランソン社製、超音波ホモジナイザーＭＯＤＥＬ−４５０Ｄ；３／４“ホーン）にセットし、出力８０Ｗで２０分間分散処理する。なお、分散処理中に摩擦熱によりスラリー温度が上昇するので、５０℃を超えないように十分冷却しながら分散処理する。カーボンナノチューブ分散液の調整条件を表１に示す。
Example 1
A BET specific surface area of 164 m 2 / g, carbon nanotube powder of 94.5% C purity, 4.5 g, 3% aqueous solution of sodium carboxymethylcellulose (manufactured by Daicel Finechem Co., Ltd.) 30.0 g, ion-exchanged water 55.5 g in a 100 ml beaker Measure and mix and agitate thoroughly with a spatula until there is no lumps. The obtained mixed slurry was set in an ultrasonic dispersing machine (Branson, USA, ultrasonic homogenizer MODEL-450D; 3/4 "horn), and subjected to a dispersion treatment at an output of 80 W for 20 minutes. As the slurry temperature rises, the dispersion treatment is performed while sufficiently cooling so as not to exceed 50 ° C. Table 1 shows the adjustment conditions of the carbon nanotube dispersion.
得られたカーボンナノチューブ分散液の粘度は４１．３ｍＰａ・ｓ、カーボンナノチューブ含有量は５．１４重量％、分散粒子径ｄ５０は４１６ｎｍ、カーボンナノチューブ長さｄ５０は１．２１μｍであった。カーボンナノチューブ分散液の特性を表２に示す。 The obtained carbon nanotube dispersion had a viscosity of 41.3 mPa · s, a carbon nanotube content of 5.14% by weight, a dispersed particle diameter d50 of 416 nm, and a carbon nanotube length d50 of 1.21 μm. Table 2 shows the characteristics of the carbon nanotube dispersion.
前記カーボンナノチューブ分散液を負極活物質の導電助剤として電極合剤に添加した非水電解質二次電池のサイクル特性は８７．７％であった。 The cycle characteristics of the nonaqueous electrolyte secondary battery in which the carbon nanotube dispersion was added to the electrode mixture as a conductive aid for the negative electrode active material was 87.7%.
実施例２〜１０
表１のカーボンナノチューブ分散液の調製条件に示すように、カーボンナノチューブ粉末の種類、含有量、分散媒種類、分散剤種類分散剤添加量、分散処理条件を種々変化させた以外は前記実施例１と同様にして、様々なカーボンナノチューブ分散液を得た。
Examples 2-10
As shown in the preparation conditions of the carbon nanotube dispersion liquid in Table 1, Example 1 except that the type, content, type of dispersion medium, type of dispersant, addition amount of dispersant, and dispersion treatment conditions were variously changed. In the same manner, various carbon nanotube dispersions were obtained.
分散媒が溶剤である実施例７，８については、カーボンナノチューブ分散液を正極活物質の導電助剤として添加した非水電解質二次電池のサイクル特性を測定した。 For Examples 7 and 8 in which the dispersion medium is a solvent, the cycle characteristics of the nonaqueous electrolyte secondary battery in which the carbon nanotube dispersion was added as a conductive aid for the positive electrode active material were measured.
比較例１〜３
表１のカーボンナノチューブ分散液の調製条件に示すように、カーボンナノチューブ粉末、分散剤添加量、分散処理条件を種々変化させた以外は、前記実施例１と同様にして、様々なカーボンナノチューブ分散液を得た。
Comparative Examples 1-3
As shown in the preparation conditions of the carbon nanotube dispersion liquid in Table 1, various carbon nanotube dispersion liquids were obtained in the same manner as in Example 1 except that the carbon nanotube powder, the amount of dispersant added, and the dispersion treatment conditions were variously changed. Got.
なお、実施例１で使用したものと同一のカーボンナノチューブ粉末を分散液とすることなく、粉末のまま用い、前記＜カーボンナノチューブ分散液を負極合剤に添加する場合＞にしたがって、負極の作製を試みた。しかしながら、得られた負極合剤スラリー（分散媒は水）の分散状態が劣悪なため、集電体銅箔への塗布が不可能となり、負極板が作製できなかった。 The same carbon nanotube powder as that used in Example 1 was used as the dispersion without using it as a dispersion, and the negative electrode was prepared according to the above <when adding the carbon nanotube dispersion to the negative electrode mixture>. Tried. However, since the dispersion state of the obtained negative electrode mixture slurry (dispersion medium is water) is inferior, application to the current collector copper foil is impossible, and a negative electrode plate cannot be produced.
前記と同様に、実施例１で使用したものと同一のカーボンナノチューブ粉末を分散液とすることなく、粉末のまま用い、前記＜カーボンナノチューブ分散液を正極合剤に添加する場合＞にしたがって、正極の作製を試みた。この場合も、正極合剤スラリー（分散媒はＮ−メチル−２−ピロリドン）の分散状態が劣悪なため、集電体アルミニウム箔への塗布が不可能となり、正極板が作製できなかった。 In the same manner as described above, the same carbon nanotube powder as used in Example 1 was used as a powder without using it as a dispersion, and according to the above <in the case where the carbon nanotube dispersion was added to the positive electrode mixture> I tried to make. Also in this case, since the dispersion state of the positive electrode mixture slurry (dispersion medium was N-methyl-2-pyrrolidone) was poor, application to the current collector aluminum foil was impossible, and the positive electrode plate could not be produced.
参考例１
負極活物質にカーボンナノチューブ分散液を添加しなかった他は、＜カーボンナノチューブ分散液を負極合剤に添加する場合＞と同様の製造法によって作製した非水電解質二次電池のサイクル特性は７９．３％であった。
Reference example 1
The cycle characteristics of the nonaqueous electrolyte secondary battery produced by the same production method as in <when carbon nanotube dispersion is added to the negative electrode mixture> except that the carbon nanotube dispersion was not added to the negative electrode active material is 79. 3%.
参考例２
正極活物質にカーボンナノチューブ分散液を添加しなかった他は、＜カーボンナノチューブ分散液を正極合剤として添加する場合＞と同様の製造法によって作製した非水電解質二次電池のサイクル特性は７８．５％であった。
Reference example 2
The cycle characteristics of the nonaqueous electrolyte secondary battery produced by the same production method as in <when carbon nanotube dispersion liquid is added as positive electrode mixture> except that the carbon nanotube dispersion liquid is not added to the positive electrode active material are 78. It was 5%.
表１にカーボンナノチューブ分散液の調整条件を、表２にカーボンナノチューブ分散液の特性を示す。 Table 1 shows the conditions for adjusting the carbon nanotube dispersion, and Table 2 shows the characteristics of the carbon nanotube dispersion.
実施例に示すとおり、本発明に係るカーボンナノチューブ分散液は、サイクル特性を飛躍的に向上させる効果を持つので、非水電解質二次電池用の正極・負極いずれの活物質の導電助剤としても好適である。 As shown in the examples, the carbon nanotube dispersion liquid according to the present invention has the effect of dramatically improving the cycle characteristics, so that it can be used as a conductive auxiliary for both the positive and negative electrode active materials for non-aqueous electrolyte secondary batteries. Is preferred.
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