JP2006172804A - Active material, coating composition containing the same, electrode plate, and nonaqueous electrolytic solution secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an active material for a secondary battery which active material is provided with characteristics superior in pot life (storage characteristics). <P>SOLUTION: The active material has a tap density of 0.67 to 0.70, and is contained in a coating composition for the secondary battery. The coating composition forms electrode plates, which is included in the nonaqueous electrolytic solution secondary battery. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an active material for a secondary battery, a coating composition for a secondary battery containing the active material, an electrode plate formed from the composition, and a non-aqueous electrolyte secondary battery having the electrode plate.

  In recent years, electronic devices and communication devices are rapidly becoming smaller and lighter, and secondary batteries used as power sources for driving these devices are also required to be smaller and lighter. For this reason, in place of the conventional alkaline storage battery, a non-aqueous electrolyte secondary battery having a high energy density and a high voltage, typically a lithium ion secondary battery has been proposed.

  A positive electrode plate (positive electrode plate) of a non-aqueous electrolyte secondary battery uses a composite oxide such as lithium manganate or lithium cobaltate as a positive electrode active material, and such a positive electrode active material and a binder (binder). Is dispersed or dissolved in a suitable wetting agent (solvent) to prepare a slurry-like coating composition, and the coating composition is applied onto a current collector made of a metal foil to form a positive electrode active material layer. It is produced by forming.

  On the other hand, the negative electrode plate (negative electrode plate) of the non-aqueous electrolyte secondary battery uses a carbonaceous material such as carbon that can occlude cations such as lithium ions released from the positive electrode active material layer during charging as the negative electrode active material. A slurry-like coating composition is prepared by dispersing or dissolving such a negative electrode active material and a binder (binder) in an appropriate wetting agent (solvent), and the coating composition is removed from the metal foil. The negative electrode active material layer is formed by coating on the current collector.

  The secondary battery is equipped with a terminal for taking out current in each of the positive electrode plate and the negative electrode plate, wound with a separator for preventing a short circuit between both electrode plates, and sealed in a container filled with a non-aqueous electrolyte solution It is assembled by doing.

The present invention relates to a secondary battery active material, and in particular, provides an active material effective for improving the properties of a coating composition for forming an electrode of a secondary battery. For example, various coating compositions for forming a negative electrode of a non-aqueous electrolyte secondary battery are known (for example, Patent Document 1). Generally, after the coating composition is prepared, the coating composition is formed on the current collector. It takes a certain time or number of days (hereinafter referred to as “storage period”) for storage and transportation of the coating composition before coating. In order to form a uniform electrode active material layer on a current collector such as a metal stay, the composition should be uniformly dissolved and dispersed without causing precipitation or precipitation of the composition during the storage period. Required.
JP 2003-173780 A

  The present invention has been made in view of the above circumstances, and is formed from an active material for a secondary battery capable of imparting excellent pot life (storage characteristics), a coating composition containing the active material, and the composition. An object is to provide an electrode plate and a nonaqueous electrolyte secondary battery having the electrode plate.

  The above object can be achieved by using a secondary battery having a tap density of 0.67 to 0.70. The tap density in this invention means the value measured by the tap density measuring method described in the following Example. The tap density is a value obtained as a result of various factors such as the type, crystal form, and size of the active material. For example, if you want to increase the tap density, you only need to increase the grain size, and conversely The particle size may be reduced. If the tap density is too large, the viscosity of the electrode coating composition increases. Therefore, the tailing length tends to be long when the electrode is produced. If the tap density is too small, the pot life of the electrode composition is increased. There is a problem that becomes shorter.

  In addition, since the tap density has an additivity, even if the active material has a tap density outside the specified range of the present invention, the active material is mixed and adjusted to the tap density specified by the present invention. Can be used as the active material of the present invention.

As the positive electrode active material, for example, it is possible to use a material used as a positive electrode active material of nonaqueous electrolyte secondary batteries conventionally, for example, LiMn ₂ O ₄ (lithium manganate), LiCoO ₂ (cobaltate Examples thereof include lithium oxides such as lithium) or LiNiO ₂ (lithium nickelate), or chalcogen compounds such as TiS ₂ , MnO ₂ , MoO _3, or V ₂ O ₅ . In particular, by using LiCoO ₂ as a positive electrode active material and a carbonaceous material as a negative electrode active material, a lithium secondary battery having a high discharge voltage of about 4 volts can be obtained.

  The positive electrode active material is preferably a powder having a particle size in the range of 1 to 100 μm and an average particle size of 3 to 30 μm in order to uniformly disperse it in the coating layer. These positive electrode active materials may be used alone or in combination of two or more.

  As a negative electrode active material, the material conventionally used as a negative electrode active material of a nonaqueous electrolyte secondary battery can be used, for example, natural graphite, artificial graphite, amorphous carbon, carbon black, or these Carbonaceous materials such as those obtained by adding different elements to these components are preferably used. When the solvent is organic, a lithium-containing metal such as lithium metal or a lithium alloy is preferably used.

  Although the particle shape of a negative electrode active material is not specifically limited, For example, a flaky shape, a lump shape, a fiber shape, and a spherical shape can be used. The negative electrode active material is preferably a powder having a particle size in the range of 1 to 100 μm and an average particle size of 3 to 30 μm in order to be uniformly dispersed in the coating layer. These negative electrode active materials may be used alone or in combination of two or more. In the present invention, the average particle diameter is a value measured by a laser diffraction particle size distribution analyzer.

  In the present invention, when the negative electrode active material whose tap density is adjusted to 0.67 to 0.70 is used, the object effect of the present invention can be further enjoyed. Preferably, graphite adjusted to a tap density of 0.67 to 0.70, particularly scaly graphite is preferable.

  The blending ratio of the positive electrode or negative electrode active material in the coating composition is 90 to 90% from the viewpoint of realizing a high battery capacity and a balance with cycle characteristics when the blending component excluding the solvent is used as a standard (solid content standard). 98.5% by weight, preferably 96 to 98.5% by weight.

  The active material layer coating composition contains at least the positive or negative electrode active material, the binder, and usually a conductive agent. In addition, a surfactant or the like may be contained.

  If it is conventionally used as a binder, for example, an electrode plate for a non-aqueous electrolyte secondary battery, a thermoplastic resin, more specifically, a polyester resin, a polyamide resin, a polyacrylate resin, a polycarbonate Resin, polyurethane resin, cellulose resin, polyolefin resin, polyvinyl resin, fluorine-based resin, polyimide resin, or the like can be used. At this time, an acrylate monomer or oligomer into which a reactive functional group is introduced can be mixed in the binder. In addition, rubber-based resins, thermosetting resins such as acrylic resins and urethane resins, ionizing radiation curable resins composed of acrylate monomers, acrylate oligomers, or mixtures thereof, and the various mixtures described above can also be used.

  The blending ratio of the binder of the active material layer coating composition is, for example, about 0.5 to 10% by weight based on the solid content in the case of a normal electrode plate for a non-aqueous electrolyte secondary battery.

  A conductive material may be added to the positive electrode or negative electrode active material layer coating composition. As the conductive material, for example, in the case of an electrode plate for a non-aqueous electrolyte secondary battery, a carbonaceous material such as graphite, carbon black, or acetylene black is used as necessary. For example, in the case of an electrode plate for a non-aqueous electrolyte secondary battery, the blending ratio of the conductive material in the coating composition is usually 1.5 to 2.5% by weight based on the solid content.

As a solvent for preparing the active material coating composition, an organic solvent such as IPA (isopropyl alcohol) toluene, methyl ethyl ketone, N-methyl-2-pyrrolidone or a mixture thereof, and water can be used. The solvent in the coating composition is usually blended so that the solid content is 40 to 85% by weight, preferably 50 to 80, more preferably 60 to 80% by weight, based on the entire composition, Is prepared in a slurry state.
When preparing a negative electrode coating composition, in particular, water can be used as a solvent, and an aqueous emulsion coating liquid can be prepared by selecting a rubber-based binder that can be colloidally dispersed in water. . When water is used, ion exchange water is usually used to prevent the influence of impurities. The present invention is particularly useful for such an aqueous emulsion coating solution.

  The active material coating composition is prepared by mixing at least an appropriately selected active material and binder, and usually an appropriately selected conductive material and other compounding ingredients in an appropriate solvent, and then homogenizer, ball mill, sand mill, roll mill or planetary. What is necessary is just to mix and disperse | distribute with dispersers, such as a Lee mixer, and to prepare in a slurry form. It is widely known that the viscosity behavior of the coating composition is correlated with the specific surface area and particle size of the active material. However, the viscosity behavior must be taken into account because conditions such as particle size distribution may be affected. On the other hand, the tap density has a correlation with the specific surface area and particle size of the active material. However, if the coating composition is prepared using the active material having the tap density, the coating suitability and pot life (storage) It is possible to obtain a coating composition having good characteristics.

The active material coating composition finally prepared is adjusted to have a viscosity of 1.7 to 3.0 as a thixotropic index. In the present invention, the thixotropy index is a value expressed using the viscosity ratio of the active material coating composition when a probe of rotor No. 4 is attached to a B-type viscometer and the probe rotates at 6 rpm and 60 rpm.
Thixotropic index = 6 rpm viscosity / 60 rpm viscosity

<Electrode plate>
The positive electrode or negative electrode active material layer coating composition prepared by the method as described above is applied to one or both sides of a current collecting holiday as a substrate and dried to form a positive electrode or negative electrode active material layer. As the current collector of the positive electrode plate, for example, an aluminum foil is usually preferably used as long as it is an electrode plate for a non-aqueous electrolyte secondary battery. On the other hand, as the current collecting holiday of the negative electrode plate, for example, if it is an electrode plate for a non-aqueous electrolyte secondary battery, a copper foil such as an electrolytic copper foil or a rolled copper foil is preferably used. The thickness of the current collector is usually about 5 to 50 μm in the case of an electrode plate for a non-aqueous electrolyte secondary battery, for example.

  The method for applying the coating composition for the active material layer is not particularly limited, but a method capable of forming a thick coating layer such as a slide die coat, a comma direct coat, a comma reverse coat and the like is suitable. However, when the thickness required for the active material layer is relatively thin, it may be applied by gravure coating or gravure reverse coating. The active material layer may be formed by repeating application and drying a plurality of times.

  As a heat source in the drying process, hot air, infrared rays, far infrared rays, microwaves, high frequencies, or a combination thereof can be used. You may dry with the heat which discharge | released the metal roller and metal sheet which support a collector in a drying process. Moreover, after drying, the active material layer can also be obtained by crosslinking reaction of the binder by irradiating an electron beam or radiation. Application and drying may be repeated a plurality of times.

  Furthermore, the density of the active material layer, the adhesion to the current collector, and the homogeneity can be improved by pressing the obtained active material layer.

  The press working is performed using, for example, a metal roll, an elastic roll, a heating roll, a sheet press machine, or the like. The pressing temperature may be performed at room temperature or humidified as long as the temperature is lower than the temperature at which the coated film of the active material layer is dried. ~ 35 ° C).

  The roll press is preferable because a long sheet electrode plate can be continuously pressed. When performing the roll press, either a stereotaxic press or a constant pressure press may be performed. The line speed of the press is usually 5 to 50 m / min. And When the pressure of the roll press is managed by linear pressure, the pressure is adjusted according to the diameter of the pressure roll, but the linear pressure is usually 0.5 kgf / cm to 1 tf / cm.

Also, normally when performing sheet ^{pressing, 4903~73550N / cm 2 (500~7500kgf /} cm 2), preferably to adjust the pressure in the range of ^{29420~49033N / cm 2 (3000~5000kgf / cm} 2). If the pressing pressure is too low, it is difficult to obtain the homogeneity of the active material layer. If the pressing pressure is too high, the electrode plate itself including the current collector may be damaged. The active material layer may have a predetermined thickness by a single press, or may be pressed several times for the purpose of improving homogeneity.

The coating amount of the active material layer is usually 20 to 350 g / m ² , and the thickness is usually 10 to 200 μm, preferably 50 to 190 μm after drying and pressing. The density of the active material layer is about 1.0 g / cc after coating, but increases to 1.5 g / cc or more (usually about 1.5 to 1.75 g / cc) after pressing. Therefore, the capacity of the battery can be increased by improving the volume energy density by performing the pressing without hindrance.

<Nonaqueous electrolyte secondary battery>
An electrode plate is obtained as described above, and a nonaqueous electrolyte secondary battery can be produced using this electrode plate.

  When producing a non-aqueous electrolyte secondary battery using the electrode plate according to the present invention, a vacuum open or the like is used to remove moisture and / or solvent in the active material layer before moving to the battery assembly process. It is preferable to perform aging such as heat treatment or reduced pressure treatment in advance.

  The electrode plate (positive electrode plate, negative electrode plate) produced by the method as described above is spirally wound through a separator such as a polyethylene porous film and inserted into an outer container. After insertion, connect the terminal connection part of the positive electrode plate (exposed surface of current collection holiday) and the positive electrode terminal provided on the upper surface of the outer container with the lead, while the terminal connection part of the negative electrode plate (exposed surface of current collection holiday) The negative electrode terminal provided on the bottom surface of the outer container is connected with a lead, and the outer container is filled with a nonaqueous electrolyte and sealed, thereby completing a nonaqueous electrolyte secondary battery including the electrode plate according to the present invention. .

When producing a lithium secondary battery, a nonaqueous electrolytic solution in which a lithium salt as a solute is dissolved in an organic solvent is used. Examples of the lithium salt include inorganic lithium salts such as LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiC 1, LiBr, or LiB (C ₆ H ₅ ) ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiC ( _{SO 2 CF 3) 3, LiOSO} 2 CF 3, LiOSO 2 C 2 F 5, LiOSO 2 C 3 F 7, LiOSO 2 C 4 F 9, LiOSO 2 C 5 F 11, LiOSO 2 C 6 F 13, LiOSO 2 C An organic lithium salt such as ₇ F ₁₅ is used.

  Examples of the organic solvent for dissolving the lithium salt include cyclic esters, chain esters, cyclic ethers, chain ethers and the like. More specifically, examples of cyclic esters include propylene carbonate, butylene carbonate, γ-butyrolactone, vinylene carbonate, 2-methyl-γ-butyrolactone, acetyl-γ-butyrolactone, γ-valerolactone, and the like.

  Chain esters include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl butyl carbonate, methyl propyl carbonate, ethyl butyl carbonate, ethyl propyl carbonate, butyl propyl carbonate, propionic acid alkyl ester, malon Examples thereof include acid dialkyl esters and acetic acid alkyl esters.

  Examples of cyclic ethers include tetrahydrofuran, alkyltetrahydrofuran, dialkyltetrahydrofuran, alkoxytetrahydrofuran, dialkoxytetrahydrofuran, 1,3-dioxolane, alkyl-1,3-dioxolane, 1,4-dioxolane and the like.

  Examples of chain ethers include 1,2-dimethoxyethane, 1,2-diethoxyethane, diethyl ether, ethylene glycol dialkyl ether, diethylene glycol dialkyl ether, triethylene glycol dialkyl ether, and tetraethylene glycol dialkyl ether. Can do.

  By defining the tap density of the active material to a specific value, it is possible to obtain an electrode coating composition, particularly a negative electrode coating composition, in which pot life and coating accuracy are ensured.

(Negative electrode material)
Active material: Flake-shaped artificial graphite (manufactured by Hitachi Chemical Co., Ltd .: MAGE)
(Samples with different tap densities shown in Tables 1 to 3 below were prepared)
SBR (styrene-butadiene-rubber emulsion: BM-400B (made by Nippon Zeon)
Thickener: Serogen EP (Daiichi Kogyo Seiyaku)

(Manufacture of negative electrode coating composition)
A flake-shaped artificial graphite (manufactured by Hitachi Chemical Co., Ltd .: MAGE) as a carbon material as an active material, an SBR emulsion (manufactured by ZEON Co., Ltd .: BM-400B) as a binder, and a CMC (carboxymethyl cellulose) (Daiichi Kogyo Seiyaku Co., Ltd.) as a thickener (Product: Serogen EP) is mixed at a blending ratio (weight ratio) of 98.0: 1.0: 1.0, kneaded with a planetary mixer, ion-exchanged water is added as appropriate, and the solid content (50 weight) %) Negative electrode coating composition was prepared.

  The resulting negative electrode coating composition was evaluated for thixotropy index, presence / absence of sedimentation, and tailing, and summarized in Tables 1 and 2 below.

(Tap density measurement method)
100 cc of the active material was placed in a 150 ml graduated cylinder and the sample weight was measured. After tapping the measuring cylinder 30 times from a height of 5 cm, the sample volume was measured, and the tap density was calculated from the following equation.
D = m / V
(D: Tap density (g / cm ³ ), m: Sample weight (g), V: Sample volume after 30 times tapping (cm ³ ))
This density measurement was performed according to JIS K 1469 except that the tapping number was 30 times.

(Thixotropic index (viscosity measurement))
A rotor No. 4 probe was attached to the B-type viscometer, and the viscosity of the coating composition for a negative electrode when the probe was rotated at 6 rpm and 60 rpm was recorded. In order to obtain an indication of the properties of the negative electrode coating composition, it was expressed using a viscosity ratio of 6 rpm and 60 rpm as a thixotropic index (thixotropic index).
Thixotropic index = 6 rpm viscosity / 60 rpm viscosity

(Pot life measurement (presence or absence of sedimentation))
100 g of the obtained negative electrode coating composition was placed in a resin container (φ60 mm × H80 mm) and allowed to stand. Seven days after standing, it was visually confirmed whether or not the negative electrode coating composition that had been uniformly dispersed was separated. Further, using a spatula, it was confirmed whether or not the composition of the negative electrode coating composition had settled on the bottom of the container. When the spatula is lifted, no precipitate is observed at the tip of the spatula, and there is no separation of the slurry. When there is no settling, when the spatula is lifted, precipitate is found at the tip of the spatula, and separation of the slurry is recognized. , “With sedimentation”.

(Measurement of tail length)
From the die head toward the collector copper foil (10 μm), discharge of the negative electrode coating composition (width 60 cm, coating amount 100 g / m ² ) and stop (0.3 seconds) were repeated 30 times continuously. Then, intermittent coating was performed (see FIG. 1). As schematically shown in FIG. 2, the length from the base of the tail end of the coating end of the intermittently applied product to its tip was measured, and the average value was taken as the tail length.

Comprehensive evaluation When sedimentation was not observed and the tailing length was less than 1.5 mm, it was evaluated as “OK”.
When sedimentation was observed or the tail length was 1.5 mm or more, it was evaluated as “NG”.

  In the case of Examples 1 to 4, the tailing was slight and there was no sedimentation. The pot life and coating accuracy were secured.

  In Comparative Examples 1 and 2, there is no problem with the length of the tail, but since sedimentation occurred, it was difficult to use as a coating liquid. In Comparative Examples 3 and 4, settling did not occur, but the length of the tail was long, causing a problem in coating accuracy.

(Example 5)
The tap density, thixotropy index, presence / absence of settling, and length when mixing two active materials with different tap densities of tap density 0.66 and tap density 0.71 at 50:50 (weight ratio) Evaluation was conducted in the same manner as in Example 1, and the results are summarized in Table 3 below.

  Even if the active material has a tap density determined to be NG by itself, the tap density usable in the present invention could be obtained by mixing them. The coating composition for negative electrodes using such an active material did not settle, and the tailing of the coating end portion was slight. The pot life and coating accuracy were secured.

Schematic conceptual diagram of intermittent coating Schematic for explaining tail length

Claims (10)

  A secondary battery active material having a tap density of 0.67 to 0.70.   A secondary battery negative electrode active material having a tap density of 0.67 to 0.70.   The active material for secondary battery negative electrodes of Claim 2 whose active material is a scaly graphite.   The coating composition for secondary batteries containing the active material whose tap density is 0.67-0.70.   A secondary battery negative electrode coating composition comprising an active material having a tap density of 0.67 to 0.70.   The coating composition for a secondary battery negative electrode according to claim 5, wherein the active material is scaly graphite.   The coating composition for lithium ion secondary battery negative electrodes according to claim 5 or 6, wherein the thixotropy index is 1.7 to 3.0.   An electrode plate comprising an active material layer containing at least a negative electrode active material having a tap density of 0.67 to 0.70 on a current collector.   The electrode plate according to claim 8, wherein the active material is scaly graphite. A non-aqueous electrolyte secondary battery comprising the electrode plate according to claim 9.

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