JP4220867B2 - Slurry for forming positive electrode of nonaqueous electrolyte secondary battery, positive electrode of nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery such as a lithium secondary battery. Furthermore, the present invention relates to a positive electrode used in the nonaqueous electrolyte secondary battery and a slurry for forming a positive electrode of a nonaqueous electrolyte secondary battery that is a raw material for the positive electrode.

In recent years, electronic devices have been made smaller and lighter, and batteries that serve as power sources have been required to be smaller and lighter. In order to meet such demands, recently, non-aqueous electrolyte secondary batteries have been developed because they have high energy density, little self-discharge, and can be lightweight. A typical example of the nonaqueous electrolyte secondary battery is a lithium secondary battery. Hereinafter, a lithium secondary battery will be described as an example.
A lithium secondary battery has a positive electrode containing a lithium-containing transition metal oxide as a positive electrode active material, a negative electrode containing a carbon material as a negative electrode active material, and a non-aqueous electrolyte containing an electrolyte salt.
A positive electrode of a lithium secondary battery may be prepared from a raw material containing a lithium-containing transition metal oxide and carbon black. In that case, the lithium-containing transition metal oxide, carbon black, and binder are used as a solvent. A method is adopted in which the slurry is dispersed to form a slurry, and the slurry is applied to a metal foil and dried.

  Since the carbon black used at that time has a low bulk density, when it is slurried, the viscosity of the slurry tends to be high, but if the viscosity of the slurry is high, it is difficult to apply to the metal foil. Specifically, the amount of the solvent is increased, the solid content concentration is lowered, and the viscosity of the slurry is lowered. However, lowering the viscosity of the slurry by lowering the concentration improves the applicability, but on the other hand, the amount of solvent that must be volatilized during drying after application increases, so that much time and energy is consumed for drying. I had to.

Then, the slurrying method which makes a viscosity low without adding a solvent is proposed (for example, nonpatent literature 1). In this method, lithium-containing transition metal oxide and carbon black are dry-mixed while applying a strong shearing force, and the resulting mixture and binder are dispersed in a solvent to obtain a slurry. In this way, the viscosity of the slurry is lowered and the particles are more highly dispersed to obtain a very homogeneous slurry. Therefore, if such a slurry is used as a raw material for electrode production, it is said that the homogeneity of the electrode is increased and the performance of the battery is improved.
However, in the above-described method, when lithium-containing transition metal oxide and carbon black are excessively dry-mixed, the battery capacity at the time of high cycle (specifically, charge / discharge of 1000 cycles or more) is reduced and the battery performance is reduced. May decrease. In addition, since no index has been found for obtaining the optimum range such as conditions for dry mixing, for example, mixing time, mixing energy, etc., there has been a tendency for excessive mixing.

By the way, in the nonaqueous electrolyte secondary battery, various methods for improving the cycle characteristics and preventing the decrease in the battery capacity at the time of high cycle have been studied. For example, Patent Document 1 proposes a method of using graphite having a specific bulk density as a negative electrode material.
JP-A-8-180873 Munehiro Kadowaki, 3 others, “High performance of battery using mechanical particle composite device”, Abstracts of 42nd Battery Discussion Meeting, Battery Engineering Committee of the Electrochemical Society of Japan, November 2001, p86-87

However, even if the graphite described in Patent Document 1 is used as the negative electrode material, the problem of the positive electrode is not solved, and the performance of the finally obtained lithium secondary battery may not be improved. Therefore, there has been a demand for a positive electrode that has high homogeneity and does not reduce battery capacity during high cycles.
The present invention has been made in view of the above circumstances, and an object thereof is to provide a slurry for forming a positive electrode of a non-aqueous electrolyte secondary battery having a high concentration and a low viscosity. It is another object of the present invention to provide a positive electrode for a non-aqueous electrolyte secondary battery with high homogeneity. Furthermore, it aims at providing the nonaqueous electrolyte secondary battery with the high battery performance at the time of a high cycle.

Positive electrode forming slurry of a non-aqueous electrolyte secondary battery of the present gun according to claim 1, a non-aqueous electrolyte secondary containing mixed powder of at least one of the positive electrode active material and the conductive carbon material is dry mixed as a raw material a positive electrode forming slurry of the following cell, the mixed powder, the amount of absorption of dibutyl phthalate per conductive carbon material weight is characterized by a 30~198cm 3 ^/ 100g. Here, the amount of dibutyl phthalate absorbed is a value measured in accordance with JIS K 6217.

In the slurry for forming the positive electrode of the non-aqueous electrolyte secondary battery as described above, the properties of the mixed powder are optimized, and the viscosity of the slurry can be lowered even at a high concentration. In addition to being able to be applied to the foil, the time and energy required for drying can be reduced. In addition, the conductive carbon material is appropriately divided and adsorbs around the positive electrode active material, thereby forming a large amount of good conductive paths.
In the above-described slurry for forming a positive electrode, the positive electrode active material is preferably a lithium-containing transition metal oxide. If the positive electrode active material is a lithium-containing transition metal oxide, a non-aqueous electrolyte secondary battery with higher battery performance can be obtained.

  Further, the positive electrode of the nonaqueous electrolyte secondary battery of the present invention is characterized in that a positive electrode layer made of the above-described slurry for forming a positive electrode of a nonaqueous electrolyte secondary battery is formed on a metal foil. It is said. In the positive electrode of such a non-aqueous electrolyte secondary battery, a large number of good conductive paths are formed and the homogeneity is high. Even if the conductive path is divided, the remaining amount of good conductive paths increases.

  The nonaqueous electrolyte secondary battery of the present invention includes the positive electrode of the nonaqueous electrolyte secondary battery described above and a negative electrode containing a carbon material as a negative electrode active material, and the positive electrode of the nonaqueous electrolyte secondary battery. The electrode and the negative electrode are in contact with a non-aqueous electrolyte solvent in which an electrolyte salt is dissolved. In such a non-aqueous electrolyte secondary battery, even when the high cycle is used, the remaining amount of good conductive paths is large, so that it is possible to prevent the battery performance of the non-aqueous electrolyte secondary battery from being deteriorated.

In the slurry for forming the positive electrode of the non-aqueous electrolyte secondary battery of the present invention, the bulk density of the mixed powder or the amount of dibutyl phthalate absorbed is specified, the aggregation state is optimized, the concentration is low, and the viscosity is low. Therefore, it can be easily applied to the metal foil. Moreover, the drying time and drying energy at the time of drying can be reduced. Furthermore, a large amount of good conductive paths can be formed around the positive electrode active material.
Since the positive electrode of the non-aqueous electrolyte secondary battery of the present invention is made from the above-mentioned slurry for non-aqueous electrolyte secondary battery electrodes, the conductivity of the electrode is increased and the expansion and contraction associated with charge / discharge Even if the conductive path is divided, the remaining amount of good conductive paths increases.
According to the nonaqueous electrolyte secondary battery of the present invention, since the positive electrode of the nonaqueous electrolyte secondary battery described above is used, it is possible to prevent a decrease in battery performance during high cycle use.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 shows a lithium secondary battery that is a non-aqueous electrolyte secondary battery of this embodiment. This lithium secondary battery is a so-called coin-type battery, and includes a disc-shaped positive electrode (positive electrode) 4 containing at least one lithium-containing transition metal oxide as a positive electrode active material, and a carbon material as a negative electrode active material. It has a negative electrode (negative electrode) 3 that is contained, and a separator 6 disposed between the negative electrode 3 and the positive electrode 4, and these are in contact with a non-aqueous electrolyte solvent in which a lithium salt (electrolyte salt) is dissolved. It is roughly structured.

In this lithium secondary battery, a negative electrode 3, a separator 6, and a positive electrode 4 are sequentially laminated on a substantially flat cylindrical battery case 1 made of stainless steel or the like, and a sealing plate 2 made of stainless steel or the like is disposed thereon, Further, an annular gasket 7 made of polypropylene is disposed between the battery case 1 and the sealing plate 2. And the upper end part of the battery case 1 is crimped inside, and the gasket 7 and the sealing board 2 are being fixed.
Glass wool filter papers 5 and 5 are inserted between the separator 6 and the positive electrode 4 and the negative electrode 3.
The nonaqueous electrolyte solvent is impregnated in the positive electrode 4, the negative electrode 3, the separator 6, and the glass wool filter papers 5 and 5.

The positive electrode 4 is formed by laminating a metal foil with at least one lithium-containing transition metal oxide as a positive electrode active material, a conductive carbon material as a conductive auxiliary, and a binder for binding them. Further, it is formed into a disk shape.
As the lithium-containing transition metal oxide, since battery performance can be improved, lithium manganate, lithium cobaltate, lithium nickelate, lithium ferrate, vanadium oxide, and lithium vanadate can be preferably used. Further, a lithium manganate partially substituted with different elements Co, Ni, Fe, Mg, Cr, Ba, Ag, Nb, Al, Cu or the like is also preferably used.
The average particle size of such a lithium-containing transition metal oxide is usually 15 to 20 μm.

  The conductive carbon material is composed of an aggregate in which particles are aggregated in a chain. A conductive path is formed by agglomeration of the conductive carbon material in a chain. Examples of such a conductive carbon material include carbon black, furnace black, and ketjen black. Among these, carbon black which is most easily available is preferable. Usually used carbon black has an average particle size of 20 to 50 nm.

  The binder is not limited as long as it binds a lithium-containing transition metal oxide and a conductive carbon material. For example, fluorine-based resins such as polyvinylidene fluoride and polytetrafluoroethylene, and imide resins An amide resin can be used.

  Such a positive electrode 4 is produced as follows. First, a mixed powder is prepared by dry-mixing at least one lithium-containing transition metal oxide and a conductive carbon material, and the mixed powder and binder are dispersed in a solvent and wet-mixed to obtain a non-aqueous electrolyte secondary. A slurry for forming a positive electrode of a battery (hereinafter abbreviated as slurry) is prepared. Next, the slurry is applied onto a metal foil made of Al foil or the like, heated and dried to form a positive electrode layer, and the metal foil provided with the positive electrode layer is formed into a desired shape to form a positive electrode Get 4.

In the method for producing the positive electrode 4 described above, the mixed powder has a bulk density of 0.25 to 1.05 g / cm ³ , preferably 0.33 to 0.77 g / cm ³ , or the weight of the conductive carbon material. absorption of dibutyl phthalate 30~198cm ³ / 100g, per preferably 41~141cm ³ / 100g.
Here, the bulk density of the mixed powder or the absorbed amount of dibutyl phthalate depends on mixing conditions such as mixing time or mixing speed during dry mixing. For example, if the mixing time is lengthened, the bulk density increases up to a certain mixing time, and becomes almost constant at longer mixing times. Further, the amount of dibutyl phthalate absorbed is substantially constant up to a certain mixing time, and when the mixing time is further increased, the amount of dibutyl phthalate absorbed decreases. The reason why the bulk density or the absorbed amount of dibutyl phthalate changes depending on the mixing time is that the aggregation state of the mixed powder changes. That is, when the mixing time of dry mixing is lengthened, the aggregate of the chain-like conductive carbon material is divided by the shearing force of mixing, and the particle size and particle size distribution change.

  FIG. 2 (a) shows the state of the raw material particles (lithium-containing transition metal oxide particles and conductive carbon material particles) before dry mixing, and FIG. 2 (b) shows the mixed powder after dry mixing. Thus, the bulk density or the absorbed amount of dibutyl phthalate satisfies the scope of the present invention. Before wet mixing, as shown in FIG. 2A, the aggregate 12 of the conductive carbon material 11 is in a long chain shape, and the conductive carbon material 11 and the lithium-containing transition metal oxide 13 are in contact with each other. However, after wet mixing, as shown in FIG. 2B, the aggregate of the conductive carbon material is appropriately divided, and the divided conductive carbon material 11 is surrounded by the lithium-containing transition metal oxide 13. Adsorbed. Thus, in the state where the divided conductive carbon material is adsorbed around the positive electrode active material, the bulk density becomes dense, the amount of dibutyl phthalate absorbed increases, and many good conductive paths are formed. As a result, the conductivity is increased. If the mixing time is too long, the aggregates of the chain-like conductive carbon material are too divided, and the amount of good conductive paths formed is reduced.

Therefore, the bulk density of the mixed powder or the amount of dibutyl phthalate absorbed as the raw material of the slurry is an index for achieving optimum mixing conditions. By setting the volume density or the amount of dibutyl phthalate absorbed in the above range, the lithium-containing transition metal A large amount of conductive carbon material that forms a good conductive path can be adsorbed around the oxide.
And the positive electrode formed from the slurry which contains such mixed powder as a raw material becomes high in homogeneity, and therefore becomes highly conductive. In addition, even if the conductive path is divided due to expansion / contraction associated with charging / discharging of the lithium secondary battery, the remaining amount of good conductive path increases.

Further, when the bulk density of the mixed powder or the absorbed amount of dibutyl phthalate is in the above range, the powder properties of the mixed powder are improved and the mixed powder is easily handled.
Furthermore, since a slurry in which such a mixed powder is dispersed in a solvent has a low viscosity despite its high concentration, it has excellent applicability when applied to a metal foil. Furthermore, since the amount of solvent distilled off during drying is small, the time and energy required for drying can be reduced.

When preparing a mixed powder by dry-mixing a lithium-containing transition metal oxide and a conductive carbon material, various mixing means can be used, but those that impart a strong shearing force are preferred. Examples of mixing means for applying a strong shear force during dry mixing include a Henschel mixer and a chopper mixer.
It is preferable that the mixing conditions such as the mixing time and the mixing speed are appropriately set in an optimal range depending on the specifications of the mixing means (capacity, blade, etc.).

  When preparing the slurry, the solvent for dispersing the mixed powder described above is preferably a solvent that can dissolve the binder, and examples thereof include N-methylpyrrolidone. Furthermore, it is preferable to previously dissolve the binder in the solvent because the production efficiency is increased.

The negative electrode 3 is composed of a carbon material powder as a negative electrode active material and a binder for binding the carbon material powder, which are layered on a metal foil and further formed into a disk shape. .
As the carbon material, for example, artificial graphite, natural graphite, coke, non-graphitizable carbon fiber, polyacene, or the like can be used. Further, as the binder, the same binder as that used in the positive electrode 4 can be used.

  As the separator 6, for example, a non-woven fabric made of a polymer material such as polyethylene or polypropylene, a porous film, glass fiber, or various polymer fibers can be used.

Examples of the non-aqueous electrolyte solvent include ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, benzonitrile, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, γ-valerolactone, 1,3-dioxolane, 4 -Methyldioxolane, N, N-dimethylformamide, dimethylacetamide, dimethyl sulfoxide, dioxane, sulfolane, 3-methylsulfolane, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, ethyl butyl carbonate, dipropyl Carbonate, diisopropyl carbonate, dibutyl carbonate, diethylene glycol, dimethyl ether, 1,2-dimethoxy ester An aprotic solvent such as tan or a mixed solvent containing two or more of these solvents can be exemplified.

Examples of the lithium salt that is an electrolyte salt dissolved in a non-aqueous electrolyte solvent include LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiSbF ₆ , Examples of LiAlO ₄ , LiAlCl ₄ , LiN (C _x F _{2x + 1} SO ₂ ) (C _y F _{2y + 1} SO ₂ ) (where x and y are natural numbers), LiCl, LiI, etc. it can.

  In the embodiment example as described above, the positive electrode 4 has a large number of good conductive paths made of a conductive carbon material around the lithium-containing transition metal oxide that is the positive electrode active material. Therefore, even if the conductive path is divided due to expansion / contraction caused by charging / discharging, the remaining amount of good conductive paths increases. Therefore, even when the high cycle is used, the remaining amount of good conductive paths is large, so that it is possible to prevent the battery performance of the nonaqueous electrolyte secondary battery from being lowered. Specifically, a practical battery capacity can be maintained up to 3000 to 4000 cycles.

  The present invention is not limited to the above-described embodiment example. For example, although the embodiment described above is a coin-type lithium secondary battery, it may be a cylindrical, square, or sheet-type lithium secondary battery.

(Example 1)
A positive electrode was produced as follows. First, 80% by weight of lithium manganate (LiMn ₂ O ₄ ) having an average particle diameter of 20 μm, which is a positive electrode active material, and 10% by weight of carbon black having an average particle diameter of 20 nm, which is a conductive carbon material, are mixed with a Henschel mixer for 10 minutes. And dry mixing at 1000 rpm. The dry mixed powder was gently filled into a 100 cm ³ constant volume container, and the bulk density was measured. Further, the amount of dibutyl phthalate (DBP) absorbed per weight of carbon black in the dry mixed powder was measured. Here, the measurement of the dibutyl phthalate absorption amount conformed to JIS K6217-4.
Then, 10% by weight of polyvinylidene fluoride (PVdF), which is a binder dissolved in N-methylpyrrolidone, is added to the dry mixed powder, and added separately so that the viscosity becomes 5.0 Pa · s. The slurry for positive electrode formation was obtained by mixing for 80 minutes while adjusting the amount of solvent used. At that time, the viscosity of the slurry is a value when measured using a Haake viscometer at a rotation speed of 30 rpm. Moreover, the stirring of the slurry was performed using a planetary mixer, and its rotation speed was set to 60 rpm.
Next, this slurry was applied on one surface of an Al foil so that the amount applied was uniform, and then heated and dried at 80 ° C. for 1 hour to form a coating film. Next, the coating film on the aluminum foil was molded by a roller press so that the film density was 2.2 g / cm ³ . Subsequently, it heat-dried at 130 degreeC for 12 hours, decompressing to 100 Pa or less, and obtained the positive electrode.

Next, 10% by weight of polyvinylidene fluoride as a binder (binder) and 90% by weight of graphite as a negative electrode active material were added to N-methylpyrrolidone and mixed to obtain a slurry for forming a negative electrode. Next, this slurry was applied onto a copper foil and dried at 80 ° C. to form a coating film. Next, the coating film on the copper foil was formed by a roll press so that the film density was 1.4 g / cm ³ . Subsequently, it was heat-dried at 130 ° C. for 12 hours while reducing the pressure to 100 Pa or less to obtain a negative electrode.

Next, the positive electrode and the negative electrode were cut so that the area was a circle of 2 cm ² , and a separator was sandwiched between the two electrodes to produce a coin-type battery. At that time, as the non-aqueous electrolyte of the coin-type battery, ethylene carbonate (EC) and dimethyl carbonate (DMC) are contained in a mixing ratio of EC: DMC = 1: 2, and LiPF ₆ is further added at 1 mol / mol. A solution containing a concentration of L was used.
And the charge / discharge test of this coin type battery was done. In this charge / discharge test, the battery capacity after 300 cycles of charge / discharge was measured to determine the battery capacity retention rate.

(Examples 2 to 8, Comparative Example 1)
A coin-type battery was obtained in the same manner as in Example 1 except that the mixing time in dry mixing at the time of producing the positive electrode was changed as shown in Table 1.
(Examples 9 to 15 and Comparative Example 2)
In the slurry mixing during the production of the positive electrode, the same as in Example 1 except that the solid content concentration was 60% by weight while the slurry viscosity was 5.0 Pa · s, and the slurry mixing time was changed as shown in Table 1. A coin-type battery was obtained.

From these results, the graphs of FIGS.
FIG. 3 is a graph showing the relationship between mixing time and bulk density. From FIG. 3, it was found that the bulk density increases with the mixing time, but the increase rate of the bulk density decreases as the mixing time increases to some extent.
FIG. 4 is a graph showing the relationship between the mixing time and the absorbed amount of dibutyl phthalate per weight of carbon black. FIG. 4 shows that the DBP absorption amount decreases with the mixing time, but the DBP absorption amount becomes almost constant when the mixing time is increased to some extent.
FIG. 5 is a graph showing the relationship between the bulk density and the solvent ratio in the slurry. From FIG. 5, it was found that the higher the bulk density, the smaller the amount of solvent added during slurry preparation.
FIG. 6 is a graph showing the relationship between the bulk density and the film density of the positive electrode. From FIG. 6, it was found that the higher the bulk density, the higher the film density of the electrode. From this, it is presumed that when the film density of the electrode during coating is large, the press pressure for adjusting the film density may be small.

FIG. 7 is a graph showing the relationship between the bulk density and the charge / discharge test result (capacity retention rate). From FIG. 7, it was found that the bulk density exhibits a high capacity retention rate in the range of about 0.24 to 1.05 g / cm ³ . That is, when the bulk density is in the range of 0.24 to 1.05 g / cm ³ , the retention rate of the battery capacity after 300 cycles is over 80%, and the battery life is increased.
FIG. 8 is a graph showing the relationship between DBP absorption and charge / discharge test results. From Figure 8, DBP absorption amount was found to exhibit a capacity retention rate of high battery in a range of approximately 30~198cm ³ / 100g. That is, the absorption amount of dibutyl phthalate in a range of 30~198cm ³ / 100g, 300 retention of the battery capacity after the cycle is above 80%, it was found that battery life is prolonged.
FIG. 9 is a graph showing the relationship between the bulk density and the slurry mixing time. From FIG. 9, it was found that when the bulk specific gravity is reduced, the mixing time of the slurry can be shortened and the production efficiency is improved.

1 is a cross-sectional view showing a lithium secondary battery according to an embodiment of the present invention. (A) is a schematic diagram which shows the state of the raw material particle before dry-mixing, (b) is a schematic diagram which shows the state of the mixed powder after dry-mixing. It is a graph which shows the relationship between mixing time and a bulk density. It is a graph which shows the relationship between mixing time and the dibutyl phthalate absorption amount per carbon black weight. It is a graph which shows the relationship between a bulk density and the solvent ratio in a slurry. It is a graph which shows the relationship between the bulk density and the film density of a positive electrode. It is a graph which shows the relationship between a bulk density and a charging / discharging test result. It is a graph which shows the relationship between the dibutyl phthalate absorption amount per carbon black weight, and a charging / discharging test result. It is a graph which shows the relationship between a bulk density and slurry mixing time.

Explanation of symbols

3 Negative electrode (negative electrode)
4 Positive electrode (positive electrode)
11 Conductive carbon material 13 Lithium-containing transition metal oxide

Claims (5)

A slurry for forming a positive electrode of a non-aqueous electrolyte secondary battery containing, as a raw material, a mixed powder obtained by dry-mixing at least one positive electrode active material and a conductive carbon material,
The mixed powder, conductive positive electrode forming slurry of the non-aqueous electrolyte secondary batteries, characterized by absorption of dibutyl phthalate per carbon material weight is 30~198cm 3 ^/ 100g. The slurry for forming a positive electrode of a non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode active material is a lithium-containing transition metal oxide. A positive electrode of a nonaqueous electrolyte secondary battery, wherein a positive electrode layer made from the slurry for forming a positive electrode of the nonaqueous electrolyte secondary battery according to claim 1 or 2 is formed on a metal foil. electrode. The positive electrode of the nonaqueous electrolyte secondary battery according to claim 3 and a negative electrode containing a carbon material as a negative electrode active material, wherein the positive electrode and the negative electrode of the nonaqueous electrolyte secondary battery are electrolyte salts. A nonaqueous electrolyte secondary battery, wherein the nonaqueous electrolyte solution is in contact with a dissolved nonaqueous electrolyte solvent. In a method for producing a slurry for forming a positive electrode of a non-aqueous electrolyte secondary battery, dry mixing at least one positive electrode active material and a conductive carbon material to prepare a mixed powder and adding a solvent to the mixed powder There,
A method for producing a slurry for forming a positive electrode of a non-aqueous electrolyte secondary battery, wherein the dry mixing time is within 60 minutes.

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