JP3243239B2 - Method for producing positive electrode for non-aqueous secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive electrode for a non-aqueous secondary battery which provides high capacity and excellent cycle characteristics.

[0002]

2. Description of the Related Art With the spread of portable telephones, notebook personal computers, and the like, attention has been paid to lithium secondary batteries that can be expected to have a high energy density. I have.

[0003] However, since electrodes that do not originally contribute to the capacity of the electrodes, such as binders and conductive materials, are included in the electrodes, there is a problem that the battery capacity per volume is limited.

Therefore, as one means for increasing the capacity per unit volume, an attempt has been made to form the electrode from a sintered body substantially made of an active material. When the electrode is formed of a sintered body made of an active material, it does not include a binder and can further reduce or eliminate the conductive material, so that the packing density of the active material can be increased, and the capacity per unit volume can be reduced. Can be increased. For example, JP-A-8-18
No. 0904 discloses a positive electrode made of a sintered body of a lithium transition metal oxide. According to this, a sintered body is obtained by press-molding a lithium transition metal oxide powder or a raw material powder thereof using a mold, and firing the molded body at a predetermined temperature in the presence of oxygen. However, this sintered body has insufficient conductivity, and the performance as a positive electrode is not satisfactory.

[0005]

In order to reduce the thickness of a lithium secondary battery, it is effective to reduce the thicknesses of the positive electrode and the negative electrode, which occupy most of the thickness. To reduce the thickness of the positive electrode made of a sintered body, it is necessary to increase the area of the sintered body in order to secure a predetermined capacity. However, when the press molding method is used, when the area of the mold is increased to increase the area of the sintered body, it is possible to uniformly fill the lithium transition metal oxide powder or the raw material powder in the mold. It becomes difficult, and the thickness and density of the molded body become uneven. As a result, the sintering reaction does not proceed uniformly inside the molded body, and the density of the sintered body is distributed. When such a sintered body is used as an electrode in a battery, the battery capacity and cycle characteristics are reduced. was there. In addition, when there is a region where sintering is insufficient, the binding force between the primary particles constituting the sintered body is reduced in that region, the mechanical strength of the sintered body is reduced, and the collapse of the electrode during charging and discharging is caused. There is a problem that the battery capacity and cycle characteristics are easily deteriorated.

Accordingly, an object of the present invention is to solve the above-mentioned problems and to provide a positive electrode for a non-aqueous secondary battery which has a larger area, a high capacity and excellent cycle characteristics.

[0007]

Means for Solving the Problems The present inventors have made it possible to make the thickness and density of a sintered body used for a positive electrode more uniform even in a large area by using a coating film instead of a molded body. And that the electrical conductivity can be an index indicating the binding force between the primary particles constituting the sintered body, and that a sufficient mechanical strength can be obtained by using a sintered body having a high electrical conductivity. Thus, the present invention has been completed.

[0008]

That is, the method for producing a positive electrode according to the present invention is a porous sintered body made of a lithium transition metal oxide, having a porosity of 15 to 60% and a conductivity of 1 mS / c.
A method for producing a positive electrode for a non-aqueous secondary battery having a capacity of at least m, comprising: a) a positive electrode material comprising a lithium transition metal oxide powder, a solvent and a urethane resin, a phenol resin, an epoxy resin, polyethylene, polypropylene, polyvinyl alcohol; , Polyvinyl butyral, and vinylidene fluoride, ethylene fluoride, ethylene oxide, a step of preparing a coating solution by adding at least one binder selected from homopolymers or copolymers of propylene oxide,
b) a step of applying the coating liquid on a substrate and removing a solvent to prepare a coating film; and c) a step of firing the coating film in an air atmosphere to sinter the positive electrode material. It is characterized by the following.

[0010] According to the production method of the present invention, by forming a coating film containing a cathode material composed of lithium transition metal oxide powder, the lithium transition metal oxide powder can be uniformly dispersed in the coating film even in a large area. Can be dispersedly maintained. Therefore, compared with the case of using a molded body by the press molding method,
The thickness and density of the sintered body can be made more uniform. In the press molding method, powder particles are likely to be non-uniformly filled at the time of molding, and in this case, sintering proceeds quickly where the particles are densely packed, so that the densely packed portion shrinks quickly. The grains grow fast, and the thickness and density become uneven. Also,
Since the positive electrode of the present invention is substantially composed of only the active material, the binding force between the primary particles constituting the sintered body reflects on the conductivity. Therefore, a sintered body having a conductivity of 0.1 mS / cm or more has a strong binding force between primary particles, and even if the volume expands or contracts due to charge and discharge, the primary particles may fall off or the electrodes may collapse. Absent.

In the present invention, baking can be carried out in the presence of a predetermined amount of a pore-forming agent in the coating film. By oxidizing and decomposing the pore-forming agent by a heat treatment in an air atmosphere, a communication hole effective as a path for ions can be formed, so that the concentration polarization of ions can be suppressed.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The positive electrode of the present invention can be suitably used for a lithium secondary battery. Hereinafter, embodiments of the lithium secondary battery will be described.

As the lithium transition metal oxide used for the positive electrode of the present invention, any known material can be used.
_{i x CoO 2, Li x NiO} 2, Li x MnO 2, Li x Mn 2
It is preferable to use O ₄ , Li _x Mn _2-y O _4, or the like. Examples of the compound of lithium and the transition metal as the raw materials include respective hydroxides, oxides, nitrates and carbonates.

The sintered body of the lithium transition metal oxide can be prepared by the following method. That is, a coating liquid is prepared by dispersing a lithium transition metal oxide powder or a raw material powder of a lithium transition metal oxide together with a binder in a solvent, applying the coating liquid to a substrate, removing the solvent, and then removing the solvent. It is fired and sintered in an atmosphere. Here, after removing the solvent, it is better to bake in the air atmosphere after peeling the coating film from the substrate,
This is preferable because the coating film does not warp and warp during firing. Examples of the substrate include an organic polymer film or sheet and a metal foil or plate, and an organic polymer film is preferable. Further, the sintered body is preferably at least 20 mm × 20 mm, particularly preferably at least 20 mm × 40 mm, from the viewpoint of exhibiting the feature of the present invention that a uniform sintered body can be obtained even in a large area.

Here, the firing temperature is a temperature at which the binder is completely oxidized and decomposed, and is 700 to 1100 ° C., preferably 800 to 1000 ° C., and the firing time is 0.1 to 100 hours, preferably 1 to 50 hours. It is.

As the binder, a thermosetting resin such as a urethane resin, a phenol resin, and an epoxy resin;
Thermoplastic resins or elastomers such as polyethylene and polypropylene, and homo- or copolymers such as vinylidene fluoride, ethylene fluoride, acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, polyvinyl alcohol, and polyvinyl butyral can be used.

In order to form a communication hole effective as an ion passage, a pore forming agent may be used.
As the pore-forming agent, a material that is insoluble in the solvent at the time of preparing the coating liquid and that completely oxidizes and decomposes at least in the air atmosphere at a temperature equivalent to the thermal decomposition temperature of the binder, for example, nylon, acrylic , Acetate, polyester or other organic short fibers (0.1 to 100 μm in diameter) or polymethyl methacrylate (PM having a diameter of 0.1 to 100 μm)
Organic polymer particles such as MA) can be used.

Further, the porosity of the obtained positive electrode is preferably 15 to 60%, more preferably 30 to 50%. While ensuring a conductivity of 0.1 mS / cm or more and a packing density of the positive electrode material, it is possible to further improve the permeability of the electrolytic solution and suppress the concentration polarization of ions. The porosity referred to here is an open porosity, which is measured by the Archimedes method described below. Archimedean method: the original sample weight W _1, under reduced pressure or boiled in water, expelling air in the pores, the cooled weight measured in water W _2, taken out of water was measured by taking a drop of water wiped by the surface Weight W ₃
Then Is required.

The conductivity of the sintered body used for the positive electrode is preferably 0.1 mS / cm or more, more preferably 1 mS / cm.
/ Cm or more. The binding force between the primary particles constituting the sintered body becomes stronger, and the mechanical strength of the electrode is further improved.

The positive electrode of the present invention can constitute a non-aqueous secondary battery using the following negative electrode active material and non-aqueous electrolyte. As the negative electrode active material, any material known as a negative electrode active material for a lithium ion secondary battery can be used. For example, natural graphite, carbon materials such as coke and glassy carbon, silicon, metal lithium, and metals such as aluminum Metals capable of forming an alloy with lithium can be given.

The non-aqueous electrolyte may be a non-aqueous electrolyte obtained by dissolving a lithium compound such as LiPF ₆ as an electrolyte in an organic solvent such as ethylene carbonate or dimethyl carbonate, or a solid solution of a lithium compound in a polymer or a lithium compound. A solid polymer electrolyte holding an organic solvent in which is dissolved can be used.

[0022]

[Embodiment 1] The positive electrode was formed by the following method. That is, with respect to 60 parts by weight of LiCoO ₂ , 3.4 parts by weight of polyvinyl butyral resin (BM-S manufactured by Sekisui Chemical Co., Ltd.), 0.85 part by weight of dioctyl adipate as a plasticizer, and toluene / 1-butyl alcohol = 4/60 parts by weight. 1 (volume ratio) 28 parts by weight of a mixed solvent were mixed and kneaded in a ball mill for 24 hours. This coating liquid is applied to a silicone-treated polyester film having a thickness of 50 μm,
After drying at 80 ° C. for 20 minutes, a coating film having a length of 300 × 150 mm was obtained. This coating film was cut into a length of 20 × width 40 mm, peeled from the polyester film, and heated to 900 ° C. in the atmosphere.
For 10 hours to obtain a positive electrode made of a LiCoO ₂ sintered body having a thickness of 300 μm ± 3% and a porosity of 41%. The shrinkage ratio before and after firing {= 1− (vertical length after firing / vertical length before firing)} was about 7%. When the conductivity of the positive electrode was measured by the method described below, it was 13 mS / c.
m.

The conductivity of the sintered body was measured by a four-terminal method.
That is, four terminals are connected in parallel at predetermined intervals on the sintered body, two outer terminals are used as terminals for supplying current, and two inner terminals are used as terminals for detecting voltage. And connected to the voltmeter. The current I was swept in the range of -1 mA to +1 mA, the voltage V was measured, the resistance R was obtained, and the conductivity σ was calculated by the following equation. σ = (R × A / 1) ^-1 where l is the distance between the two voltage detection terminals, and A is the cross-sectional area of the sintered body cut along a plane perpendicular to the current direction.

Next, the negative electrode was formed by the following method. 90 parts by weight of crystalline silicon powder having a purity of 99.9% and a particle size of about 1 μm manufactured by Kojundo Chemical Co., Ltd., and an n-methyl-2-pyrrolidone solution of polyvinylidene fluoride (14 wt.
%) Of 70 parts by weight to obtain a uniform coating liquid. This coating solution is applied on a copper foil of a current collector, dried at 80 ° C. for 20 minutes, punched out to a size of 20 × 40 mm, and baked at 800 ° C. for 3 hours in a nitrogen atmosphere to be integrated with the current collector. A negative electrode was obtained.

As the electrolytic solution, a solution prepared by dissolving 1 mol / L of lithium hexafluorophosphate in a mixed solvent of propylene carbonate and dimethyl carbonate at a volume ratio of 1: 1 was used. The positive electrode and the negative electrode were laminated via a separator made of a porous polyethylene film, housed in a battery can, and then injected with the electrolytic solution and sealed to produce a battery cell.

After leaving the battery cell at room temperature for 24 hours,
When a charge / discharge test was performed, the discharge capacity at the first cycle of this battery cell was 55 mAh, and the capacity retention at the 50th cycle {= (discharge capacity at the 50th cycle / discharge capacity at the first cycle) × 100} was as follows. 90%.

Embodiment 2 FIG. A positive electrode was prepared in the same manner as in Example 1 except that 3 parts by weight of spherical polymethyl methacrylate having a diameter of 5 μm was added as a pore-forming agent to the coating solution of Example 1, and LiCoO _{2 having} a porosity of 43% was prepared. A positive electrode made of a sintered body was obtained. Using this as a positive electrode, a battery cell was constructed in the same manner as in Example 1, and a charge / discharge test was performed.

The shrinkage ratio of the sintered body of LiCoO ₂ before and after firing was about 7%, and the conductivity was 5 mS / cm. Further, the discharge capacity of the battery cell in the first cycle is 60 mAh, 5
The capacity retention at the 0th cycle was 90%.

Comparative Example 1 In Example 1, firing was performed at 900 ° C. for 3 hours in an air atmosphere to obtain a thickness of 300 μm ± 3%,
A battery cell was formed in the same manner as in the example except that a sintered body of LiCoO _{2 having} a porosity of 43% was used as a positive electrode, and a charge / discharge test was performed. The shrinkage of this sintered body before and after firing was about 2%, and the electrical conductivity was 0.04 mS / cm. Further, the discharge capacity in the first cycle of this battery cell is 60 m.
Ah, the capacity retention at the 50th cycle was 10%.

Comparative Example 2 To 100 parts by weight of LiCoO ₂ powder, 10 parts by weight of polyethylene powder was added as a molding aid, and the mixture was filled in a mold and pressed, and the length was 20 × 40 mm in width.
An attempt was made to prepare a molded body, but the powder could not be uniformly filled, and the molded body could not be prepared.

From the above results, since the higher the conductivity of the sintered body, the higher the shrinkage ratio before and after firing, the binding between the primary particles constituting the sintered body progresses further, and the binding force between the primary particles increases. It can be seen that is increasing. Moreover, the result that the sintered body with a higher electric conductivity had a higher capacity retention was obtained. this is,
It is considered that even if the electrode repeatedly expands and contracts due to charge and discharge, the high binding force between the primary particles suppressed the falling off of the primary particles and the collapse of the electrodes. Moreover, since a sintered body is used, the packing density of the active material is high, and the capacity per unit volume is high.

[0032]

As described above, as described above, the positive electrode of the present invention
A porous sintered body made of a lithium transition metal oxide, having a porosity of 15 to 60% and a conductivity of 0.1 mS
/ Cm or more, the internal resistance of the battery can be reduced, and a non-aqueous secondary battery having high capacity and excellent cycle characteristics can be provided.

Further, in the manufacturing method of the present invention, since the coating containing the positive electrode material made of lithium transition metal oxide is baked to form a sintered body, a positive electrode having a uniform thickness and density can be formed. In addition, it is possible to provide a positive electrode having an excellent cycle characteristic, a high capacity, and a larger area.

In the manufacturing method of the present invention, since the coating film containing the pore-forming agent is baked, the concentration polarization of ions can be suppressed, and the internal resistance of the battery can be reduced.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2000-82464 (JP, A) JP-A-2000-1224 (JP, A) JP-A-2000-11994 (JP, A) JP-A-2000-11993 (JP, A A) JP-A-11-31498 (JP, A) JP-A-11-31534 (JP, A) JP-A-8-180904 (JP, A) JP-A 9-120813 (JP, A) JP-A-60 -133656 (JP, A) JP-A-61-171071 (JP, A) WO 98/28804 (WO, A1) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/02-4 / 04 H01M 4/58 H01M 10/40

Claims (2)

(57) [Claims] 1. A positive electrode for a non-aqueous secondary battery, comprising a porous sintered body made of a lithium transition metal oxide, having a porosity of 15 to 60% and a conductivity of 1 mS / cm or more. A method for producing, comprising: a) a positive electrode material comprising a lithium transition metal oxide powder;
With a solvent, urethane resin, phenol resin, epoxy resin, polyethylene, polypropylene, polyvinyl alcohol, polyvinyl butyral, and vinylidene fluoride,
At least one selected from homo- or copolymers of fluorinated ethylene, ethylene oxide, and propylene oxide
A step of preparing a coating liquid by adding a kind of binder; b) a step of applying the coating liquid on a substrate and removing a solvent to prepare a coating film; And sintering the positive electrode material to obtain a positive electrode for a non-aqueous secondary battery. 2. The method according to claim 1, wherein, in the step of preparing the coating liquid, a pore-forming agent which gives a pore by being thermally decomposed is added.