JP2665479B2 - Rechargeable battery - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly, to a high volume efficiency, excellent heavy load charge / discharge characteristics, and easy manufacture. The present invention relates to a certain non-aqueous electrolyte secondary battery.

[Prior arts and problems to be solved by the invention] Generally, batteries using metallic lithium as a negative electrode active material have advantages such as light energy density, light weight, small size, and long-term storage stability. Has been put to practical use. However, the secondary battery using the negative electrode active material has a new problem that the primary battery does not have. That is, a secondary battery using metallic lithium as a negative electrode has a short charge-discharge cycle life, and has a low charge-discharge efficiency during charging and discharging. This is due to the deterioration of the lithium negative electrode due to the fact that the metallic lithium deposited on the negative electrode has a dendritic (dendritic) shape.

The present inventors, as a means of solving the disadvantages of using metallic lithium as the negative electrode, as a negative electrode for a secondary battery, a composite in which lithium was previously absorbed in a carbon fiber molded body having a specific lattice spacing was used. (Japanese Patent Application Laid-Open No. Sho 62)
No. -268058). A secondary battery using this negative electrode has excellent properties such as no generation of dendrites, a long charge / discharge cycle life, high reliability, and measurement.

On the other hand, as the separator of the non-aqueous electrolyte battery, for example, nonwoven fabrics such as polypropylene nonwoven fabric, vinylon-rayon blended nonwoven fabric, vinylon nonwoven fabric, polyester nonwoven fabric, and glass fiber nonwoven fabric,
For example, there are synthetic resin microporous membranes such as polyethylene microporous membrane and polypropylene microporous membrane. In particular, a polypropylene nonwoven fabric or a polypropylene microporous membrane is generally used because of its stability against an electrolytic solution and mechanical strength.

However, a composite in which lithium has been previously absorbed in a molded body of carbon fiber or carbon powder has a lower volume efficiency than that of metallic lithium or the like. Therefore, when manufacturing a secondary battery, it was found that when the nonwoven fabric or the like as described above was used as a separator, the thickness of the separator was extremely large and the volume efficiency was low, so that the separator could not be put to practical use. .

On the other hand, when a synthetic resin microporous membrane is used as a separator, the wettability to the electrolyte and the liquid retention are very poor, and therefore, there is a problem in the operability during production and the charge / discharge characteristics under heavy load. I understand.

In order to solve these drawbacks when using a synthetic resin microporous membrane as a separator, there is a method of forming a multilayer structure with a nonwoven fabric and / or further using an electrolyte impregnating material. obtain. However,
These methods still have low volumetric efficiencies, as with nonwoven fabrics. This is also not practical.

When the surface of the microporous membrane is chemically treated with a chromic acid-sulfuric acid mixed solution or the like to improve the wetting, the wetting is not substantially improved or the treatment is performed even if it is improved. It takes a long time, and furthermore, it is necessary to carry out a post-treatment in order to prevent the incorporation of moisture and heavy metals.

[Means and actions for solving the problem]

The present inventors have proposed a mold using each of carbon fiber and carbon powder having a specific lattice spacing (hereinafter, referred to as a mold).
Both may be collectively referred to as a carbon molded body. In addition, the above-mentioned problems of a secondary battery having a composite in which lithium has been stored in advance as a negative electrode have been solved, and a secondary battery which has high volume efficiency, excellent heavy load charge / discharge characteristics, and is easy to manufacture. As a result of diligent and intensive studies to develop batteries, for example, by using a membrane that has been subjected to surface treatment by plasma or sputter etching on a microporous membrane made of synthetic resin such as polypropylene as a separator, volume efficiency is high. It has been found that a secondary battery having excellent heavy-load charge / discharge characteristics and easy to manufacture can be obtained, and has reached the present invention.

That is, the present invention provides, at least, a rechargeable lithium-based negative electrode, a rechargeable battery having a rechargeable positive electrode and a separator impregnated with an organic electrolyte, in which the negative electrode is formed using a carbon fiber or carbon powder. A secondary battery, which is a composite in which lithium is stored in advance, and wherein the separator is a synthetic resin microporous membrane surface-treated by either plasma or sputter etching.

The carbon fiber and carbon powder used in the carbon molded article of the present invention are not particularly limited, but are preferably lattice spacing (d0
02) is a carbon fiber and a carbon powder of 3.38 to 3.56Å, particularly preferably 3.40 to 3.50Å.

In the molded body using the carbon fiber and the carbon powder, the lattice spacing (d002) is derived from the lattice spacing of the carbon fiber and the carbon powder used, and is preferably from 3.38 to 3.56. Å, particularly preferably 3.40 ~
3.50Å.

When a carbon fiber molded body or a carbon powder molded body having a lattice spacing (d002) of more than 3.56 ° is used, the battery voltage decreases, and the discharge voltage decreases simultaneously with the start of discharge. On the other hand, when a carbon fiber molding or a carbon powder molding having a lattice spacing (d002) of less than 3.383.3 is used, for example, the lattice spacing (d002) exceeding 3.563.5
In addition to lowering the battery voltage as in the case of using a carbon fiber molded body or a carbon powder molded body having a carbon fiber, the carbon molded body swells when lithium is absorbed, and it becomes difficult to finally maintain the original shape. .

In the present invention, the lattice spacing (d002) is expressed as X-rays.
This is a value measured by a method using CuKα and using high-purity silicon as a standard substance. This method is described in, for example, "Carbon Fiber"
(March 1986) pp. 733-742.

The carbon fiber used for the carbon fiber molded body is made of raw materials,
Although there is no particular limitation on the production method, typical examples of practical raw materials include pitch, polyacrylonitrile, rayon and the like. The fiber diameter of the carbon fiber is not particularly limited, but is practically 0.01 to 100 μm, preferably 0.1 to 50 μm.

Carbon fibers that have been subjected to various surface treatments can also be used.

The carbon powder used in the present invention is obtained, for example, by pulverizing and classifying carbon fibers and carbon blocks having the lattice spacing (d002) described above. The average particle size of the carbon powder is generally 0.1 to 30 μm, preferably 0.5 to 20 μm.

In the present invention, the average particle size of the carbon powder is determined from the particle size distribution obtained by the light transmission method by the liquid phase sedimentation method according to the Stokes sedimentation method, and the cumulative distribution is determined as the particle size corresponding to 50%. Value. For this purpose, for example, a “centrifugal automatic particle size distribution measuring device manufactured by Dig Mfg. Co., Ltd.” is suitably used.

The molded body of carbon fiber used in the present invention, carbon fiber, with or without a binder, practically,
For example, it is molded into a form such as a plate, a net, a woven fabric, a nonwoven fabric, and a papermaking, but is not necessarily limited thereto.

The molded body of the carbon powder is usually formed into a plate or sheet using a binder.

When a binder is used, the amount of the binder is 50% by weight or less of the carbon molded body, preferably 2 to 30% by weight, particularly preferably 5 to 20% by weight.

The thickness of the carbon molded body differs depending on the desired battery capacity and battery shape and cannot be specified unconditionally, but is practically usually 0.05 to 50 mm, preferably about 0.1 to 20 mm. In addition, one carbon molded body may be used, but several carbon molded bodies may be used by being stacked.

The negative electrode in the present invention is a composite (hereinafter, also referred to as a composite) in which lithium is occluded in the carbon molded body.
It is. When the carbon molded body itself is used as a negative electrode, it has disadvantages such as low energy density and short charge / discharge cycle life.

There is no particular limitation on the method of storing lithium in the carbon molded body, but usually, an electrochemical method or a physical method is used. That is, for example, as an electrochemical method, a method in which a current is applied between both electrodes as described in JP-A-62-28058 in an organic electrolyte using a carbon molded body and metallic lithium as electrodes, respectively. And JP-A-63-10
As described in Japanese Patent No. 462, there is a method of short-circuiting both electrodes with a conductor.

Examples of the physical method include a method of contacting a carbon molded body with metallic lithium as described in JP-A-62-28056.

The positive electrode used in the secondary battery of the present invention may be any one that can be charged and discharged, such as a transition metal chalcogen compound, a conjugated polymer compound, or activated carbon, but is not limited thereto. is not.

Transition metal chalcogen compounds include TiO ₂ , Cr
_{3 O 5, V 2 O 5} , V 3 O 6, sulfides such as MnO ₂ and Mo ₃ O, Ti
Examples include sulfides such as S ₂ , VS ₂ , FeS and MoS ₃ and selenium compounds such as NbSe ₃ .

Further, as the conjugated polymer compound, polyacetylene,
Polyparaphenylene, polypyrrole, polythiophene, polyaniline, and the like can be used.

These positive electrodes are generally used in the form of a membrane or a plate by an ordinary method.

The lithium salt used in the organic electrolyte of the secondary battery is not particularly limited, but typical examples thereof include LiCl, LiClO ₄ , LiBF
_4, such as LiPF _6, LiAsF ₆ and LiSbF ₆ can be mentioned. Among them, LiClO ₄ , LiPF ₆ and LiBF ₄ are preferred.

These lithium salts are usually used alone,
Two or more types can be used in combination.

A solution in which these lithium salts are dissolved in a solvent is used as an organic electrolyte for a secondary battery.

This solvent may be any organic solvent capable of dissolving the lithium salt, but is preferably an aprotic and high dielectric constant organic solvent, such as nitrile, carbonate, ether, nitro compound, amide, sulfur-containing compound, and hydrochloric acid. Hydrocarbons, ketones and esters can be used.

Representative examples of these include acetonitrile, propionitrile, pyropyrene carbonate, ethylene carbonate, tetrahydrofuran, dioxane, 1,3-dioxolan, 4-methyl-1,3-dioxolan, 1,2-dimethoxyethane, nitromethane, N, Examples include N-dimethylformamide, dimethylsulfoxide, sulfolane, and γ-butyrolactone.

One of these organic solvents may be used, or two or more of them may be used as a mixed solvent.

The concentration of the lithium salt in the lithium salt solvent is usually 0.1.
It is about 1 to 5 mol /, preferably about 0.5 to 1.5 mol /.

The separator used in the secondary battery of the present invention is a synthetic resin microporous film (hereinafter, sometimes referred to as a microporous film) surface-treated by plasma or mass sputter etching.

The microporous film in the present invention is an integrated structure having a thin film-like structure having many micropores 9. Thus, these micropores overlap each other to form a through-hole, or are closed by a non-porous portion of the thin film-like structure to become a non-through-hole.

Further, in the microporous membrane, one hole is divided into a plurality of small microholes (hereinafter, referred to as microholes) by many membrane-shaped partitions before the treatment, but after the treatment, one of the many partitions is formed. Although the part or the front part is erased, the loss of the partition is particularly remarkable in the surface layer of the microporous membrane.

The microporous membrane of the present invention is not particularly limited as long as it is made of a synthetic resin, but is preferably made of a polyolefin such as polyethylene and polypropylene for practical use.

This microporous membrane has a thickness of 100 μm or less, preferably 30 to 7
The thing of 0 μm is used. The pore size and porosity of the micropores of the microporous membrane before the treatment are not particularly limited, but are preferably in the range of 0.1 to 1 μm as the average pore size from a practical viewpoint, and 20 to 60 as the porosity. % Is preferred.

In the present invention, the parallel pore diameter and the porosity are values obtained by observing micropores present in the microporous membrane with a scanning electron microscope, and “average” means an arithmetic average.

The pore diameter is defined as the diameter of a circle when the micropores are assumed to be a circle, and the porosity is defined as the ratio of the area of the micropores to the surface area.

Commercially available products can be used as such a microporous membrane. Typical examples of commercially available products include Duraguard, NF sheet, and Cellpore (all of which are trade names).

The plasma processing apparatus is not particularly limited, and any of a capacitive coupling type and a coil coupling type can be used.

As the gas used for the plasma treatment, air, nitrogen, hydrogen, argon, helium, and the like are practically used, respectively, but there is substantially no difference depending on the type of gas.

The plasma processing conditions vary depending on the film material, pore size, porosity, and the like, and cannot be specified unconditionally. However, in general, a suitable processing state can be easily realized by changing the processing time. The preferred treatment state here means that the average pore diameter and the porosity of the micropores present in the micropores are respectively increased as compared to before the treatment, and the respective increase rates are usually 10%.
The condition does not exceed 0%, preferably does not exceed 50%.

Processing time varies depending on the type of film, processing conditions, etc.
Although it cannot be specified unconditionally, it is preferably 30 seconds to 5 minutes per side for practical use, and it is preferable to perform double-sided treatment.

The apparatus used for the sputter etching treatment is not particularly limited.

The gas used for this treatment is air, nitrogen, oxygen, hydrogen argon and helium, respectively.
In particular, there is no practical difference depending on the type of gas.

The processing conditions vary depending on the material of the film, the pore size, the porosity, and the like, and cannot be specified unconditionally.
It is preferable to perform the treatment for two minutes, and it is preferable to perform the double-sided treatment.

Thus, a suitable processing state can be realized. Here, the preferred treatment state is a state in which the respective increasing rates of the average pore diameter and the porosity do not exceed 100%, and preferably do not exceed 50%, as in the case of the plasma treatment.

Of both processes, plasma treatment is preferred from the viewpoint of the uniformity of the film surface treatment.

A current collector can be used similarly to a conventional battery, and is preferable. As the current collector, a current collector used in a conventional battery can be used. That is, an electrochemically inactive conductor is used for the electrolytic solution and each of the positive electrode and the negative electrode. For example, metals such as nickel, titanium, stainless steel and the like can be used in the form of plates, foils and nets. The positive electrode current collector and the negative electrode current collector may be different materials, or may be the same material. Its thickness is 0.
It is about 001 to 10 mm, preferably about 0.01 to 5 mm.

〔Example〕

The following examples further illustrate the present invention. The present invention is not limited to these examples.

Example 1 A polyacrylonitrile-based carbon fiber molded body having a lattice spacing (d002) of 3.44% (fiber diameter 8 μm, weight 25.0 mg, diameter 14)
mm, thickness 0.4mm) and metal lithium disk (diameter 18mm, thickness)
1.5 mm) were placed facing each other with a glass fiber nonwoven fabric interposed, and sandwiched and pressed by a stainless steel plate from both sides. This was added to a mixed solvent of propylene carbonate and 1,3-dioxolane (1: 1 volume ratio) at a concentration of 1.0 mol /
Was impregnated with a solution in which lithium perchlorate was dissolved. Next, the external DC power supply “Hokuto Denko Co., Ltd., battery charging / discharging device, HJ-201B” (the same applies to the following examples and comparative examples) has a plus terminal on a stainless steel plate on the metal lithium side and a minus terminal on carbon fiber. Each was connected to a stainless steel plate on the side of the molded body, and a current was passed for 9.5 hrs at a constant current of 0.32 mA / cm ² to obtain a composite in which 1.2 mg of lithium was absorbed.

The thus obtained composite was used as a negative electrode, 20 parts by weight of vanadium pentoxide, and acetylene black 10 as a conductive agent.
Parts by weight, a disc-shaped molded body (weight: 10 parts by weight of a polytetrafluoroethylene powder as a binder)
A solution prepared by dissolving lithium perchlorate in a mixed solvent of propylene carbonate and 1,3-dioxolane (1: 1 volume ratio) so that the concentration becomes 1.0 mol / is used as an electrolyte. A secondary battery was fabricated using the impregnated polypropylene microporous membrane as a separator. A microporous polypropylene membrane having a thickness of 50 μm, an average pore diameter of 0.3 μm, and a porosity of 30% as a separator was subjected to a plasma treatment using a plasma basic research apparatus model BP-1 manufactured by Samco International Laboratories, Inc. One used.

In addition, this processing apparatus is a capacitive coupling type, the processing condition is an oscillation frequency of 13.56 MHz, and the processing condition is an air atmosphere.
A double-sided treatment was performed at a pressure of 20 Pa, a high-frequency output of 20 W, a plate current of 300 mA, and a treatment time of one minute per one side. By this treatment, the average pore diameter and the porosity of the microporous membrane became 0.33 μm and 35%, respectively.

FIG. 1 shows the surface state of the microporous film subjected to the plasma treatment. That is, FIG. 1 is a scanning electron micrograph of the surface of the plasma-processed microporous film (the same applies to FIGS. 2 to 4).

The photographing conditions of the electron micrograph are as follows (the same applies to the following examples). Electron microscope: Akashi scanning electron microscope (ALPHA-25) Acceleration voltage 25KV Negative electrode Tungsten Magnification 5000 times Photographing: Polaroid 665 used In this way, the secondary battery obtained was discharged with a discharge voltage of 1.50.
When a constant current discharge of 5.0 mA / cm ² was performed until the voltage reached V,
A discharge capacity of 3.4 mAh was obtained, and the volumetric efficiency was 4.6 Ah /.

Next, 5 mA / cm ² of 2.00V lower limit voltage at a constant current, where the upper limit voltage is 3.90 V, was subjected to charge-discharge cycle test, 4
Both the discharge capacity at the 0th cycle and the 120th cycle
It was 3.2 mAh.

Example 2 A secondary battery was manufactured in the same manner as in Example 1, except that the plasma treatment time was set to 5 minutes per one side and the double-sided treatment was performed. By this treatment, the average pore diameter and the porosity of the microporous membrane became 0.35 μm and 40%, respectively. FIG. 2 shows the surface state of the microporous film after the plasma treatment.

5.0 mA / cm ² until the discharge voltage reaches 1.5 V
When the constant cell discharge was performed, a discharge capacity of 3.4 mAh was obtained, and the volumetric efficiency was 4.6 Ah /.

Next, at a constant current of 5 mA / cm ² , the lower limit voltage is 2.00 V and the upper limit voltage is
When the charge and discharge cycle test was performed at 3.90 V, 40
The discharge capacity at both the cycle and the 120th cycle
It was 3.2 mAh.

Example 3 A secondary battery was manufactured in the same manner as in Example 2 except that plasma treatment was performed in an argon atmosphere.

By this treatment, the average pore diameter and the porosity of the microporous membrane became 0.35 μm and 40%, respectively. FIG. 3 shows the surface condition of the microporous film subjected to the plasma treatment.

5.0 mA / cm ² until the discharge voltage reaches 1.50 V
When a constant current discharge was performed, a discharge capacity of 3.4 mAh was obtained, and the volumetric efficiency was 4.6 Ah /.

Next, the lower limit voltage at a constant current of 5.0 mA / cm ² 2.00V, where the upper limit voltage is 3.90 V, was subjected to charge-discharge cycle test, none of the discharge capacity of the 40th cycle, and 120 cycle at 3.2mAh there were.

Example 4 As a separator, the thickness was 50 μm, and the average pore diameter was 0.3 μm.
And a polypropylene microporous membrane with a porosity of 30%,
Using an IB-3 type ion coater manufactured by Eiko Engineering Co., Ltd., a secondary battery was produced in the same manner as in Example 1 except that a microporous film subjected to sputter etching was used.

The sputter etching conditions were as follows: air atmosphere, pressure 0.2 Torr, ion current 6 mA, processing time per one side
Double-sided treatment was performed for 10 minutes.

By this treatment, the average pore diameter and the porosity of the microporous membrane became 0.35 μm and 40%, respectively. FIG. 4 shows the surface state of the microporous film subjected to the sputter etching.

When the thus obtained secondary battery was discharged at a constant current of 5.0 mA / cm ² until the discharge voltage became 1.50 V,
3.3mAh discharge capacity is obtained, and volumetric efficiency is 4.5Ah /
Met.

Comparative Example 1 Untreated 50μ thick separator as a secondary battery separator
m, an average pore diameter of 0.3 μm, and a porosity of 30%, except that a polypropylene microporous membrane having a porosity of 30% was used to produce a secondary battery in the same manner as in Example 1. FIG. 5 shows the surface condition of the microporous film.

5.0 mA / cm ² until the discharge voltage reaches 1.50 V
When a constant current discharge was performed, only a discharge capacity of 2.8 mAh was obtained.

Comparative Example 2 A secondary battery was produced in the same manner as in Example 1 except that a polypropylene nonwoven fabric (thickness: 200 μm, fiber diameter: 2 μm) was used as a separator of the secondary battery. When this secondary battery was discharged at a constant current of 5.0 mA / cm ² until the discharge voltage reached 1.50 V, a discharge capacity of 3.6 mAh was obtained.
Volumetric efficiency was only 3.7 Ah /.

Comparative Example 3 As a separator for a secondary battery, a polypropylene microporous membrane having a thickness of 50 μm, an average pore diameter of 0.3 μm, and a porosity of 30% was treated with a mixture of chromic acid and sulfuric acid (polymerization ratio 1: 1) at 60 ° C. for 1 hour. A secondary battery was produced in the same manner as in Example 1 except that the above-described battery was used. When this secondary battery was discharged at a constant current of 5.0 mA / cm ² until the discharge voltage became 1.50 V,
A discharge capacity of 2.8 mAh was not obtained.

Example 5 Propylene carbonate was used as an electrolyte for a secondary battery.
A secondary solution was prepared in the same manner as in Example 1 except that a solution prepared by dissolving lithium perchlorate in a mixed solvent with 1,2-dimethoxyethane (1: 1 volume ratio) so that the concentration became 1.0 mol / was used. A battery was manufactured.

5.0 mA / cm ² until the discharge voltage reaches 1.50 V
, A discharge capacity of 3.3 mAh was obtained and the volumetric efficiency was 4.5 Ah /.

Example 6 As a separator of a secondary battery, a plasma treatment was performed under the same conditions as in Example 1, a thickness of 26 μm, and an average pore diameter of 0.5 μm.
A secondary battery was produced in the same manner as in Example 1 except that a polypropylene microporous membrane having a porosity of 45% was used. By this treatment, the average pore diameter and porosity of the microporous membrane are:
It was 0.55 μm and 55%, respectively.

5.0 mA / cm ² until the discharge voltage reaches 1.50 V
, A discharge capacity of 3.4 mAh was obtained and the volumetric efficiency was 4.6 Ah /.

Example 7 A carbon block “SEG-RB” manufactured by Nippon Carbon Co., Ltd. having a lattice spacing (d002) of 3.41 mm was pulverized into carbon powder. The average particle size of this carbon powder was measured using an automatic particle size distribution analyzer “CAPA-500, manufactured by Digyard Co., Ltd.”
It was 3.5 μm. After 90 parts by weight of this carbon powder and 10 parts by weight of polytetrafluoroethylene powder were kneaded, a molded carbon powder having a weight of 50 mg, a diameter of 15 mm and a thickness of 0.18 mm was obtained by pressure molding.

The obtained carbon powder compact and metallic lithium disk (diameter 18
mm, thickness 1.5 mm) were placed facing each other with a glass fiber non-woven fabric interposed therebetween, and pressed and pressed by a stainless steel plate from both sides. Add propylene carbonate and 1,
Concentration in a mixed solvent with 3-dioxolane (1: 1 volume)
Impregnated with a solution in which lithium perchlorate was dissolved to 1.0 mol /, the carbon powder molded body was used as a negative electrode, and one metal lithium was connected as a positive electrode to an external DC power supply, and the current density was 1.3 mA / cm. Apply 7.0hrs at constant current of ² , 4.1mg
A composite in which lithium was absorbed was obtained.

A secondary battery was fabricated in the same manner as in Example 1, except that the composite thus obtained was used as a negative electrode.

5.0 mA / c until the discharge voltage reaches 2.00 V
When a constant current discharge of m ² was performed, a discharge capacity of 2.9 mAh was obtained, and the volumetric efficiency was 6.5 Ah /.

By comparing FIGS. 1-4 with FIG. 5, the surface-treated microporous membrane used as a separator in the present invention is characterized in that the pores themselves are enlarged and many membrane-like partitions around the micropores are formed. You can see that some have been lost.

〔The invention's effect〕

The secondary battery of the present invention is a nonaqueous electrolyte secondary battery having high volumetric efficiency, excellent heavy load discharge characteristics, and easy production.

[Brief description of the drawings]

The drawing is an electron micrograph showing the surface condition of the microporous membrane. FIGS. 1-4 are for a microporous membrane that has been surface treated, and FIG. 5 is for a microporous membrane before treatment.

Continuation of the front page (56) References JP-A-61-78053 (JP, A) JP-A-62-268058 (JP, A) JP-A-58-94752 (JP, A) "Polymer processing" Vol. 31 Issue 1 (1983) pp. 26-31

Claims (1)

(57) [Claims] 1. A secondary battery having at least a chargeable / dischargeable lithium-based negative electrode, a chargeable / dischargeable positive electrode, and a separator impregnated with an organic electrolyte, wherein the negative electrode is previously formed on a molded body using carbon fiber or carbon powder. A secondary battery, which is a composite in which lithium is stored, and wherein the separator is a synthetic resin microporous membrane surface-treated by either plasma or sputter etching.