JP2003151550A - Nonaqueous secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous secondary battery which has a high capacity and which is superior in charging-discharging cycle characteristics by making use of characteristics of poly-sulfide carbon used as a positive electrode active substance. SOLUTION: In the nonaqueous secondary battery having the positive electrode using poly-sulfide carbon as the positive electrode active substance, a negative electrode and an electrolytic solution, the positive electrode is constituted of a multi-layered structure of a layer composed of an electroconductive powder and a binder formed on a current collector, and a layer containing the poly-sulfide carbon and the binder laminated on it. That which is expressed as a general formula (CSx )n (in the formula, x is 0.9 to 2.0 and n is 4 or more) is preferable as the poly-sulfide carbon, and at least one kind selected from a group composed of graphite, acetylene black, carbon black, Ketchen black, active carbon or metal powder is preferable as the electroconductive powder.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous secondary battery, and more particularly to a non-aqueous secondary battery which uses polysulfide carbon as a positive electrode active material and has a high capacity and excellent charge / discharge cycle characteristics.

[0002]

2. Description of the Related Art Currently, a lithium ion secondary battery is mainly used as a power source for portable equipment. The reason is that the lithium-ion secondary battery can be made lighter and have a higher voltage than conventional secondary batteries represented by NiCd secondary batteries and nickel-hydrogen secondary batteries. Are mentioned.

However, the current lithium ion secondary battery uses a metal oxide such as LiCoO ₂ for the positive electrode,
Since graphite is used for the negative electrode and the theoretical capacity is approaching, the capacity limit is approaching, while on the other hand, the safety tends to decrease due to the high energy density. Also, recent IC
With the lowering of the voltage, the demand for secondary batteries that have a high capacity and can be driven at a low voltage is increasing.

When polysulfide carbon is used as a positive electrode active material, its theoretical capacity is as large as 670 mAh / g, and it is being considered as a battery candidate that can meet the above-mentioned requirements. However, in a lithium secondary battery using a non-aqueous electrolyte with this polysulfide carbon as a positive electrode active material, a conductive auxiliary agent such as carbon black is usually added to polysulfide carbon having low conductivity to improve electron conductivity. The positive electrode is formed by applying and holding them with a binder,
In the charging / discharging mechanism, the polysulfide carbon of the positive electrode active material dissolves in the electrolytic solution, so the conductive auxiliary agent and the binder also separate from the positive electrode along with the dissolution of the polysulfide carbon, clogging the separator, and the surface of the negative electrode. There is a problem in that they are deposited to raise the impedance of the battery and hinder the smooth progress of charging and discharging.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems in the prior art and to provide a non-aqueous secondary battery having high capacity and excellent charge / discharge cycle characteristics. .

[0006]

The present invention is a non-aqueous secondary battery having a positive electrode using polysulfide carbon as a positive electrode active material, a negative electrode, and an electrolytic solution, the positive electrode being formed on a current collector. The above problem is solved by forming a multilayer structure including a layer containing the conductive powder and the binder, and a layer containing the carbon polysulfide and the binder laminated on the layer.

That is, in a normal positive electrode having a single positive electrode mixture layer, the positive electrode mixture layer contains a conductive auxiliary agent and a binder in addition to polysulfide carbon as a positive electrode active material. The conductive aid is intended to impart electron conductivity to the low conductive carbon polysulfide, but in the system in which the carbon polysulfide of the positive electrode active material is dissolved in the electrolytic solution, the positive electrode mixture layer It has been found that it is more important to have higher electron conductivity with the current collector than inside the.
In addition, since the binder also polycarbon sulfide of the positive electrode active material is dissolved in the electrolytic solution, comparing the importance of mechanically holding the positive electrode mixture layer with the adhesion to the current collector, the former It was found that the minimum amount of binder should be used. From this, in the positive electrode mixture layer containing polysulfide carbon as the positive electrode active material, it is possible to reduce as much as possible the conductive auxiliary agent and the binder that are separated from the positive electrode along with the dissolution of the polysulfide carbon of the positive electrode active material,
Those materials that have come off may clog the separator,
It is considered that it can be prevented from being deposited on the surface of the negative electrode to increase the impedance of the battery and hinder the smooth progress of charge / discharge.

However, since the electron conductivity and the adhesiveness with the current collector are lacking in the layer having a small amount of the conductive auxiliary agent and the binder as described above, the present inventors have repeatedly studied and newly added the electron conductivity. It has been found that the above-mentioned problems can be solved by forming a layer on the current collector that has a large value and has excellent adhesion to the current collector.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The layer for ensuring the electronic conductivity and the adhesion to the current collector is composed of a conductive powder and a binder. Examples of the conductive powder include: Graphite, acetylene black, carbon black, Ketjen black, activated carbon, metal powders such as nickel and aluminum are used, and in order to further improve electron conductivity, the specific surface area of the conductive powders is large. preferable.

As the binder, for example, polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, amorphous polyether, polyacrylamide, fluororubber, diene rubber, carboxymethylcellulose, soluble in a solvent The same as those conventionally used, such as polyaniline, polypyrrole, or a copolymer of monomers constituting these compounds or a compound formed by crosslinking, can be used, and the same two layers may be used. Although different materials may be used, it is preferable to use a material having a stronger binding action for the layer on the side in contact with the current collector. In the layer composed of the conductive powder and the binder, the ratio thereof is not particularly limited, but the mass ratio is preferably 1: 0.2 to 1: 7, and the thickness thereof is 0. It is preferable to make the thickness equivalent to 0.5 to 5 mg / cm ² .

As the current collector for the positive electrode, for example, foils of aluminum, nickel, copper, stainless steel, punching metal, expanded metal, net, etc. can be used, but aluminum foil is particularly suitable.

In the present invention, the composition of the carbon polysulfide used as the positive electrode active material is slightly different depending on the synthesis conditions, but when the composition is represented by the formula (CS _x ) _n , x is 0.9 to 0.9. It is preferably 2.0 and n is 4 or more. That is, when x is smaller than 0.9, a large amount of C—S—C bonds that are not involved in charging / discharging is included in addition to the disulfide bonds involved in charging / discharging, and the capacity tends to decrease. Is more than 2.0, a large amount of polysulfide segment represented by —S _y — (y ≧ 3) is contained in the molecule, and the stability of the molecule may be decreased, and the reversibility during charge / discharge may be decreased. is there. That is, x is 0.9 to
A carbon polysulfide in the range of 2.0 is preferable because the existence ratio of disulfide bonds involved in charge / discharge in the molecule becomes high, and among them, carbon polysulfide in which x is 1.0 to 1.7 is a molecule. The presence ratio of the disulfide bond involved in charge / discharge is particularly high, which is preferable. And n is 4
The above is preferable because when n is 3 or less, it is difficult to produce a polysulfide carbon having a disulfide bond, and even if it can be produced, the capacity as an active material is low and the polysulfide is dissolved in an electrolytic solution. May be less useful. Considering the workability, this n is preferably 100 or more, and no matter how large it is, no problem arises, but normally n of up to about 100,000 is practically suitable.

Examples of the active material for the negative electrode include alkali metals such as lithium and sodium, alloys thereof with aluminum and the like, graphite, pyrolytic carbons, cokes, glassy carbons, fired bodies of organic polymer compounds, Mesocarbon microbeads, carbon fibers, activated carbon, carbonaceous materials such as graphite, oxides containing tin (tin) or silicon (silicon), nitrogen compounds of lithium cobalt, etc. can be used, and particularly lithium, lithium alloys, lithium A carbonaceous material capable of doping / dedoping ions is preferable.

The electrolyte solution is prepared by dissolving an electrolyte salt such as a lithium salt in an electrolyte solution solvent composed of a non-aqueous solvent such as an organic solvent.

Examples of the electrolytic solution solvent include dimethylsulfate.
It has a sulfur atom in the molecule such as ruphoxide and sulfolane.
Solvent, R₁O (CH₂CH₂O)_zR₂(R₁, R
₂Is an alkyl or alkenyl group having 1 to 4 carbon atoms
Yes, R₁And R₂Can be the same or different
May be. z is 1 ≦ z ≦ 14)
Preference is given to lenoxide-based solvents, mixed solvents thereof, etc.
In addition, these solvents have an affinity with carbon polysulfide.
There is. And the R₁O (CH₂CH₂O) _zR₂so
Specific examples of the ethylene oxide-based solvent represented are:
For example, Monoglyme, Jiglyme, Triglyme, Te
Examples include trag lime.

As the electrolytic solution solvent, only the solvent having an affinity with the above-mentioned carbon polysulfide may be used, but the solvent having an affinity with the carbon polysulfide may be, for example, 1,3-dioxolane, tetrahydrofuran or 2-methyl. -The addition of a low-viscosity solvent such as tetrahydrofuran or diethyl ether is preferable because the ion conductivity is further improved and the cycle characteristics are improved. The amount of the low-viscosity solvent added is such that the low-viscosity solvent is 50% by volume or less of the total electrolytic solution solvent,
In particular, it is preferable to occupy 40% by volume or less.

As an electrolyte salt to be dissolved in the above electrolyte solution solvent
For halo of alkali metals such as lithium and sodium
Genate or perchlorate, trifluoromethanesulfonic acid
A salt of a fluorine-containing compound represented by a salt is preferably used.
Be done. Specific examples of such an electrolyte salt include, for example,
LiF, LiCl, LiClO_Four, LiPF₆, LiB
F_Four, LiAsF₆, LiSbF₆, LiCF₃S
O₃, LiC_FourF₉SO₃, LiCF₃CO₂, Li₂
C₂F_Four(SO₃)₂, LiN (RfSO₂) (Rf ′
SO₂), LiN (RfOSO₂) (Rf'OS
O₂), LiC (RfSO ₂)₃, LiC_nF_{2n + 1}SO
₃(N ≧ 2), LiN (RfOSO₂)₂[Where Rf
And Rf ′ are fluoroalkyl groups] and the like.
Use each alone or in combination of two or more
You can The concentration of the electrolyte salt in the electrolyte is particularly limited.
Although not specified, 0.3 mol / l or more is preferable.
More preferably 0.4 mol / l or more, and
1.7 mol / l or less is preferable, and 1.5 mol / l or less
Lower is more preferable.

The electrolyte is usually used in a liquid state,
If necessary, it may be gelled with a gelling agent and used. Examples of such gelling agents include linear polymers such as polyethylene oxide and polyacrylonitrile, or copolymers thereof, and polyfunctional monomers that polymerize upon irradiation with actinic rays such as ultraviolet rays and electron beams (for example, pentaerythritol). Tetraacrylate, ditrimethylolpropane tetraacrylate, ethoxylated pentaerythritol tetraacrylate, dipentaerythritol hydroxypentaacrylate,
Polymers obtained from tetrafunctional or higher-functional methacrylates such as dipentaerythritol hexaacrylate) and polymers obtained from monomers that polymerize by utilizing the reaction between active hydrogen of an amine compound and an isocyanate group of urethane are used.

[0019]

EXAMPLES Next, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to only those examples. In the following examples and the like,% when indicating the concentration of a solution or dispersion and% when indicating composition, yield, etc. represent mass% unless otherwise noted.

Example 1 First, polysulfide carbon used as a positive electrode active material was synthesized as follows. 100 g of sodium sulfide nonahydrate (Na ₂ S.9H ₂ O) was dissolved in 300 ml of a mixed solvent of ethanol and water mixed at a volume ratio of 1: 1 and 53.4 g of elemental sulfur was added thereto. At room temperature for 1 hour. Next, the solvent was removed in vacuo, the residue was dissolved in 700 ml of N-methyl-2-pyrrolidone, 17.2 g of hexachlorobutadiene was added, and the mixture was reacted at room temperature for 1 hour. Then, it was repeatedly washed with pure water, acetone, and ethanol three times in this order, and dried in vacuum for 15 hours while maintaining at 40 ° C. to obtain a brown solid compound as an intermediate product. The yield of this brown solid compound was 32 g.

Elemental analysis of the obtained compound is carried out,
The average composition was determined. For C, N and H, fully automatic elemental analyzer [Viber EL manufactured by Sebel Hegna Co., Ltd.
(Trade name)], the analysis was performed under the conditions of sample decomposition furnace temperature: 950 ° C., reduction furnace temperature: 500 ° C., helium flow rate: 200 ml / min, oxygen flow rate: 20-25 ml / min.
In addition, S was analyzed by the flask combustion method-measurement of barium acetate using trimethylene blue as an indicator. As a result, C: 7.0%, S: 92.3%,
It was found that N: 0.2% or less and H: 0.3% or less. This can be expressed by a general formula as (CS _4.9 ) _n . Also, trace amounts of N and H were detected as impurities, which are considered to be residual components of the solvent.

Next, 10 g of the intermediate product composed of the brown solid compound was added to a ship-shaped alumina (aluminum oxide).
The container was placed in the container, the alumina container containing the intermediate product was placed in the core of an alumina heating furnace, and the atmosphere was replaced with argon gas having a purity of 99.999% by volume until the oxygen concentration was 100 ppm or less. The temperature was changed as shown below, and heat treatment was performed at 200 ° C. That is, the temperature was raised from room temperature to 60 ° C. in 0.5 hours, kept at 60 ° C. for 1 hour, then raised to 120 ° C. in 1 hour, and
The intermediate product was maintained at 20 ° C. for 1 hour, heated to 200 ° C. for 1.5 hours, and maintained at 200 ° C. for 6 hours to remove a part of sulfur in the intermediate product, thereby converting the intermediate product into polysulfide carbon. Changed to.

After the treatment, the reaction product was taken out from the alumina container after cooling to room temperature to obtain about 3 g of carbon black polysulfide having a black metallic luster. As a result of elemental analysis, the above carbon polysulfide was represented by the general formula and was (CS _1.55 ) _n . Since hexachlorobutadiene used as a reaction component in the production of the above-mentioned carbon polysulfide is a compound having 4 carbon atoms, the n value of the above (CS _1.55 ) _n is 4 or more, and mainly a multiple of 4 molecules is synthesized. it is conceivable that.

Next, this carbon polysulfide was subjected to X-ray diffraction measurement by using a powder X-ray diffractometer [RINT2000 (trade name) manufactured by Rigaku Corporation] using CuKα rays. The measurement conditions are voltage: 40 kV, current: 150 mA,
Scan speed: 2 ° / min, sampling: 0.02 °,
Total number of times: 5 times. The X-ray diffraction pattern of the carbon polysulfide obtained by this X-ray diffraction measurement is 25.5 °.
It only had a broad peak at. Further, in this polysulfide carbon, peaks corresponding to unsaturated bonds of carbon and disulfide bonds of sulfur were observed by Raman analysis, but peaks corresponding to bonds corresponding to the polysulfide segment were not observed.

In producing the positive electrode, first,
As described below, a layer for ensuring electron conductivity and adhesion was formed on the current collector. That is, acetylene black having a specific surface area of 60 m ² / g as a conductive powder and fluororubber as a binder are dispersed in toluene in a mass ratio of 2: 3, and the dispersion is prepared from an aluminum foil by a doctor blade method. 2m in mass after drying on the current collector
It was applied so as to be g / cm ² and dried to form a layer composed of conductive powder and a binder on the current collector. Next, the carbon polysulfide synthesized as described above and carbon black were mixed in a mass ratio of 97: 1, and the resulting mixture was dissolved in polyvinylidene fluoride as a binder in n-methyl-2-pyrrolidone. A binder was added to the solution so that the binder ratio was 2% with respect to the total solid content, and the mixture was stirred to prepare a polysulfide carbon-containing paste. This polysulfide carbon-containing paste was applied onto a current collector made of aluminum foil by a doctor blade method on a current collector and a layer made of a conductive powder and a binder, and the mass after drying was 16 mg / cm ² . Thus, the positive electrode having a two-layer structure was manufactured by cutting the film into a predetermined size after drying and pressing. The structure of this positive electrode will be described with reference to FIG. 1. In the positive electrode 1, a layer 3 made of conductive powder and a binder is formed on a current collector 2 made of aluminum foil. A layer 4 containing carbon sulfide as a positive electrode active material and a binder is laminated on the layer 3 to be formed.

Next, using the above positive electrode, a 2016 type coin type (coin type having a thickness of 2.0 mm and a diameter of 16 mm) non-aqueous secondary battery was produced. That is, the above positive electrode is fixed to a positive electrode can via an aluminum spacer, while, on the negative electrode side, a metal lithium foil is fixed to a negative electrode can via a nickel spacer to form a negative electrode, and the positive electrode and the negative electrode are separated from each other. A separator made of a polyethylene non-woven fabric was placed in, and after injecting the electrolytic solution, a gap between the positive electrode can and the negative electrode can was sealed with an annular gasket made of polypropylene to produce a coin-shaped non-aqueous secondary battery. As the electrolyte solution, tetraglyme and 1,3-dioxolane in a volume ratio of 5
1 m of LiCF ₃ SO ₃ was added to the mixed solvent mixed at 0:50.
What was prepared by dissolving ol / l was used. Then, the battery manufactured as described above was subjected to discharge with a current density of 0.3 m.
A / cm ² , 1.5V termination, current density during charging 0.3m
Charge / discharge was repeated under charge / discharge conditions of constant current / constant voltage charging at A / cm ² , 2.6 V, and the initial discharge capacity and the discharge capacity after 20 cycles were measured. The results are shown in Table 1 below, in terms of discharge capacity per 1 g of polysulfide carbon of the positive electrode active material.

Example 2 As a conductive powder for forming a layer comprising a conductive powder and a binder, the specific surface area used in Example 1 was 60 m ² / g.
Instead of acetylene black, the specific surface area of 156 m ² /
Example 1 except that g of carbon black was used
A positive electrode and a battery were prepared and charged and discharged in the same manner as in.

Example 3 As a conductive powder for forming a layer composed of a conductive powder and a binder, the specific surface area of 110 m ² / g was used instead of the acetylene black having a specific surface area of 60 m ² / g used in Example 1.
A positive electrode and a battery were prepared and charged / discharged in the same manner as in Example 1 except that the nickel powder in Example 1 was used.

Comparative Example 1 The same polysulfide carbon as in Example 1 and acetylene black having a specific surface area of 60 m ² / g as a conductive additive were mixed in advance in a mass ratio of 75:15, and the mixture was used as a binder. Polyvinylidene fluoride with n-methyl-
A positive electrode mixture-containing paste was prepared by adding a binder dissolved in 2-pyrrolidone so that the binder ratio was 10% with respect to the total solid content, and stirring the mixture. The obtained positive electrode mixture-containing paste was applied onto the current collector made of the aluminum foil by a doctor blade method so that the mass after drying would be 16 mg / cm ² , dried, and pressed, and then dried to a predetermined size. It cut and produced the positive electrode. Then, a battery was prepared and charged and discharged in the same manner as in Example 1 except that the positive electrode was used.

Comparative Example 2 The same polysulfide carbon as in Example 1 and carbon black having a specific surface area of 156 m ² / g as a conductive additive were mixed in advance in a mass ratio of 97: 1, and the mixture was used as a binder. Polyvinylidene fluoride n-methyl-2
-A solution containing pyrrolidone was added so that the binder ratio was 2% based on the total solid content, and the mixture was stirred to prepare a paste containing a positive electrode mixture. The obtained paste containing the positive electrode mixture was applied by a doctor blade method onto a current collector made of aluminum foil so that the mass after drying was 16 mg / cm ² , dried and pressed, and then cut into a predetermined size. Then, a positive electrode was produced. Then, a battery was prepared and charged and discharged in the same manner as in Example 1 except that the positive electrode was used.

Regarding the batteries of Examples 1 to 3 and Comparative Examples 1 and 2, the initial discharge capacity per 1 g of polysulfide carbon of the positive electrode active material and the discharge after 20 cycles were obtained from the above-mentioned discharge capacity measurement results. The capacity is shown in Table 1.

[0032]

[Table 1]

As is clear from the results shown in Table 1, Examples 1 to 1 in which the positive electrode has a two-layer structure of a layer made of conductive powder and a binder and a layer containing carbon sulfide as a positive electrode active material and a binder The battery of 3 has a large initial discharge capacity,
Moreover, the discharge capacity after 20 cycles was large, and the capacity was high and the charge / discharge cycle characteristics were excellent.

On the other hand, the positive electrode has a single layer structure (that is,
The battery of Comparative Example 1 having a structure in which the positive electrode mixture layer is composed of only one layer has a large initial discharge capacity, but a large decrease in discharge capacity due to repeated charging and discharging, and a discharge capacity after 20 cycles. Was smaller, and the charge / discharge cycle characteristics were inferior to the batteries of Examples 1 to 3. In addition, the battery of Comparative Example 2 in which the positive electrode has a single-layer structure and the amount of the conductive additive and the binder used is small, the discharge capacity of Example 1 is high even though the amount of carbon polysulfide as the positive electrode active material is large. It was smaller than the batteries of Examples 1 to 3 and the discharge capacity was significantly reduced after 20 cycles, and the charge and discharge cycle characteristics were significantly inferior to the batteries of Examples 1 to 3.

[0035]

As described above, according to the present invention, in the non-aqueous secondary battery having the positive electrode using polysulfide carbon as the positive electrode active material, the negative electrode, and the electrolytic solution, the positive electrode is formed on the current collector. A non-aqueous secondary battery having a high capacity and excellent charge / discharge cycle characteristics by having a multi-layer structure composed of a layer composed of the conductive powder and the binder and a layer containing the polysulfide carbon and the binder laminated on the layer. Could be provided.

[Brief description of drawings]

FIG. 1 is a cross-sectional view schematically showing a main part of a positive electrode of a non-aqueous secondary battery of the present invention.

[Explanation of symbols]

1 positive electrode 2 Current collector 3 Layer consisting of conductive powder and binder 4 Layer containing carbon polysulfide and binder

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Toshihiro Nakai             Hitachima, 1-88, Torora, Ibaraki City, Osaka Prefecture             Within Kucsel Co., Ltd. (72) Inventor Yoshishi Iizuka             Hitachima, 1-88, Torora, Ibaraki City, Osaka Prefecture             Within Kucsel Co., Ltd. F term (reference) 5H029 AJ03 AJ05 AK05 AL06 AL07                       AL08 AL11 AL12 AM04 AM07                       BJ12 DJ08 DJ16 EJ01 EJ04                       HJ02                 5H050 AA07 AA08 BA16 BA17 CA11                       CB02 CB07 CB08 CB09 CB11                       CB12 DA10 DA11 EA02 EA08                       FA17 HA02

Claims (3)

[Claims] 1. A non-aqueous secondary battery comprising a positive electrode using polysulfide carbon as a positive electrode active material, a negative electrode, and an electrolytic solution,
The above-mentioned positive electrode is a non-aqueous structure characterized in that it has a multi-layered structure consisting of a layer made of conductive powder and a binder formed on a current collector, and a layer containing polysulfide carbon and a binder laminated on the layer. Secondary battery. 2. The carbon polysulfide has the general formula (CS
The non-aqueous secondary battery according to claim 1, wherein _x ) _n (wherein x is 0.9 to 2.0 and n is a number of 4 or more). 3. The conductive powder is at least one selected from the group consisting of graphite, acetylene black, carbon black, Ketjen black, activated carbon and metal powder.
The non-aqueous secondary battery according to claim 1 or 2, which is a seed.