JPH09147918A - Secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery, in which reduction in high output capacity/low output capacity is suppressed even if an electrode formed of a powder molding is thick, by limiting a relationship between conductivity and viscosity of a nonaqueous electrolytic solution in a nonaqueous secondary battery. SOLUTION: In a secondary battery provided with a positive electrode, a negative electrode, and a nonaqueous electrolytic solution containing LiPF6 as electrolyte, a ratio (p)=σ/η of the conductivity σ (10<-3> S/cm) to the viscosity η(cps) of the nonaqueous electrolytic solution is 1 or more. A mixture of a high dielectric constant solvent and a low viscosity solvent is desirable for a solvent constituent of the nonaqueous electrolytic solution. A combination of cyclic carbonate and chain carbonate is especially suitable, as it increases a high output capacity.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a secondary battery using a positive electrode, a negative electrode, and a non-aqueous electrolyte.

[0002]

2. Description of the Related Art In recent years, with the spread of portable devices such as video cameras and notebook personal computers, demand for small and high capacity secondary batteries has been increasing. Most of the secondary batteries currently used are nickel-cadmium batteries using an alkaline electrolyte, but the battery voltage is as low as about 1.2 V, and it is difficult to improve the energy density. Therefore, in order to improve the voltage, a secondary battery using lithium metal for the negative electrode has been studied.

However, in a secondary battery in which lithium metal is used for the negative electrode, lithium repeatedly grows in dendrites due to repetition of charge and discharge, and disadvantages such as a short circuit and a shortened life are likely to occur. Therefore, a secondary battery has been proposed in which various carbon materials are used for the negative electrode and the lithium ion is used by inserting and extracting lithium ions. As a secondary battery composed of a negative electrode utilizing the occlusion / release of lithium ions to / from the above carbon material, JP-A-57-208079.
JP-A-58-93176, JP-A-58-93176
192266, JP-A-62-90863,
JP-A-62-122066 and JP-A-3-6685.
No. 6, for example, is known.

The positive or negative electrode of these secondary batteries is
It is composed of a molded body containing at least an active material capable of inserting and extracting lithium ions. Further, as the electrolytic solution, a nonaqueous electrolytic solution such as an organic solvent is used because an aqueous solution cannot be used as the electrolytic solution because of a high voltage.

[0005]

However, in the positive electrode or the negative electrode, when the molded body becomes thicker, the discharge capacity per unit weight of the active material (mAh / g) becomes larger than the discharge capacity when discharged at a low output current. However, there was a problem of a significant decrease. Here, if the discharge capacity when discharged at a high output current is the high output capacity and the discharge capacity when discharged at a low output current is the low output capacity, it is important that the high output capacity / low output capacity is small. It becomes a problem. An object of the present invention is to provide a secondary battery with a small decrease in high output capacity / low output capacity.

[0006]

The present invention has the following constitution in order to solve the above problems.

[In a secondary battery using a positive electrode, a negative electrode and a non-aqueous electrolyte solution containing LiPF ₆ as an electrolyte, the ratio of the conductivity σ [10 ^-3 S / cm] and the viscosity η [cPoise] of the non-aqueous electrolyte solution. p
(P = σ / η [10 ⁻³ S / cm / cPoise]) is 1 ≦
A secondary battery satisfying the relationship of p. "

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The secondary battery of the present invention relates to an electrolytic solution for a secondary battery comprising a positive electrode made of a powder compact, a negative electrode, and a non-aqueous electrolytic solution. The method is not particularly limited.

The negative electrode and the positive electrode used in the present invention are not particularly limited, and electrodes generally used in secondary batteries can be used. Among them, a mixture containing a powdered active material and a binder is used. The molded body of is preferably used. In the case of an electrode made of a powder compact, the reason why the discharge capacity at a high output current decreases when the compact is thick is that the movement of ions in the electrolyte in the minute gaps of the compact is poor. Guessed. Therefore, as a result of intensive studies, the inventors have considered that the ionic conductivity and viscosity of the electrolytic solution are important in order to improve the movement of ions in the minute gaps of the molded electrode. Then, the relationship between the high output capacity / low output capacity and the electric conductivity or viscosity of the electrolytic solution was examined, but they were not related by themselves. However, by taking the ratio of conductivity and viscosity of this electrolyte,
It was found that high output capacity / low output capacity was improved. That is, the ratio p (σ / η [10 ⁻³ S / cm / cPo) of the conductivity σ [10 ⁻³ S / cm] of the non-aqueous electrolyte and the viscosity η [cPoise].
It has been found that the decrease of high output capacity / low output capacity can be suppressed when ise]) is within the above specific range.

Materials used for the negative electrode of the present invention include:
Although not particularly limited, for example, a carbon material or the like is preferable because it has a good lithium ion storage / release property. The raw material and manufacturing method of the carbon material can be used without particular limitation. Examples of raw materials include coke and pitch such as petroleum and coal, plants such as wood, low molecular weight organic compounds such as natural gas and lower hydrocarbons, polyacrylonitrile, synthetic polymers such as phenol resin and furfuryl alcohol resin. Then, a carbon material is obtained through a treatment called carbonization or graphitization in which these are fired at 700 to 3000 ° C. depending on the raw material and application. Natural graphite can also be used. Properties of the carbon material include density, crystal thickness (Lc), crystal plane spacing (d002), electric resistance, strength, elastic modulus, etc., which are appropriately determined according to the intended electrode characteristics of the secondary battery. It should be, and is not particularly limited.

A preferred embodiment is also a case where the above-mentioned carbon material has a powdery form obtained by finely cutting carbon fibers and is a molded product. In the case of using the powdery one obtained by finely cutting the carbon fiber as described above, the voids increase in the electrode as compared with the case of using the carbon fiber as the long fiber, and thus the high output capacity / low capacity which is the object of the present invention is obtained. It is possible to preferably obtain a secondary battery having a large output capacity. Further, when a powdery carbon material is used, an electrode can be easily prepared by adding a binder and a conductive material together with a material to make a slurry and applying the slurry. The powdery carbon material preferably has an average length of 5 mm or less,
Furthermore, 0.1 mm or less and 0.05 mm or less are more preferable. Taking the form of fibers or fiber structures is a preferred embodiment. Here, the “average length” can be determined, for example, by measuring the length in the fiber axis direction of 20 or more powdered carbon materials by observation with an SEM or the like. Further, the method of “cutting” is not particularly limited, and a method such as cutting or crushing is used.

As the carbon fiber, PAN-based carbon fiber obtained from polyacrylonitrile (PAN), pitch-based carbon fiber obtained from pitch of coal or petroleum, cellulose-based carbon fiber obtained from cellulose, gas of low molecular weight organic substance Examples of the vapor-grown carbon fibers obtained include carbon fibers obtained by firing polyvinyl alcohol, lignin, polyvinyl chloride, polyamide, polyimide, phenol resin, furfuryl alcohol, and the like. Among these carbon fibers, depending on the characteristics of the electrode and battery in which the carbon fibers are used, it is necessary to appropriately select the carbon fibers that satisfy the characteristics. Of these, PAN-based carbon fibers, pitch-based carbon fibers, and vapor-grown carbon fibers are preferable. In particular, PAN-based carbon fibers and pitch-based carbon fibers are preferable, and among these, "Torayca" T series or "Torayca" manufactured by Toray Industries, Inc.
PAN-based carbon fibers such as M series and pitch-based carbon fibers obtained by firing mesophase pitch coke are more preferably used.

The particle diameter or fiber diameter of the carbon material used for the negative electrode of the present invention is not limited, but is 0.1.
-100 μm is preferable, and 1-20 μm is particularly preferably used.

In particular, the nonaqueous electrolytic solution of the present invention is preferably used when a powdery carbon material obtained by cutting PAN-based carbon fibers is used for the negative electrode.

In the negative electrode used in the present invention, it is also preferable to add a binder to the active material in order to improve moldability. Examples of such a binder include, but are not particularly limited to, high molecular compounds such as polytetrafluoroethylene, polyvinylidene fluoride, polyacrylonitrile, polyimide, polyethylene, polypropylene, epoxy resin, and phenol resin. These binders are used by being mixed with the powder and the active material, and the specification form is not particularly limited, such as being used in the form of a slurry with the active material dissolved in a solvent or dispersed as an emulsion. .

In the negative electrode used in the present invention, a current collector is used to conduct electricity from the negative electrode to the terminal. As such a current collector, metals such as copper, stainless steel, nickel, titanium, and platinum can be used in the form of white, net, lath, and the like, but these are not particularly limited. . Also, as a method of bringing the negative electrode and the current collector into contact with each other, the production method is not particularly limited, such as directly pressing the fibrous or powdery mixture containing the negative electrode active particles onto the current collector. . Furthermore, the distance from the current collector to the surface of the negative electrode, which corresponds to the thickness of the negative electrode, is not particularly limited, but the effect of the nonaqueous electrolytic solution of the present invention is great when the distance is 20 μm or more.

In addition to the above carbon material active material, it is also preferable to add a conductive agent to the negative electrode used in the present invention in order to improve electron conductivity. The addition of the conductive agent reduces the resistance in the electrode, which is effective in improving the battery capacity, and is effective not only in improving the low output capacity but also in particularly improving the high output capacity. As such a conductive agent, a metal, a semiconductor or a semimetal, which is a material having a low electric resistance, is used, but a carbonaceous material or a carbon material such as graphite or carbon black is particularly preferably used. Among them, acetylene black, Ketjen black, aniline black, artificial graphite and the like are preferably used. The shape of the conductive agent is not particularly limited, such as powder and fiber, but in the case of powder, the particle size is 0.1 to 10
It is preferably 0 μm, more preferably 1 to 50 μm.
The amount of the conductive agent added is preferably 0.1 to 20% by weight.

The carbon material used for the negative electrode of the present invention can use a metal as a current collector in order to enhance the current collecting effect. The form of the metal current collector is not particularly limited, such as a foil shape and a fiber shape.

The solvent component of the non-aqueous electrolytic solution used in the present invention is not particularly limited as long as it satisfies the electric conductivity-viscosity ratio p of the above claims, but 1≤p≤.
The case of 3 is preferable in that the output capacity is large. In the present invention, a mixture of a high dielectric constant solvent and a low viscosity solvent is preferably used as the electrolytic solution. Examples of the high dielectric constant solvent include cyclic carbonates such as propylene carbonate, ethylene carbonate and butylene carbonate, cyclic esters such as γ-butyrolactone, tetramethylsulfolane, dimethylsulfoxide, N-methylpyrrolidone, dimethylformamide and the like. The derivative is not particularly limited. Examples of the low-viscosity solvent include chain carbonates such as dimethyl carbonate, ethylmethyl carbonate and diethyl carbonate, chain ethers such as dimethoxyethane, ethoxymethoxyethane and diethoxyethane, cyclic ethers such as tetrahydrofuran, dioxolane and dioxane, and derivatives thereof. Is used, but is not particularly limited. The composition ratio of these high-dielectric constant solvent and low-viscosity solvent is not particularly limited either, and the ratio p of the electric conductivity and viscosity of the electrolytic solution is 1
It is appropriately determined so as to be in the range of ≦ p. In particular, it is preferable to use a cyclic carbonate and a chain carbonate as the solvent of the non-aqueous electrolytic solution because the output capacity is large. In addition to the above-mentioned solvents, addition of up to 10% by volume as a trace component is a preferred embodiment of the solvent of the non-aqueous electrolyte used in the present invention. In this case, examples of the additive used include various organic compounds or inorganic compounds.

The electric conductivity and viscosity of the non-aqueous electrolytic solution used in the present invention may be values measured using a commercially available electric conductivity meter or rotational viscometer. The measurement is performed at 20 ° C.

The positive electrode active material used in the present invention is not particularly limited. For example, lithium cobaltate, lithium nickelate, lithium manganate,
Lithium niobate, transition metal oxides such as lithium vanadate, molybdenum sulfide, transition metal chalcogens such as titanium sulfide, or a mixture thereof, or a disulfide compound such as mercaptothiadiazole, or
Polyalkylene oxide and polyalkylene sulfide,
Examples thereof include heteropolymers such as polyaniline, polythiophene and polypyrrole, and conjugated polymer compounds such as polyacetylene, polydiacetylene, polyparaphenylene and polyphenylenevinylene. The above-described substances capable of inserting and extracting lithium ions or anions can be used as the positive electrode active material without any limitation, and the oxidation potential of these substances is preferably 2.5 V or more with respect to lithium. The particle size of the positive electrode active material powder is 0.1 to 1
The thickness is 00 μm, preferably 1 to 50 μm.

It is also preferable to add a binder to the active material and the conductive agent in the positive electrode used in the present invention in order to improve the moldability. The same compound as the negative electrode is used,
The form of use is not particularly limited.

In the positive electrode used in the present invention, a current collector is used to make it conductive to the terminal. As such a current collector,
Metals such as aluminum, titanium, platinum, nickel,
It can be used in the form of foil, net, lath, etc., but these are not particularly limited. Further, as a method of bringing the positive electrode into contact with the current collector, the powder mixture containing the positive electrode active material is directly pressed onto the current collector, the slurry containing the positive electrode active material is applied to the current collector, and the solvent is dried and then pressed. The production method is not particularly limited. Further, the distance from the current collector corresponding to the thickness of the positive electrode to the surface of the positive electrode is not particularly limited,
When the thickness is 20 μm or more, the effect of the nonaqueous electrolytic solution of the present invention is great.

The secondary battery of the present invention is widely used for small portable electronic devices such as video cameras, personal computers, word processors, radio-cassettes, mobile phones, etc., because of its light weight, high capacity and high energy density. It is possible.

[0025]

EXAMPLES Specific embodiments of the present invention will be described below with reference to examples, but the present invention is not limited thereto.

Example 1 (1) Preparation of Carbon Negative Electrode Pitch coke "LPC-A" (manufactured by Nippon Steel Chemical Co., Ltd.) powder and Teflon powder in a weight ratio of 90/10.
And mixed with the nickel mesh of the current collector to obtain 10 mg of the carbon material negative electrode. The negative electrode had a diameter of 1.6 cm and a thickness of 20 μm.

(2) Preparation of positive electrode Commercially available lithium carbonate (Li ₂ CO ₃ ) and basic cobalt carbonate
(2CoCO ₃ .3Co (OH) ₂ ) was weighed so that the molar ratio Li / Co = 1/1, mixed by a ball mill, and heat-treated at 900 ° C. for 20 hours to obtain LiCoO ₂ . Grind this with a ball mill,
Using artificial graphite as the conductive material and Teflon (PTFE) as the binder, mix so that the weight ratio is LiCoO ₂ / artificial graphite / PTFE = 80/15/5, and press-mold with nickel mesh of the collecting electrode. Thus, 20 mg of a positive electrode was obtained.

(3) Preparation of secondary battery In the above (2), the negative electrode obtained in the above (1) was used as a separator through a porous polypropylene film (Celguard # 2500, manufactured by Daicel Chemical Industries, Ltd.). A coin type secondary battery was prepared by stacking the prepared positive electrode. As the electrolytic solution, propylene carbonate / dimethyl carbonate containing 1M LiPF ₆ (volume ratio 50/50) was used.

(4) Evaluation The conductivity of the above-mentioned electrolytic solution was 4 × 10 ⁻³ S / cm when measured at 20 ° C. using a personal SC meter (Model SC82 manufactured by Yokogawa Electric Co., Ltd.). there were. The viscosity of this electrolytic solution was measured at 20 ° C. using a rotational viscometer (B-type viscometer BL manufactured by Tokyo Keiki Co., Ltd.) to obtain 3.4 cPo.
It was ise. The conductivity-viscosity ratio of this electrolyte is p =
It was 2.5 [10 ^-3 S / cm / cPoise].

Further, the secondary battery obtained in the above (3) was charged at a constant potential of 4.1 V at a current of 4 mA for 5 hours and discharged at a constant current of 0.4 mA per positive electrode. The discharge capacity of was set as the low output capacity. On the other hand, the discharge capacity per positive electrode when the constant current discharge of 8 mA was performed under the same charge condition was defined as the high output capacity. Then, high output capacity / low output capacity was obtained as a high output capacity ratio. This high output capacity ratio was 0.80, and the decrease in capacity at high output current was suppressed.

Comparative Example 1 A secondary battery was prepared in the same manner as in Example 1 except that propylene carbonate containing 1M LiPF ₆ was used as the electrolytic solution.
evaluated. The electric conductivity of the electrolyte is 3 × 10 ⁻³ S / cm, the viscosity is 9 cPoise, and the ratio of the electric conductivity to the viscosity p = 0.4 [10 ⁻³ S
/ Cm / cPoise], the high output capacity ratio was 0.05, and the capacity decrease at high output was large.

Example 2 (1) Preparation of carbon negative electrode Commercially available pitch coke "M-coke" (manufactured by Nippon Mining Co., Ltd.) powder was replaced with polyvinylidene fluoride (PVdF) powder in a weight ratio of 90/10. And mixed with N-methylpyrrolidone to form a slurry. This slurry was applied to a copper foil, dried, and pressed. The negative electrode had a diameter of 1.6 cm and a thickness of 20 μm.

(2) Preparation of Positive Electrode LiCoO ₂ was synthesized in the same manner as in Example 1, and after pulverization, artificial graphite was used as the conductive material and polyvinylidene fluoride (P) was used as the binding material.
VDF) and weight ratio of LiCoO ₂ / artificial graphite / PVDF =
80/15/5 were mixed, and N-methylpyrrolidone was used as a solvent to form a slurry. This slurry was applied on an aluminum foil of a collecting electrode, dried and then pressed to obtain a molded positive electrode. The positive electrode material had a weight of 40 mg, a diameter of 1.6 cm and a thickness of 100 μm.

(2) Preparation and Evaluation of Secondary Battery A coin type secondary battery was prepared in the same manner as in Example 1 except that ethylene carbonate / dimethyl carbonate containing 1M LiPF ₆ (volume ratio 30/70) was used as the electrolytic solution. Created and evaluated. The conductivity of this electrolyte is 9.1 × 10 ⁻³ S / cm,
The viscosity is 2.1 cPoise, the ratio of conductivity to viscosity p = 2.7
It was [10 ^-3 S / cm / cPoise]. The high output capacity ratio of the secondary battery was 0.80, and the capacity decrease at high output was suppressed.

Example 3 A secondary battery was prepared in the same manner as in Example 2 except that the negative electrode carbon material used was a powdered carbon fiber "Torayca T-300" (manufactured by Toray Industries, Inc.) cut to 30 μm. And evaluated. The high output capacity ratio of this secondary battery was 0.80, and the reduction in capacity at high output was suppressed.

Example 4 Carbon fiber "Torayca T-300" (manufactured by Toray Industries, Inc.) was cut into 30 μm powder carbon as a negative electrode carbon material, acetylene black as a conductive agent, and polyvinylidene fluoride as a binder in a weight ratio. The mixture was mixed at 80/15/5, and N-methylpyrrolidone was added to obtain a slurry. Example 2 except that this slurry was used and the electrolytic solution was ethylene carbonate / dimethyl carbonate containing 1 M LiPF ₆ (volume ratio 50/50).
A secondary battery was prepared and evaluated in the same manner as in. The high output capacity ratio of this secondary battery was 0.88, and the reduction in capacity at high output was suppressed.

[0037]

According to the present invention, it is possible to suppress the decrease in high output capacity / low output capacity even when the electrode made of a powder molded body is thick.

Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location // D01F 9/22 D01F 9/22

Claims (16)

[Claims] 1. A secondary battery using a positive electrode, a negative electrode, and a nonaqueous electrolytic solution containing LiPF ₆ as an electrolyte, wherein the nonaqueous electrolytic solution has a conductivity σ [10 ⁻³ S / cm] and a viscosity η [cPoise] of Ratio p (p
= Σ / η [10 ⁻³ S / cm / cPoise]) satisfies the relationship of 1 ≦ p. 2. The secondary battery according to claim 1, wherein the negative electrode is a powder molded body. 3. The secondary battery according to claim 1, wherein the positive electrode is a powder molded body. 4. The secondary battery according to claim 1, wherein a cyclic carbonate and a chain carbonate are contained as a solvent component of the non-aqueous electrolytic solution. 5. The secondary battery according to claim 1, wherein the solvent component of the non-aqueous electrolyte contains a cyclic carbonate and a cyclic ether. 6. The secondary battery according to claim 1, wherein the solvent component of the non-aqueous electrolyte contains a cyclic carbonate and a chain ether. 7. The negative electrode contains at least a carbon material capable of inserting and extracting lithium ions.
The secondary battery according to any one of the preceding claims. 8. The carbon material of the negative electrode is carbon fiber or a material obtained by processing carbon fiber.
The secondary battery according to any one of the preceding claims. 9. The secondary battery according to claim 8, wherein the carbon material is a powdery carbon material obtained by finely cutting carbon fibers. 10. The secondary battery according to claim 8, wherein the carbon fiber of the negative electrode is a fired body of a polymer compound containing polyacrylonitrile as a main component. 11. The secondary battery according to claim 8, wherein the carbon material of the negative electrode is a structure selected from a unidirectional molded body made of carbon fiber, a cloth and a felt. 12. The secondary battery according to claim 9, wherein the powdery carbon material obtained by finely cutting the carbon fiber of the negative electrode has an average length of 0.1 mm or less. 13. The secondary battery according to claim 9, wherein the powdery carbon material obtained by finely cutting the carbon fiber of the negative electrode has an average length of 0.05 mm or less. 14. The secondary battery according to claim 1, wherein the negative electrode contains at least a carbon material capable of inserting and extracting lithium ions and a conductive agent. 15. The secondary battery according to claim 1, wherein the positive electrode contains at least a transition metal compound capable of inserting and extracting lithium ions. 16. The molded body, wherein the positive electrode is a molded body in which a mixture of a transition metal compound capable of inserting and extracting lithium ions, a conductive agent, and a binder is provided on a current collector. The secondary battery described.