JPH02227968A - Secondary battery - Google Patents

Abstract

PURPOSE:To heighten the mechanical strength of a positive electrode and a negative electrode by using cobalt oxide added with carbon black and vapor- deposited graphite in the positive electrode and a sodium alloy added with carbon black and vapor-deposited graphite in the negative electrode. CONSTITUTION:Cobalt oxide added with carbon black and graphite obtained by a vapor deposition method is used in a positive electrode, and a sodium alloy added with carbon black and graphite obtained by a vapor deposition method in a negative electrode. Vapor-deposited graphite and carbon black act as a conductor and an electrolyte retainer and a cushion to deformation of the electrode attendant on charge-discharge cycles. They also increase the specific surface area of the electrode and make the electrode porous. The mechanical strength of both electrodes is heightened, and capacity and energy density are also increased. The reversibility of the battery is increased, self discharge rate is lowered, and cost is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a secondary battery with high energy density, low self-discharge rate, long cycle life, and good performance.

[Conventional technology]

Non-aqueous secondary batteries using transition metal oxides or sulfides as positive electrodes and alkali metals or their alloys as negative electrodes are well known. Non-aqueous secondary batteries such as
758, secondary batteries using a fusible alloy as the negative electrode material, batteries using an alloy negative electrode manufactured by melting lithium and metallic aluminum as in U.S. Patent No. 3,607,413, and JP-A-58 -73968, which uses a simple alkali metal as a negative electrode, has been reported. On the other hand, the invention of a secondary battery using a conductive polymer doped with an alkali metal etc. as a negative electrode was disclosed in JP-A-56-136469, and in JP-A-61-245474, an alkali metal and conductive polymer were used as a negative electrode. Secondary batteries and the like using the mixture as a negative electrode have been proposed.

The present inventors also proposed a secondary battery using a mixture of a conductive material and a conductive polymer as an electrode in JP-A-59-112585, and in JP-A-60-72692, they proposed a secondary battery using a mixture of a conductive material and a conductive polymer as an electrode. He has invented and published a secondary battery that uses a ternary mixed material consisting of aluminum and lithium for the negative electrode.

However, when a secondary battery using an alkali metal alone as a negative electrode is charged and discharged at a high current density using the above method, dendrites are formed and the positive and negative electrodes are short-circuited, making subsequent charging and discharging impossible. Furthermore, even when an alkali metal alloy is used for the negative electrode,
Even if the formation of dendrites is suppressed to some extent, repeated charging and discharging will cause the alloy electrode to become finer and collapse. Therefore, unless the amount of electricity charged and discharged is suppressed, a long cycle life cannot be achieved. In addition, when a conductive polymer doped with alkali metal etc. is used for the negative electrode, although the reversibility of charging and discharging is good, the capacitance density per electrode weight and per electrode volume is low, and a secondary battery with sufficient energy density It cannot be. Therefore, recently, secondary batteries with high capacity and long cycle life have been developed using mixtures of alkali metal alloys and conductive polymers as negative electrode active materials (Japanese Unexamined Patent Publication No. 61-245474). ). However, the conductive polymers used as one of the constituent elements of the negative electrode, such as polyacetylene and polybaraphenylene, are expensive to manufacture, making it difficult to obtain an inexpensive secondary battery.

Furthermore, in high capacity density batteries, the reversibility of the alkali metal itself as a negative electrode is not sufficient in lithium metal-based batteries.

As a solution to the above problem, the present inventors previously proposed the use of a negative electrode containing a sodium alloy, carbon black, and/or graphite (Japanese Patent Application No. 62-291804
issue).

[Problem to be solved by the invention]

However, the electrodes of the above-mentioned secondary batteries have a drawback that they have low mechanical strength and are easily disintegrated.

The inventors of the present invention have found that it is possible to further eliminate the above-mentioned drawbacks (as a result of intensive research, it has been found that if graphite produced by a vapor phase method is used as the graphite added to the positive and negative electrodes, the positive and negative electrodes can be combined without reducing the electrical conductivity). It was discovered that mechanical strength was improved.

The present invention was made based on the above discovery, and an object of the present invention is to provide a secondary battery that is inexpensive, has high mechanical strength of both positive and negative electrodes, and has excellent battery performance.

[Means to solve the problem]

In order to achieve the above object, in the secondary battery of the present invention, a cobalt oxide to which carbon black and vapor-phase graphite are added is used for the positive electrode, and a sodium alloy to which carbon black and vapor-phase graphite are added is used for the negative electrode. use

The amount of carbon black added is 05 to 3wt for the positive electrode.
%, 3 to 10 vL% for the negative electrode, and 0.1 to 2 vt% for the positive electrode, and 1 to 5 wt% for the negative electrode.

Among the components of the positive electrode in the secondary battery of the present invention, cobalt oxide primarily acts as an active material that transfers and extracts charges.

There are no particular restrictions on the method for producing the cobalt oxide, but in order to effectively demonstrate the performance of the secondary battery, the electrode potential of the positive electrode active material must be within a range where an effective potential difference (voltage) can be maintained with respect to the negative electrode. It is necessary to use a cobalt oxide that operates at high temperatures and has a composition that does not cause large side reactions with the electrolyte used.

Therefore, it is best to use a positive electrode in which Na is combined with cobalt oxide in advance.In particular, the composition ratio of Na and cobalt oxide is such that the molar ratio of Na and Co is 1:1 when the battery is charged.
A range of 5 to 4;5 is preferred.

Furthermore, it is important that the main components of the sodium alloy as a component of the negative electrode are sodium and lead, or sodium and tin. The reason for this is that the above-mentioned metal is a metal that is easily alloyed with sodium electrochemically, and furthermore, sodium can be reversibly taken in and taken out.

Among the constituent elements of the negative electrode used in the secondary battery of the present invention, the sodium alloy mainly acts as an active material that transfers and extracts electric charges.During charging, alkali metal ions are reduced from the electrolyte side and migrate into the sodium alloy. It is also believed that some of the sodium in the sodium alloy is oxidized and transferred into the electrolyte during the discharge.

The main component of sodium alloy is an alloy of sodium/-4 and lead or tin, but in reality, alloys are easy to make (and the ones that can electrochemically and reversibly take in and out large amounts of alkali metal ions are sodium) It is an alloy of lead and lead.

As for the composition of the above-mentioned alloy, the molar ratio of sodium to lead is preferably within the range of 1021 to 1:2 when the battery is being charged. The reason for this is that sodium and the other metal can maintain an alloyed state from the charging state to the discharging state of the battery, or the open circuit potential is higher than the potential of sodium alone, and sodium dendrites occur. This is because the amount of electricity charged and discharged in each cycle can be kept as large as possible, and at the same time, it is within a range where sodium is not depleted in the negative electrode before the specified discharge state.

The vapor phase graphite and carbon black used in the positive and negative electrodes of the secondary battery of the present invention not only act as a conductive aid and an impregnating agent for the electrolytic solution, but also have large specific surface areas and pores, which are the characteristics of carbon black itself. Depending on its nature, it plays a major auxiliary role because it quickly promotes the electrode reaction of the entire electrode. Furthermore, vapor-phase graphite absorbs the force that tends to cause the electrode to expand and contract during charging and discharging, and acts as an elastic agent to prevent deformation and collapse. It has the advantage that a small amount of binder can compensate for the need for a large amount of binder to maintain the properties and prevent collapse.

An appropriate amount of binder is added to the positive and negative electrodes of the present invention in order to maintain electrode strength, but it is important that the binder hardly reacts with the electrolyte used in the secondary battery, and that a small amount of binder When used, the binding properties of the electrode itself must be maintained to a sufficient extent to withstand use as a secondary battery. Examples of binding agents that meet the purpose of the present invention include polyethylene, polypropylene, and EPM (ethylene propylene copolymer), E
PDM (ethylene propylene rubber), PTFE! I
Among them, EPDM and PTFE, which are used in a relatively small amount and have a large binding effect, are preferable as binders for the positive electrode. However, since PTFE reacts with the sodium alloy, which is the main active material of the negative electrode,
EPDM is preferable as the binder for the negative electrode. EPDM here is a type of synthetic rubber, and is a copolymer of ethylene and propylene, into which an unsaturated compound having a double bond is introduced as a third component.

Next, the compounding ratio of each component of the positive electrode and negative electrode of the present invention will be explained.

As explained above, the vapor-phase graphite and carbon black used in the present invention function as a conductive agent and an impregnating agent for electrolyte solution, as a cushioning agent for electrode deformation during charging and discharging, and as a cushioning agent for electrode deformation during charging and discharging. In order to increase the specific surface area and make the electrode porous, it is necessary to disperse vapor-phase graphite and carbon black into the positive and negative electrodes, but if there are too many vapor-phase graphite and carbon black, vapor-phase graphite And since carbon black itself has a small function as an electrode active material, it actually reduces the electric capacity density.

Therefore, the effective amount of vapor-phase graphite added to the positive electrode is
0°1-2vt% of the weight of the positive electrode excluding the binder and current collector
Within this range, the effective amount of carbon black added to the positive electrode is in the range of 0.5 to 3 wt%.

In addition, the effective amount of vaporized graphite added to the negative electrode is within the range of 1 to 5 wt% of the weight of the negative electrode excluding the binder and current collector, and the effective amount of carbon black added to the negative electrode is:
It is within the range of 3 to 10 wt%.

Naturally, a binder is also required to keep the mixture in a bound state, but the amount added is smaller than when carbon black alone is mixed into the electrode when compared with the same electrode performance.

The binder itself is an obstacle to the electrode reaction, except for maintaining the shape of the electrode, so it is better to have as little amount as possible.

In the present invention, the ideal binder amount is the sum of the transition metal oxide and/or sulfide, vapor-phase graphite, and carbon black in the positive electrode, or the alkali metal alloy, vapor-phase graphite, and carbon black in the negative electrode. If it is 100wt%,
The content is preferably 5 wt% or less. If there is a large amount of binder, deformation and collapse of the electrode can be prevented, but the capacitance density will decrease due to the above-mentioned reasons.

Furthermore, if the amount of binder is small, the capacitance density increases, but the cycleability decreases.

Therefore, the optimum amount of binder cannot be determined unconditionally because it varies depending on the intended use and shape of the battery.

Next, the types, grades, etc. of vapor phase graphite and carbon black used in the positive electrode and negative electrode of the present invention will be explained.

The vapor-phase graphite used in the present invention is so-called pyrolytic graphite produced by a vapor-phase method, and the method disclosed in Japanese Patent Application No. 61-233758 is recommended, for example. However, it is not particularly limited to this. Vapor-phase graphite is usually produced by spraying an organic compound solution containing a predetermined amount of a transition metal compound into a thermal furnace heated to 1000 to 1500'C.The organic compound in the solution is thermally decomposed and graphite is produced. can get. Benzene is often used as the organic compound.

Furthermore, there are no particular limitations on the type and grade of the carbon bra used in the present invention.

For example, it may be furnace black, channel black, thermal black (including acetylene black), or lamp black.

However, the higher the electrical conductivity (the larger the specific surface area and the larger the amount of liquid absorbed), the higher the carbon bra.

Excellent. Therefore, acetylene black is preferred.

Also, no matter what kind of carbon black you use, carbon black itself tends to adsorb various substances, including those that cause harm to batteries. etc. must be removed as much as possible.

Next, the electrolyte will be explained.

In the positive electrode and negative electrode used in the secondary battery of the present invention, the introduction and withdrawal of sodium ions dominates the electrode reaction. Furthermore, since the sodium metal used in the negative electrode itself has high reactivity with moisture and oxygen, it goes without saying that the electrolyte must not contain large amounts of moisture, oxygen, etc.

Other materials that significantly inhibit battery reactions cannot be used as the electrolyte.

Therefore, it is necessary to use a non-aqueous electrolyte in which sodium salt is dissolved in the secondary battery of the present invention. The type of solvent is not particularly limited, but effective non-aqueous solvents include ether compounds, such as tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 4-methyl-1,3- Dioxolane, 1,3-dioxolane, dioxane, 2-methoxy-1,3-dioxolane, anisole, m-trifluoromethylanisole, 1,2-dimethoxyethane, l,1-dimethoxyethane, tiglyme, 12-crown-4 etc. can be mentioned.

Ether compounds are preferable as non-aqueous solvents because they have low reactivity with sodium and sodium alloys, have excellent ability to dissolve sodium salts, and have a relatively wide potential stability window.

On the other hand, for the purpose of improving the electrical conductivity of the electrolytic solution, a mixed system of ether compounds or other non-aqueous solvents may be used.

The electrolyte in the electrolytic solution is a sodium salt, dissolves well in a solvent, and has an electrical conductivity higher than that suitable for use as a battery, so for example, N a B F a, N a CQO4, Na
P F a, NaAsF, , Na5OsCF, , NaB
Et,,NaBBu,.

NaBPh, , NaBEt, Bu.

NaBEt, Bu, etc. can be mentioned.

Among the above, the one that is particularly excellent as an electrolyte is NaPF.
,,NaAsF6,NaCQO,,N a B F 4
Considering toxicity, safety, etc., NaPF and NaBF are preferable.

〔Example〕

An example in which this invention is applied to a coin-type secondary battery will be described below.

Example 1 (Manufacture of positive electrode) Journal of 5olid Chesistr
According to y 6 + 532-537 (1973),
Nao produced by heating from Na, O, and C0104
, aqCo Ot and vapor phase graphite (manufactured by Showa Denko K.K.)
, Acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd., trade name Denka Black).

Polylatrafluoroethylene (manufactured by Daikin Industries, Ltd., trade name: Polyflon) at a weight ratio of 95:0.67:1.
The mixture was blended at a ratio of 33:3 and thoroughly mixed using a high-speed mixer.

A small amount of xylene was added dropwise to this mixture, and the mixture was thoroughly kneaded in a mortar until it became glutinous. This glutinous mixture was molded using a tablet molding machine into a pellet having a diameter of 15 mm and a thickness of 400 μm to obtain a pellet-like positive electrode.

(Manufacture of negative electrode) A high-purity sodium rod immersed in paraffin oil is taken out, the dirty surface is scraped off, and the sodium salt and granular lead are mixed at an atomic ratio of 2.7:l, and an electric Using a furnace, it was melted at 500°C for 3 hours, and then the temperature was lowered to 350°C and annealed for 20 hours.

After the alloy temperature was returned to room temperature, it was ground in a mortar. In addition to this, vapor phase graphite (manufactured by Showa Denko Co., Ltd.) and acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd., trade name Denka Black)
A predetermined amount of was added and mixed well.

Next, a predetermined amount of EPDM (manufactured by Japan Synthetic Rubber Co., Ltd., trade name JSR-EP57P) dissolved in cyclohexane was mixed with the above mixture and kneaded, so that the weight ratio of sodium alloy, graphite powder, carbon black, and EPDM was 90:2:
A 6:2 mixture was obtained.

This mixture was molded into tablets with a diameter of 15 mm and a thickness of 300 μm.
A pellet-shaped negative electrode was obtained.

Note that all of the above operations were performed under an argon gas atmosphere.

(Battery experiment) Using the above positive electrode and negative electrode, a 1.2-dimethoxyethane solution of NaPF with a concentration of 1 mol/Q was used as the electrolyte,
The coin-type battery shown in Figure 1 was assembled.

In the figure, 1 is a positive electrode, 2 is a nickel wire mesh, 3 is a polypropylene nonwoven fabric, 4 is a polypropylene microporous film, 5 is an insulating backing, and 6 is a negative electrode.

A battery test was conducted using this coin type battery.

First, from the discharge direction, the battery voltage is 1°7 at a constant current value of 3mA.
Discharge until the battery voltage reaches V, then rest for 30 minutes, then charge with a current of 5 mA until the battery voltage reaches 3゜l■, wait for a rest time, then discharge again. The test was repeated.

As a result, the relationship between the number of repeated charging/discharging cycles and the discharge capacity was as shown in (■) shown in FIG. 2, and it was found that the battery had a very high capacity and good reversibility.

3 of the 50th cycle and 100th cycle of this battery
The self-discharge rates at day 0 were 3.3% and 4.8%, respectively.

Example 2 (Production of negative electrode) High purity sodium salt and granular lead were mixed in an atomic ratio of 2°5:1.
After melting at 500°C for 4 hours, 350°C
It was annealed at °C for 15 hours and cooled to room temperature. After thoroughly crushing this alloy in a mortar, a predetermined amount of vapor phase graphite powder (manufactured by Showa Denko K.K.) and carbon black (manufactured by Cabot Corporation, trade name: Black Bar 2000) were mixed, and then EPDM dissolved in xylene ( Made by Japan Synthetic Rubber Co., Ltd.:
Product name JSR-EP57P) is mixed with the above mixture, kneaded, sodium alloy, graphite powder, carbon black,
A mixture was prepared with an EPDM weight ratio of 90:4:4:2.

After removing excess xylene from this mixture under reduced pressure,
A 75-mesh nickel wire mesh was layered on top of the mixture as a reinforcing material, and the mixture was formed into a sheet by a roller press method so that the total thickness was 320 μl.

The negative electrode produced by the above method was cut out to have a diameter of about 15IllII+ to obtain a negative electrode for a secondary battery.

(Battery experiment) Using the above negative electrode, the positive electrode obtained in Example 1, and a 1,2-dimethoxyethane electrolyte of NaPFa with a concentration of 1 mol/C, the coin type battery shown in Figure 1 was assembled and its performance was evaluated. I looked into it.

The test method was the same as in Example 1.

As a result, the relationship between the number of repetitions of charging and discharging and the discharge capacity was as shown in (■) shown in FIG. 2, and like the battery of Example 1, the battery had a very high capacity and good reversibility.

Furthermore, the self-discharge rates during 30 days at the 50th cycle and the 100th cycle were 4.0% and 53%, respectively.

Example 3 Instead of the electrolyte used in the battery experiment of Example 1, 1so
1/12 concentration of NaPF at a volume ratio of 1:l of 1°2-
Example 1 except that a solution of dimethoxyethane and diethylene glycol dimethyl ether was used.
The experiment was conducted in exactly the same manner.

As a result, the relationship between the number of repetitions of charging and discharging and the discharge capacity was as shown in Figure 2 (■).

The self-discharge rate of this battery over 30 days at the 50th cycle and the 100th cycle was 3.5% and 4.0%, respectively.
%Met.

Examples 4.5, 6, Comparative Examples 1, 2.3 Positive electrodes and negative electrodes with the same thickness as in Example 1 were manufactured by changing the mixing ratio of the positive electrode and negative electrode in Example 1 as shown in Table 1. A battery experiment was conducted in the same manner as in Example 1.

In that case, the maximum amount of discharged electricity and the amount of discharged electricity are lOmAh
Table 1 shows the number of cycles required to divide .

Further, the change in the number of repeated charging and discharging and the discharge capacity in the case of Comparative Example 1 is shown in (■) in FIG.

Note that the cycle life in Table 1 is the number of cycles when the amount of discharged electricity reaches 60% of the maximum amount of discharged electricity.

〔effect〕

As described above, the secondary battery of the present invention has high capacity and high energy density, has good reversibility (low self-discharge rate, and can be used at low cost), and can be used as a main power source for portable equipment, a backup battery, etc. Used as a large or small secondary battery that can be used as a power source, a power source for household electrical appliances, a drive power source for electric vehicles, a load leveling device, a power source for identification cards, etc. I can do it.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of a coin-type secondary battery cell, and FIG. 2 is a diagram showing the relationship between the number of repetitions of charging and discharging and the amount of discharged electricity. 1...Positive electrode, 2...Nickel wire mesh, 3...Polypropylene nonwoven fabric, 4...Polypropylene microporous film, 5...Insulating backing, 6...
・Negative electrode.

Claims (2)

[Claims] (1) A secondary battery characterized in that the positive electrode uses cobalt oxide to which carbon black and vapor-grown graphite are added, and the negative electrode uses a sodium alloy to which carbon black and vapor-grown graphite are added. (2) The amount of carbon black added is 0.5 to 3 for the positive electrode.
The secondary battery according to claim 1, wherein the amount of vapor grown graphite added is 0.1 to 2 wt% in the positive electrode and 1 to 5 wt% in the negative electrode.