JPH01260767A - Secondary battery - Google Patents

Abstract

PURPOSE:To obtain a battery having high energy density and long cycle life by using a mixed sinter of MnO2 and NaOH as a positive active material, a material capable of reducing a sodium ion in charging as a negative active material, and a sodium ion conductive nonaqueous electrolyte as an electrolyte. CONSTITUTION:In a positive electrode 1 using a mixed sinter of MnO2 and NaOH, a sodium ion is inserted into the active material by charging and the sodium ion inserted into the active material is reversibly released by discharging. In a negative electrode 6, a sodium ion is reduced by charging and the sodium is oxidized to a sodium ion and released by discharging, and this reaction is more reversible compared with a lithium system. A battery having better reversibility and larger capacity density compared with that using gamma-MnO2 or beta-NnO2 alone is manufactured. Sodium having lower reactivity with an electrolyte than lithium is used. A secondary battery having high energy density and long cycle life is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Field of Industrial Application The present invention relates to a secondary battery with high energy density, long cycle life, and good performance.

(2) Prior art Until now, alkali metals such as Ll and Na have been used as negative electrode active materials, and LiCj)04. Development of a secondary battery using a so-called non-aqueous electrolyte in which LiBF4°L iAs Fe and the like is dissolved is progressing.

However, although this type of secondary battery exhibits a relatively high electric capacity, its cycle life is short, and it is not at a level that can be used satisfactorily.

The reason for this is that the positive and negative electrodes have low I'+1 reversibility with respect to charging and discharging.

For example, for positive electrodes, oxides and chalcogen compounds with layered structures or tunnel structures such as TI, V, Cr, Mo, etc. are known, but in compounds with these structures, alkali metal ions are released from the compound during charging and discharging of the battery. Entering or exiting a layer or tunnel. For this reason, the crystal structure of the cathode active material itself is subject to repeated expansion and contraction, which destroys the particles between the active material particles and causes the electrode to collapse, reducing reversibility.Also, the alkali metal ions inserted during discharge It has been pointed out that the alkali metal ions are strongly bound in the oxide, and the alkali metal ions cannot be sufficiently released during the next charge, leading to a gradual decrease in battery capacity and poor cycle life.

On the other hand, in negative electrodes, lithium, which is the lightest and has the highest electric capacity density, is often used as the active material, but in the case of secondary batteries,
It has the drawbacks of forming dendrites during charging, leading to a short circuit with the counter electrode, and because lithium itself is highly active, it reacts with electrolytes and solvents, resulting in low charge/discharge efficiency and short cycle life. In order to improve those shortcomings,
Methods of preventing direct contact between lithium and the electrolyte are being considered, such as using a lithium alloy instead of lithium alone or creating an ion-conductive film on the surface of the negative electrode.

(3) Problems to be solved by the invention However, with the above configuration, the cycle life is poor when the capacitance density is increased as a secondary battery, and the energy density is high, the cycle life is long, and the performance is good. It was not possible to obtain a suitable secondary battery.

(4) Means for Solving the Problem The inventors used a fired mixture of M n 02 and NaOH as the positive electrode active material, and maintained the electrode reaction by reducing sodium ions to the negative electrode active material by charging. using things,
It has been found that the above problems can be solved by using a sodium ion conductive non-aqueous electrolyte as the electrolyte, leading to the present invention.

M n 02 and Na which are positive electrode active materials as constituent elements
As for the mixed fired product with OH, the manufacturing method is also an important factor. Regarding the raw materials M n 02 and NaOH, there is no need to specify the grade and manufacturing method. For example, M
n02 may be an electrolytically synthesized product or a chemically synthesized product, and the crystal structure itself may be α type, β type, γ type, δ type, spinel type, or any other type. . However, it is preferable that the original crystal structure itself is capable of inserting a large amount of sodium ions, and from this point of view, β-type, γ-type, or a mixture thereof is preferable.

On the other hand, as a method for mixing and firing M n O2 and NaOH, it is necessary to use the following method.

First, the molar ratio of M n 02 and NaOH to be mixed and fired is 0.2 to 1.2.
It is better to be within the range of .

Is it outside this range? The effect of improving li performance is small.

Although it is preferable to remove moisture from the M n O2 and NaOH used, this step is not necessarily necessary.

Next, M n 02 and NaOH need to be mixed as well as possible, and kneading both powders is also a good way to mix them well.

What is particularly important is the firing temperature; if it is too high or too low, the effect of mixing and firing will be almost negated.

That is, the firing temperature must be 200°C or more and 450°C or less; if it is less than 200°C, no firing effect will be obtained;
Furthermore, if the temperature exceeds 450°C, the reaction between NaOH and MnO2 will proceed too much, making it impossible to obtain a high-capacity active material. Among these, the temperature at which the firing effect is particularly suppressed and the firing is performed efficiently is within the range of 250°C to 380°C.

On the other hand, as negative electrode active materials, in addition to sodium alone, sodium alloys, composites of sodium alloys and conductive polymers,
There are composites of sodium alloys and carbon materials such as carbon black, sodium ion insertion compounds of conductive polymers, etc., but preferred are composites of sodium alloys and conductive polymers, sodium alloys and carbon black, etc. It is a composite with carbon material.

Preferred examples of the conductive polymer include polyparaphenylene, polyacetylene, polythiophene, and ladder polymers, and examples of the carbon material include carbon black, graphite, and activated carbon. I don't buy these either in powder or fibrous form.

As the sodium ion conductive electrolyte, an electrolytic solution in which a sodium salt is dissolved in a non-aqueous solvent is generally used, such as NaPFe, NaBF4゜NaAsF NaCff ONaCF35o4B'
4.

A sodium salt of 1,2-dimethoxyethane, propylene carbonate, 2-methyltetrahydrofuran, tetrahydrofuran, ethylene carbonate, dioxane, γ-butyrolactone, or the like is dissolved in a non-aqueous solvent. Furthermore, it is possible to use a solid electrolyte that conducts sodium ions.

(5) Function In the secondary battery that is a component of the present invention, in the positive electrode, M
By using a fired product of n02 and NaOH,
Sodium ions are inserted into the active material by discharging, and the inserted sodium ions are reversibly released by charging. On the other hand 1. In the negative electrode, sodium ions are reduced by charging, and are oxidized and released into sodium ions by discharging with better reversibility than lithium-based ions. It is possible to produce a battery with good reversibility and high capacity density. Furthermore, by using a sodium-based material that has a lower reaction activity with the electrolyte than lithium, a high-performance secondary battery with good battery energy density and cycle life can be obtained.

(6) Example Hereinafter, an example in which the present invention is applied to a coin-type secondary battery will be described.

[Example 1] (Manufacturing method of positive electrode) After thoroughly mixing γ-type M n O2 and NaOH in equimolar amounts, the temperature was raised from room temperature to 350°C in about 1 hour in an argon atmosphere, and the mixture was heated at 350°C. It was held for 20 hours. After firing, the mixture was naturally cooled to room temperature, and in an argon atmosphere, 10% by weight of carbon black and 3% by weight of polytetrafluoroethylene were added and mixed well. Diameter 15m+s, thickness 4
It was molded using a tablet molding machine to give a total of 50 tablets. Next, in order to increase the binding strength, the mixture was heated at 250° C. for 2 hours to obtain a pellet positive electrode.

(Manufacturing method of negative electrode) Place the molar ratio of sodium and lead in an iron crucible at 2.5:l, and gradually raise the temperature from room temperature under argon to approximately 5:1.
After melting at 00°C for 4 hours, it was annealed at 340°C for 15 hours and cooled to room temperature.

After thoroughly crushing this alloy in a mortar, a predetermined amount of artificial graphite powder manufactured by Showa Denko was mixed in, and then EPD dissolved in xylene was mixed.
M (manufactured by Japan Synthetic Rubber Co., Ltd., trade name JSR-EP25X) was mixed and kneaded with the above mixture. Note that the above-mentioned predetermined amounts are such that the weight ratio of sodium alloy, graphite powder, and EPDM is 82:14.
: The blending ratio was set to be 4.

Next, the above mixture was molded using a tablet molding machine to a diameter of 15 m+* and a thickness of 300 ρ to obtain a pellet negative electrode.

(Battery test) Using the negative electrode and positive electrode prepared in the above method, add 1 mo to the electrolyte.
Using a 1,2-dimethoxyethane solution of NaPFe at a concentration of 1)/l, the coin type battery shown in FIG. 1 was assembled and a battery test was conducted.

First, discharge from the discharge direction at a constant current value of 1.7n+A until the battery voltage reaches 1.7V, then after a 30-minute rest time, the battery voltage reaches 3.0V with a current of 1.7mA.
The battery was charged to 3V, and after a rest time, the battery was discharged again, which was a repeated test of charging and discharging.

As a result, the maximum amount of discharged electricity was 13.8mAh, and the average discharge voltage was 2.55V. Also, the discharge capacity is IOmAh
The cycle life until breaking was 852.

In addition, when this battery was subjected to a self-discharge test at 40℃ for 20 days at the 100th cycle, the self-discharge rate was 1.
It was 2.8%.

[Example 2] (Production of positive electrode) Electrolytic manganese dioxide (1, C, N (L, 3) and Na
Mix OH in a molar ratio of 1:0.5 and store in dry air for 30 min.
The temperature was raised to 0°C and held at 300°C for 20 hours. After firing, it was naturally cooled to room temperature, and 1.5fflf1% of EPDM (ethylene propylene rubber) in which 10% by weight of carbon black was dissolved in xylene was added under an argon atmosphere. *
.

It was molded using a tablet molding machine to a thickness of 450 cm. Next, in order to remove xylene, it was dried under reduced pressure at 130° C. for 2 hours and used as a positive electrode.

(Manufacturing method of negative electrode) Sodium and lead were mixed at an atomic ratio of 3.75:l, melted at 500°C for 4 hours, annealed at 350°C for 15 hours, and cooled to room temperature. After thoroughly crushing this alloy in a mortar, a predetermined amount of pyrolytic graphite powder was mixed therein, and then EPDM dissolved in xylene was mixed and kneaded with the above mixture.

Note that the above-mentioned predetermined amounts are sodium alloy, graphite powder, and E.
The blending ratio was such that the weight ratio of PDM was 85:12:3.

Next, after removing excess xylene from the mixture under reduced pressure, a 75-mesh nickel wire gauze 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 380 mm.

The electrode formed by the above method was cut out to have a diameter of about 15 cm to form a negative electrode.

(Battery test) LiA was added to the positive electrode, negative electrode, and electrolyte prepared by the above method.
Using a solution of sFe dissolved in a mixed solvent of 1,2-dimethoxyethane and 2-methyltetrahydrofuran in a volume ratio of 1:1, the coin-type battery shown in FIG. 1 was assembled and its performance was investigated.

The test method was the same as in Example 1.

As a result, the maximum discharge amount of electricity was 13.5+nAh,
The number of cycles until the amount of discharged electricity becomes less than 1O+++Ah is 5.
It was 92 times.

Moreover, the self-discharge rate after 100 cycles was 11.596.

[Example 3] A battery experiment was conducted by constructing a cell exactly the same as in Example 1, except that polybaraphenylene was used instead of the negative electrode graphite powder.

As a result, the maximum amount of discharged electricity was 13.7IIIAh, and the number of cycles until the amount of discharged electricity was less than lo■Ah was 64.
It was seven times.

Furthermore, the self-discharge rate at the 100th cycle was 12.5%.

(7) Effects As described above, the secondary battery of the present invention has high capacity and high energy density, has good reversibility, has a low self-discharge rate, can be made at low cost, and can be used as a main power source for portable equipment. We provide excellent secondary batteries, both large and small, for use as backup power supplies, power supplies for household electrical appliances, drive power supplies for electric vehicles, load leveling, and power supplies for identification cards. It is something to do.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of a coin-shaped secondary battery cell used to evaluate the battery of the present invention.

Claims (1)

[Claims] The components include a positive electrode, a sodium ion conductive nonaqueous electrolyte, and a negative electrode that absorbs sodium ions during charging and releases sodium ions during discharge, and the positive electrode active material is a mixed fired product of MnO_2 and NaOH. A non-aqueous electrolyte secondary battery featuring: