JP2839627B2 - Rechargeable battery - Google Patents

Description

The present invention relates to a secondary battery having good performance such as high energy density, low self-discharge rate, and long cycle life.

[Related Art] Conventionally, secondary batteries using lithium metal, which is one of alkali metals, as a negative electrode have been attracting attention for a long time.
M. Hughes, et al, Journal of Power.Sources, 12 , pp.83-14
4 (1984) provides a review.

Among them, lithium metal is so active that it reacts with a solvent to form an insulating film and further causes dendrite growth, indicating difficulty in application to a negative electrode for a secondary battery. For this reason, a secondary battery with good performance using an alkali metal has not been developed yet, and none has been put to practical use until it is comparable to current lead batteries and nickel cadmium storage batteries.

As described above, the biggest cause of the practical use of the alkali metal negative electrode is the short-circuit phenomenon due to the reaction between the alkali metal and the electrolytic solution and the dendrite growth caused by the reaction.

As a method of solving the above problems, the activity is reduced by replacing the alkali metal negative electrode with an alkali metal alloy, or the alkali metal surface is covered with an ion conductive film so that the alkali metal does not come into contact with the reactive solution directly. There are methods to do this, but this is not always a sufficient measure.

Further, when an alkali metal alloy is used, there is a problem that if charge and discharge are repeated, the alloy particles themselves become finer and an expected cycle life cannot be obtained.

Therefore, the present inventors have previously proposed a non-aqueous secondary battery using a negative electrode in which an alkali metal alloy, a carbon material and a binder are combined (Japanese Patent Application No. 63-169384).

The binder is added to prevent electrode collapse due to alloy miniaturization, and the carbon material is added to increase specific surface area and porosity and improve charge / discharge characteristics at high current. It is.

[Problem to be Solved by the Problem] However, although the above secondary battery is a battery showing high performance, since the carbon material is added, the electrode bulk density is smaller than that of the alloy alone, and the volume per electrode volume is small. There is a limit on the capacitance density.

In this case, it was considered that there was no problem in using an alkali metal alloy alone as a negative electrode, especially for applications that do not require high-speed current.However, with an alloy having a too high bulk density, the entire electrode did not work, and instead the electric capacity Turned out not to rise.

Therefore, an attempt was made to pulverize the alkali metal alloy and to form the fine particles together with the binder.

When general polyethylene or polypropylene is used as the binder, the amount of use is required to be 5% by weight or more.
As a result, the binder becomes a resistance of the electrode reaction, and a smooth battery reaction cannot be obtained. If the content is less than 5% by weight, the electrode gradually collapses due to volume expansion and contraction of the alkali metal alloy during charging and discharging, and the reversibility is not improved.

However, if a carbon material is mixed into the negative electrode to form a composite as proposed by the present inventors, the electrode area increases, and the alkali metal alloy acts as a buffer material for volume expansion and contraction. It has been demonstrated that the resulting secondary battery has a rather high performance except that the capacity density is limited. However, it has been found that mixing the carbon material into the negative electrode causes other disadvantages.

In other words, most carbon materials are aggregates of primary particles, and there are many types such as carbon black, activated carbon, and graphite powder. The cycle life is not semi-permanent due to floating in the electrolyte, discharging to the electrolyte hole of the separator, or contacting the counter electrode through the separator to cause a short phenomenon.

As described above, when the carbon material is mixed in the negative electrode, advantages are obtained.
It has drawbacks and requires a large amount of binder to prevent the carbon material from collapsing during long cycles. However, an increase in the amount of binder used causes problems such as an increase in resistance as described above. Therefore, at present, the type of the carbon material to be mixed with the negative electrode is selected and adjusted according to the purpose of use of the secondary battery.

The present inventors have conducted intensive studies in order to obtain a secondary battery having excellent performance using a negative electrode that does not require adjustment of the formulation or the like depending on the purpose of use, and as a result, it has been found that a fibrous substance can be added instead of a carbon material. I found something useful.

The present invention has been made based on the above findings, and has as its object to provide a secondary battery having a large effective electrode area and good performance.

[Means for Solving the Problems] To achieve the above object, in a secondary battery of the present invention, a fibrous substance and an electrolyte are mixed with an alkali metal alloy,
A negative electrode to which a binder is added and molded is used.

Fibrous material used for the negative electrode of the secondary battery of the present invention,
It is necessary not to alloy with the alkali metal used for the negative electrode, and examples thereof include stainless steel fiber, carbon (including graphite) fiber, glass fiber, and the like. Metal fiber and carbon fiber are preferable, and nickel fiber is particularly preferable. is there. The fibrous substance may be used by mixing several kinds.

The method of mixing the fibrous substance in the negative electrode is not particularly limited, but when producing an alkali metal alloy, the fibrous substance is mixed in advance, and heated to contain the fibrous substance. Or a method of producing an alloy containing a fibrous substance by a melting method, or a method of mixing a fibrous substance with an alkali metal alloy powder at the time of molding, adding an electrolyte and a binder thereto, and molding the same. Although any method may be used, the latter is more convenient because the operation is simple.

Also, the electrolyte mixed with the negative electrode must be one that dissolves in the battery solvent when the battery is assembled,
The degree of dissolution is not particularly defined. This is because the electrolyte mixed in the negative electrode is dissolved in the battery solvent and becomes porous,
This is because the electrolyte may penetrate into the inside of the negative electrode to accelerate the electrode reaction, and the electrolyte used in the negative electrode does not necessarily need to be completely dissolved in the battery solution.

However, in view of the electric conductivity of the electrolytic solution in the electrode, it is naturally preferable that the electrolyte is completely dissolved.

As the type of the electrolyte, it is optimal to use the same alkali metal salt as the alkali metal of the negative electrode. However, a solid electrolyte may be used as long as it is soluble in the solvent of the battery electrolyte.

As the alkali metal salt, when the negative electrode is a sodium alloy, for example, NaPF ₆ , NaBF ₄ NaAsF ₆ , NaCF ₃ SO ₃ , NaCl, NaB (CH
₄ ) ₄ etc., and when the negative electrode is a lithium alloy, LiP
Examples include, but are not limited to, F ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiCl, LiB (CH ₄ ) _{4 and the} like.

The solid electrolyte is not particularly limited, but is preferably a mixture of each alkali metal salt and polyolefin oxide. For example, those obtained by complexing NaCF ₃ SO ₃ or LiBF ₄ with polyethylene oxide can be mentioned.

As the alkali metal used for the alkali metal alloy used in the present invention, sodium or lithium is used. As an alloy in the case of sodium, an alloy with lead or tin is preferable because of good reversibility when used for an electrode, and in the case of lithium, an alloy with at least one metal of aluminum, lead, and tin Is preferred.

As the binder for the negative electrode, polyethylene, polypropylene, polytetrafluoroethylene, olefin-based copolymer rubber, or the like can be recommended, but it has no reactivity with the alkali metal of the negative electrode, and can be used even in a small amount. Olefin-based copolymer rubber is most preferred because of its large adhesion effect.

The olefin copolymer rubber is, for example, ethylene-propylene rubber (EPR), ethylene-butene rubber (EB
R), ethylene-propylene-diene rubber (EPDM), and the like. Particularly, EPDM having a small amount and a large binding effect is preferable. EPDM here is a kind of synthetic rubber, which is a copolymer of ethylene and propylene, into which an unsaturated compound having a double bond is introduced as a third component.

If the olefin copolymer rubber is added too much, it not only lowers the electric capacity of the entire negative electrode, but also increases the electric resistance of the entire electrode, increases the polarization of the battery reaction, and causes a decrease in performance. If the amount is too small, the effect as a binder is not exhibited. Therefore, the amount to be added is in the range of 0.3 to 5% by weight, particularly preferably 0.5 to 3% by weight.

The positive electrode used in the battery of the present invention is not particularly limited, but it is preferable that the positive electrode has an appropriate potential difference with respect to the negative electrode, and has a high electric capacity density capable of reversibly transferring electric charges. For example, inorganic oxides, chalcogenides, conductive polymers, carbon materials, and the like can be given. Further specific examples include sodium cobalt oxide, sodium manganese oxide, lithium manganese oxide, lithium cobalt oxide,
Vanadium oxide, chromium oxide, polyaniline, polypyrrole, graphite, activated carbon and the like can be mentioned.

As a particularly excellent combination among them, when a sodium alloy is used for the negative electrode, the positive electrode is preferably sodium-cobalt oxide. When a lithium alloy is used for the negative electrode, lithium manganese oxide or lithium cobalt oxide is suitable for the positive electrode.

Some of the positive electrodes have a high electrical conductivity in the active materials listed above, and some do not necessarily require a conductive additive.However, for the purpose of improving the specific surface area of the electrode, a carbon material, particularly carbon black, is added. Is common. In addition, polytetrafluoroethylene, olefin copolymer rubber, or the like can be used as the binder.

[Example] Next, the secondary battery of the present invention will be described with reference to examples and comparative examples.

Example 1 <Production of Negative Electrode> An alloy having an atomic ratio of Na: Pb of 2.2: 1.0 obtained by a melting method as an anode active material under an argon atmosphere was pulverized to a particle size of 1
Fine particles of 50 μm or less were obtained.

Next, 9 parts by weight of nickel fiber (manufactured by Nippon Seisen Co., Ltd., average diameter 35 μm, average length 2000 μm) was added to 100 parts by weight of the alloy powder by fine cutting with a high-speed rotary mixer. This is mixed well, and 3
A solution obtained by dissolving parts by weight of EPDM in xylene was added, and mixed thoroughly in a mortar. This mixture was dried under reduced pressure to remove xylene, and then mixed and pulverized again with a high-speed rotary mixer. 18 parts by weight of NaPF ₆
Was added and mixed well. Next, a predetermined amount of the mixture was collected, and a rectangular electrode of 40 × 330 mm was formed by a press method so as to include a nickel-made extra band metal as a current collector.

<Production of Positive Electrode> Na ₂ Co ₃ and Co ₃ O ₄ were heated and reacted at 820 ° C. for 50 hours in an oxygen atmosphere to synthesize sodium-cobalt oxide of Na _0.7 CoO ₂ . This Na _0.7 CoO ₂ , Ketjen Black and tetrafluoroethylene were mixed in xylene at a weight ratio of 96: 1: 3 in xylene, and a stainless steel expanded metal was used as a current collector, in the same manner as the negative electrode. Thus, a rectangular electrode of 40 × 290 mm was formed.

<Battery Experiment> The above positive electrode and negative electrode were wound in a glove box in an argon atmosphere with a polypropylene microporous film (manufactured by Polyplastics) sandwiched therebetween as a separator, and NaPF was added to the electrolyte at 1 mol / mol. ₆
1,2-dimethoxyethane and tetraglyme in a volume ratio
Using a solution dissolved in a mixed solution mixed at a ratio of 9: 2,
AA batteries were assembled. In addition, the battery seal | sticker used the laser solution method.

The voltage immediately after assembly of this battery was 2.5V. The battery was discharged at room temperature at a current of 300 mA, and stopped when the battery voltage reached 1.8 V. After a rest time of 30 minutes in an open circuit, the battery was charged at a current of 300 mA, and the battery voltage was lowered. When the voltage reached 3.3 V, the charging was terminated. After resting for 30 minutes, the battery was discharged again. Thereafter, discharging and charging were repeated with a rest time.

As a result, the maximum discharge capacity of this battery at 300 mA discharge was 6
At 30 mAh, the discharge average voltage was 2.8V.

When charging and discharging were repeated 100 times, the current efficiency of charging and discharging was 100%, the discharge capacity was 605 mAh, and the secondary battery was high in energy and excellent in sile performance.

Furthermore, after charging this battery during the cycle experiment,
The circuit was opened, left in a constant temperature bath at 40 ° C. for 10 days, and the self-discharge rate was examined. The charge current value was kept constant at 300 mA, and the discharge current value was changed to 50 mA, 100 mA, 300 mA, 500 mA, or 1000 mA for each cycle, the discharge capacity at that time was measured, and the discharge rate characteristics were examined.

As a result, the self-discharge rate at 40 ° C for 10 days is 3.5%, the capacity at 50mA discharge is 655mAh, and at 100mA is 645mAh, 300mA
630mAh, 500mA 590mAh, 1000mA discharge 538mAh
And the discharge rate characteristics were extremely good.

Example 2 <Production of Negative Electrode> An alloy obtained by a melting method in an argon atmosphere so that the atomic ratio of Na and Pb was 2.2: 1.0 was pulverized into fine particles having a particle size of 150 μm or less. To 100 parts by weight of the alloy powder, 9 parts by weight of nickel fiber (about 25 μm in diameter and 200 μm in length) was added, mixed well, transferred to a crucible, and heated to 500 ° C. or higher, at which the Na—Pb alloy melted. After 2 hours, the heating was stopped, the mixture was cooled naturally, and then pulverized well. To this mixed powder, 3.5% of EPDM dissolved in xylene was added and mixed well. After removing xylene and pulverizing again, a powder of a complex of NaBF ₄ and PEO (polyethylene oxide, average molecular weight, 30,000) (one with eight NaBF ₄ atoms per eight atoms of PEO) was mixed with 100 parts of the above mixture. 10 parts by weight with respect to parts by weight were added and mixed well.

This is a form that contains nickel expanded metal
It was pressed into a rectangle of 40 × 280 mm to obtain a negative electrode.

<Production of Positive Electrode> The same positive electrode as that of Example 1 was used, and only the thickness and length of the electrode were changed, and a 40 × 250 mm shape was formed by a press method.

<Battery experiment> Example 1 was repeated except that a solution obtained by dissolving NaBF ₄ in a mixed solvent of 1.2-dimethoxyethane and tetraglyme at a volume ratio of 1: 1 so as to be 1 mol / was used as the electrolyte. AA batteries were assembled in exactly the same manner, and a battery performance test was performed. 3. As a result, the maximum discharge capacity at 300 mA is 640 mAh
The discharge capacity at the 100th cycle was 610 mAh. Further, the results of the self-discharge test and the discharge rate characteristic test performed in the middle cycle are as follows. Self-discharge rate is 3.4%, and capacity at 50mA discharge is 670mAh, 658mAh at 100mA, 640mAh at 300mA, 575mAh, 1000mA at 500mA
It was 470mAh.

Example 3 <Manufacture of Negative Electrode> In an argon atmosphere, a negative electrode active material was formed by electrolysis using Al.
An alloy having a 3: 1 atomic ratio of Li to Al obtained by depositing Li on the substrate was pulverized into fine particles of 150 μm or less.

Next, 10 parts by weight of nickel fiber was added to 100 parts by weight of the alloy powder, and a well-mixed mixture was further mixed with a solution obtained by dissolving 3 parts by weight of EPDM in xylene with respect to the alloy.

Next, this mixture was dried under reduced pressure to remove xylene, and then mixed and pulverized again with a high-speed rotary mixer. To this mixture, 10 parts by weight of electrolyte LiPF ₆ was added and mixed well. Next, this mixture was obtained by molding a 40 × 330 mm rectangular electrode by a press method by enclosing a thin (20 μm thick) punching metal made of nickel as a current collector.

[Manufacture of positive electrode] LiOH-MnO ₂ oxide (Sanyo Technical Review)
VoL20. No3.11 manufactured by the method of January 1988, p.69) and Ketjen black and tetrafluoroethylene in a weight ratio of 90:
A mixture of 5: 5 in a ratio of 40 to 40% with a stainless steel (SVS430) expanded metal as current collector is included.
It was press-molded to a size of × 290 mm.

<Battery Experiment> An AA battery was assembled in the same manner as in Example 1 using a 1 mol / liter solution of LiPF ₆ in 2-methyl-tetrahydrofuran as an electrolyte.

 The battery performance test was performed in exactly the same manner as in Example 1.

As a result, the maximum discharge capacity at 300 mAh discharge is 610 mAh.
The discharge capacity at the 100th cycle was 565 mAh.

The self-discharge rate is 2.9% and the discharge rate characteristics are 645mAh at 50mA discharge, 635mAh at 300mA, 300mA at 100mA.
At 608 mAh, at 580 mAh at 500 mA, and at 520 mAh at 1000 mA.

Example 4 A lithium cobalt oxide of Li _0.7 C ₀ O ₂ was used in place of the positive electrode used in Example 3, and 1 mol / Li of Li
Except for using γ- butyrolactone solution of PF ₆ was experimented assembled battery in the same manner as in Example 3.

However, in the battery performance test, the operating voltage range is the upper limit of 4.1 V and the lower limit
2.5V.

As a result, the average discharge voltage at the time of 300 mA discharge was 3.2 V, and the maximum discharge capacity was 615 mAh.

However, the discharge capacity fell below 500 mAh at the 15th cycle, and no other characteristic tests were performed.

[Example 5] <Production of negative electrode> Li obtained by a melting method under an argon atmosphere as a negative electrode active material was used.
An alloy having an atomic ratio of Pb of 3: 1 was used.

After finely pulverizing this alloy, nickel fibers were mixed, and EPDM was further used as a binder, followed by thorough mixing. After removing xylene, a solid electrolyte composed of LiSO ₃ CF ₃ and PEO (LiS
O ₃ Li of CF ₃ was 0 to well pulverized and mixed added 10 parts by weight per the mixture 100 parts by weight and are) contained 1/6 of PEO.

This was pressed under pressure to obtain a rectangular negative electrode of 40 × 180 mm.

<Production of Positive Electrode> The same positive electrode as in Example 3 was used, and only the electrode shape was 40 × 150 mm.

<Battery Experiment> A battery performance test was performed in exactly the same manner as in Example 3.

As a result, the maximum discharge capacity at 300 mA discharge is 1 at 620 mAh.
The discharge capacity at the 00th cycle was 520 mAh.

The self-discharge rate is 4.2% and the discharge rate characteristic is 5%.
Capacity at 0mAh discharge is 652mAh, 643mAh at 100mA, 300mA
It was 620mAh at 500mA, 575mAh at 500mA, and 508mAh at 100mA.

Example 6 A battery was assembled and tested in the same manner as in Example 1 except that the Na alloy used was an alloy having an atomic ratio of Na to Sn of 2.2: 1.0. As a result, the maximum discharge capacity at 300 mA discharge is 100 at 597 mAh.
The capacity at the cycle was 480 mAh.

The self-discharge rate is 3.6% and the discharge rate characteristic is 50m
The capacity at the time of Ah discharge was 615 mAh, 612 mAh at 100 mA, 595 mAh at 300 mA, 540 mAh at 500 mA, and 470 mAh at 1000 mA.

Example 7 The procedure was the same as in Example 5, except that an alloy of Li and Sn having an atomic ratio of 1: 3 was used instead of the alloy of Li and Pb.

As a result, the maximum discharge capacity at 300 mA discharge is 1 at 570 mAh.
The discharge capacity at the 00th cycle was 470 mAh.

The self-discharge rate is 4.3%.
610mAh at 0mA discharge, 600mAh, 300mA at 100mA
568 mAh at 500 mA, 530 mAh at 500 mA, and 460 mAh at 1000 mA.

Example 8 Instead of nickel fibers, graphite fibers (average diameter 15 μm, average length 100 manufactured by Showa Denko KK) manufactured by vapor phase growth method
(μm) was used in the same manner as in Example 1 except that 5 parts by weight was used with respect to 100 parts by weight of the alloy.

As a result, when discharged at a current of 300 mA, the maximum amount of discharged electricity is 600 mAh, and the discharge capacity at the 100th cycle is 580 mAh.
Met.

The self-discharge rate is 3.7%, and the discharge rate characteristics are: 630mAh at 50mA current value, 620mAh at 100mA current value, 600mAh at 300mA, 580mA at 500mA discharge
h, a capacity of 540 mAh was obtained even at 1000 mA discharge.

It was added NaPF ₆ as the electrolyte in Comparative Example 1 Example 1 in the negative electrode, but was evaluated assembling the battery without at all added electrolyte such as NaPF _6.

As a result, the maximum discharge capacity at 300 mA discharge is 1 at 595 mAh.
The capacity at the 00th cycle was 520 mAh, but the charge / discharge efficiency was 80
%.

As a result of a self-discharge test performed in the middle, the self-discharge rate was 4.2%.

On the other hand, when the discharge rate characteristics were examined, the discharge capacity at 50 mA discharge was 680 mAh, but 650 mAh at 100 mAh and 5 at 300 mA.
The current dependency was large at 510 mAh at 95 mAh, 500 mA discharge, and 380 mAh at 1000 mA discharge.

Comparative Example 2 A battery was assembled in the same manner as in Example 1 except that nickel fibers were inserted into the negative electrode in Example 1, but the nickel fibers were not inserted, and the battery performance was examined.

As a result, the maximum discharge capacity at the time of 300 mA discharge was 618 mAh, but at the 100th cycle, the capacity dropped to 320 mAh.

When a self-discharge test was performed in the middle cycle, the self-discharge rate was 5.3%. The discharge rate characteristics were not examined because the capacity was significantly reduced in each cycle.

〔The invention's effect〕

As described above, the secondary battery of the present invention has a high energy density, good reversibility, low self-discharge, and excellent discharge rate characteristics. large.

Claims (7)

(57) [Claims] 1. A secondary battery comprising a positive electrode, a negative electrode and a non-aqueous electrolyte, wherein a fibrous substance and an electrolyte are mixed with an alkali metal alloy, a binder is added thereto, and a molded negative electrode is used. Features a secondary battery. 2. The method according to claim 1, wherein the fibrous substance is a metal fiber.
The secondary battery according to any one of the preceding claims. 3. The fibrous substance is a carbon fiber.
The secondary battery according to any one of the preceding claims. 4. The secondary battery according to claim 1, wherein the electrolyte is an alkali metal salt dissolved in a solvent of the non-aqueous electrolyte used for the secondary battery. 5. The electrolyte according to claim 1, wherein the electrolyte is a solid electrolyte dissolved in a solvent of a non-aqueous electrolyte used for a secondary battery.
The secondary battery according to (2) or (3). 6. The method according to claim 1, wherein the alkali metal alloy is an alloy of sodium and lead or sodium and tin.
The secondary battery according to (3), (4), or (5). 7. The method according to claim 1, wherein the alkali metal alloy is an alloy of lithium and at least one of aluminum, lead and tin. 5) The secondary battery as described above.