JPS60167280A - Electrochemical device capable of recharging - Google Patents

Abstract

PURPOSE:To remarkably increase charge-discharge cycle life by combining a positive electrode comprising activated carbon with a negative electrode comprising lithium alloy. CONSTITUTION:Activated carbon having appropriate pore size, large surface area, high purity, high conductivity, and good molding capability is used for a positive electrode 6 so as to form a large electric double layer. A lithium alloy is used for a negative electrode 3. A metal alone or metal alloy capable of storing and releasing lithium by charge and discharge respectively is used as alloying material with lithium. By combining these positive and negative electrodes, charge-discharge cycle life is remarkably increased.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present BA relates to rechargeable electrochemical devices for use in mobile DC power sources, backup power sources, and the like.

Conventional Structures and Problems Conventionally, rechargeable secondary batteries include nickel-cadmium batteries and lead-acid batteries, and capacitors using electric double layers are used as backup power sources for electronic devices. Particularly in recent years, the market for backup power sources has expanded, and there is a demand for highly reliable, high energy rechargeable electrochemical devices of this type.

On the other hand, although lithium batteries using organic electrolytes have been put into practical use as secondary batteries for a long time, the voltage per single cell is
It is high energy and leak resistant.

It has established itself in the market as a high-energy, high-reliability battery with excellent storage characteristics.

If this lithium battery could be recharged, it would be ideal as a main or backup power source for electronic devices, which are becoming smaller and lighter.

Against this background, over the past few years, lithium secondary batteries have been actively developed using lithium metal as the negative electrode and organic electrolyte.

However, the current state is that the charge/discharge cycle life is not sufficient and the battery is not commercially available.

The reason for this is that the negative electrode, lithium gold, does not have sufficient reproducibility of charge and discharge when used alone, and lithium tent lights are generated on the surface of the lithium electrode, making it unable to withstand long charge and discharge cycles. It is in. Furthermore, in the positive electrode, there is no active material that exhibits excellent charge/discharge cycle characteristics.

Therefore, attempts have been made to use lithium metal for the negative electrode and an activated carbon electrode that utilizes an electric double layer for the positive electrode.

Activated polarization has a very large effective surface area, and positive polarization allows anions to be adsorbed to the electrolyte layer on the surface of activated carbon and charged with electrical energy as an electric double layer. Charging and discharging becomes possible through the adsorption and desorption of anio-y from this electric double layer. Activated carbon itself is not an active material and hardly involves redox reactions, so it does not self-deplete and can be repeatedly charged and discharged in a stable state. However, not just any activated carbon will work; characteristics such as electric capacity and charge/discharge cycle life will vary depending on the surface area, purity, pore size, etc. of activated carbon, resulting in high capacity.

Activated carbon with a long life cycle has been desired. On the other hand, lithium metal used as a negative electrode undergoes an oxidation-reduction reaction in which lithium becomes ions or metals, but lithium ions liberated into the electrolyte due to discharge do not necessarily return to their original positions during charging. As a result, lithium metal may precipitate in the separator, causing a short circuit between the positive and negative electrodes, or lithium metal may be liberated, resulting in a dendritic or foam-like lithium electrode, which may result in insufficient charge/discharge life. It wasn't.

However, if the charging/discharging depth of lithium metal is made shallow, lithium metal can be easily returned during charging, and even hundreds of cycles can be achieved. However, this alone was not sufficient from the perspective of long-term reliability, and fundamental improvements were needed.

OBJECTS OF THE INVENTION The present invention uses activated carbon for the positive electrode and 1 lithium metal for the negative active material, respectively, to improve the material of the negative electrode in rechargeable electrochemical devices using an IIE water electrolyte, such as an organic electrolyte. The aim is to improve charge/discharge cycle life.

Structure of the Invention In order to achieve the above object, the present invention has a positive electrode having a vertical strip ≠
≠ Activated carbon is used, and the negative electrode uses alloyed lithium that absorbs and releases lithium during charging and discharging.

The activated carbon used for the positive electrode must have an appropriate pore size, a large surface area, and have excellent purity, conductivity, and formability so that a large number of electric double layers can be easily formed. under the necessary conditions. Ordinary activated carbon particles have a maximum surface area of 1600 horns f79 or less when measured by the BKT method, which is not necessarily large, and therefore the electric capacity cannot be improved much. In addition, in activated carbon obtained from ordinary charcoal, coconut shell charcoal, or petroleum-based carbon materials, impurities and chlorine ions mixed in with the activation process remain, and these act as catalysts during charging and discharging. The electrolyte was decomposed and the charge/discharge cycle life was reduced.

First, activated carbon fibers generally have better surface area and purity than activated carbon particles, and among them, those obtained by activating novolac type phenolic tree BW have an even larger surface area than other activated carbon fibers.
00 no 1? /g to 2600 ar? /9 is possible. Furthermore, the metal content is extremely low, and anions such as chloride ions are hardly introduced during the activation process, resulting in high purity. Moreover, since it is fibrous and made of aluminum, it can be made into fabrics such as ah cloth and non-woven fabric, and cloth without using a binder.

Furthermore, those obtained from noporazoku type phenolic resins are
It has the highest fiber strength, excellent heat resistance, and has a uniform density when made into cloth.

For these reasons, activated carbon fibers have a larger electric capacity than activated carbon particles, and the electrolyte is less likely to be decomposed, improving the charge/discharge cycle life. Among them, activated carbon fibers made from novolac type phenolic resin are the best.

Furthermore, activated carbon fibers alone do not necessarily provide good electrical contact with the positive electrode case. Therefore, it is necessary to provide a metal current collector inside the activated carbon fiber or on the surface that contacts the positive electrode case, but in this case, 'Ag.

A reliable and good method is to thermally spray a metal such as Ni or Ti onto one side of an activated carbon sheet and weld all the adhering feces to the positive electrode case. %, plasma spraying has strong metal adhesion strength and good metal purity.

On the other hand, a lithium alloy is used as the negative electrode. The materials for this lithium alloy are basically a single metal or an alloy that has the function of absorbing and releasing dilithium during charging and discharging, and lithium. It is possible to use a material that has lithium occluded by charging it in a liquid, or one that has been alloyed with lithium using a normal alloy formulation.

Preferably, the metals are bismuth, lead, M, and cadmium, and the alloy is an alloy of at least two metals selected from these four types. In particular, bismuth, lead,
An alloy made of tin and cadmium has a low melting point of 100'C or less and is soft, and although the reason is not clear, it is easy to absorb lithium and it is also easy to ionize and liberate lithium electrochemically. By occluding lithium in this alloy, lithium is injected into the alloy lattice, but even when charged and discharged, lithium does not accumulate on the metal surface unlike when lithium metal is used alone. Moreover, as lithium enters the alloy, the lithium that participates in charge and discharge does not form lithium branches or lumps on the alloy, so the charge and discharge cycle life can be dramatically improved.

This alloy is generally known as a low melting point alloy and is used in solders and other products. Typical examples are bismuth, lead, tin,
An alloy with a weight ratio of cadmium of 50:24:12:14 is known as a Wood alloy.

For the negative electrode material used in the present invention, a material with a composition close to that of this wood alloy is optimal, but it is not necessarily limited to this. However, it is fully usable. Among these components, cadmium has a particularly strong binding effect, and those to which cadmium is added have strong strength even after lithium is occluded.

Furthermore, it is necessary to establish electrical continuity between this alloy and the sealing plate, but this alloy alone cannot necessarily provide sufficient electrical contact. This is because by occluding lithium in the alloy, the alloy hardens and its elasticity decreases, and the sealing plate cannot be electrically welded. Therefore, nickel and titanium.

It is preferable to press a metal current collector made of a screen or lath plate of stainless steel, aluminum, etc. to the alloy. This metal current collector maintains the elasticity of the alloy and provides good electrical contact. Further, by further improving the electrical conductivity with the metal current collector and combining it with a negative electrode made of a lithium alloy, it is possible to realize an electrochemical device whose charge/discharge cycle life is significantly improved.

In addition, as the electrolyte, propylene carbonate, γ-
Li(JO4°LiBF4.
LiAd (J4.LiPF6fl, etc.) alone or a mixture thereof can be used as a solute.

Description of examples Hereinafter, the present invention will be explained in more detail with reference to Examples.

Example 1 A fibrous novolak phenol resin was processed into a sheet with a thickness of 0.8 mm, and then activated at about 1000°C to obtain activated carbon fibers with a specific surface area of 2300 npg by the BET method. .

Next, a current collector was formed on one side of this fiber sheet by plasma spraying aluminum using a Plasmatron spraying device manufactured by Plasmadyne. Note that argon gas was used as the plasma atmosphere gas during thermal spraying, and aluminum was coated to a thickness of 100 to 300 μm. In this manner, a woven fabric sheet of activated carbon fiber having a sprayed aluminum layer on one side was punched out to a size of 14 mm in diameter to form a positive electrode.

Next, an alloy (wood alloy) with a weight ratio of Bi, Pb, Sn, and Cd of 60:24:14:12 was made into a 5-thickness alloy.
This is rolled into a 0 μm sheet, immersed in an electrolytic solution having the same composition as the subsequent electrolytic solution, and electrolytically reduced using the lithium electrode as a counter electrode to occlude lithium into the alloy to obtain a lithium alloy. The amount of lithium absorbed was approximately 10% by weight based on the alloy. Dry this in dry air and
After crimping a 60-mesh screen made of nickel wire of 60 g, it was punched out to form a negative electrode material. 1 mol/L i B F4 in propylene carbonate as an electrolyte
l! A solution was prepared at a concentration of .

An electrochemical device as shown in FIG. 1 was made using these materials. Its size is 20n in diameter.

The thickness is 1.6 mm.

First, an insulating sealing ring 1 made of polypropylene is combined with a stainless steel sealing plate 2 that also serves as a negative terminal.
Place the opening on top. Then, a negative electrode 3 made of a lithium alloy is placed in the sealing plate 2, and a nickel screen 4 as a negative electrode current collector is spot welded to the sealing plate 2. Next, a dish-shaped separator 5 is inserted, and the electrolytic solution described above is poured into it.

Thereafter, a positive electrode 7 made of a woven fabric sheet of activated carbon fiber with a current collector 6 made of a sprayed aluminum layer formed on one side was placed in the center of a positive electrode case 8 made of nickel-free stainless steel, and spot welded. After that, the electrolyte solution mentioned above is injected. The above-described assembled sealing plate is fitted into this positive electrode case, and the case opening is caulked inward to seal it.

The element thus obtained is designated as A.

Example 2 The fiberized rayon was treated in the same manner as in Example 1.

Other elements were made in the same manner. Let this be B.

Example 3 Fiberized polyacrylonitrile was treated in the same manner as in Example 1, and element C was produced in the same manner.

Example 4 An alloy with a weight ratio of Bi and Pb of 60:50 was prepared, and a negative electrode of the same size was produced using the same process as in Example 1, and a device was manufactured using the same process as in Example 1. made.

Example 5 An alloy with a composition of Pb and Sn in a weight ratio of 50:50 was prepared, and a negative electrode of the same size was made using the same treatment as in Example 1. Element E was made using the same process as in Example 1, except for the same process as in Example 1. Had made.

Example 6 An alloy with a composition of 60:30:20 of Bi, Pb, and sn0 was prepared, and a negative electrode of the same size was made using the same process as in Example 1. The device was fabricated in the same manner as in Example 1. I made F.

Example 7 Element 0 was produced in the same manner as in Example 1 except that an alloy having a weight ratio of Bi to Cd of -to:30 was used.

Example 8 Using an alloy with a weight ratio of Pb and Cd of 70:30,
Element H was produced in the same manner as in Example 1 in other respects.

Comparative Example 1 Element 1 was produced in the same manner as in Example 1 except that the negative electrode was made of lithium metal alone.

Example 9 20% by weight of fluororesin was added to activated carbon particles with a specific surface area of 120077//f by the BET method, and kneaded to a thickness of 0.
．． After forming into a sheet of 8 NM, an aluminum lath plate was crimped and punched to a diameter of 14 mW to form a positive electrode. Element J was made in the same manner as in Example 1 in all other respects.

Comparative Example 2 Element K was fabricated using the same positive electrode as Hoeimantsu, and using lithium metal alone as the negative electrode, but otherwise the same as in Example 1.

These devices were repeatedly charged and discharged for 1 hour at a constant current of 0.3 mA, and the number of cycles until the discharge sustaining voltage fell below 1.5 V was determined. The results are shown in the table below.

As is clear from the table, the comparative examples were maintained.

In addition, the number of charge/discharge cycles is improved in case of using activated carbon fiber for the positive electrode and lithium metal for the negative electrode, although the number of charge/discharge cycles is improved by using activated carbon particles for the positive electrode. low.

On the other hand, J made the negative electrode an alloy and the positive electrode chlorinated activated carbon particles.
In the case of using lithium metal alone as the negative electrode, it is better than K, but compared to humans, f is lower than that of combining activated carbon fibers and alloys? A synergistic effect is exhibited Furthermore, by comparing A, B, and C, it can be seen that among activated carbon fibers, the one obtained from novolank type phenolic resin is the best.

On the other hand, according to a comparison of negative electrode alloys A, D, E, F, G, and H, there is no significant difference in characteristics even if the alloy composition changes. However, when comparing the properties among them, wood alloy A, which has almost the same composition, is the best, followed by G and H, which contain cadmium, and alloys without cadmium are slightly inferior.

Then, add 1 mol/l of LiBF4 as an electrolyte □
Although propylene carbonate was used, it is not necessarily limited to this, and solvents such as γ-butyrolactone, 1,2-dimethoxyethane, methyl carbonate,
Tetrahydrofuran, dioxolane, etc. alone or as a mixture, as a solute LiClO4, LiAj? (J4
．． Any lithium salt such as LiPF6 can be used.

Although the above embodiments have been described using a flat coin-shaped structure, similar effects can be expected with other button-shaped or cylindrical element structures.

Effect of the invention Obtainable.

[Brief explanation of the drawing]

The drawing is a longitudinal cross-sectional view of an electrochemical element in an example of the present invention. 2... Sealing plate, 3...: Negative electrode, 4...
...-Negative electrode current collector, 5... Separator, 6...
...-Positive electrode, 7...Positive electrode current collector, 8...
...Positive electrode case.

Claims (1)

[Scope of Claims] (1) A rechargeable electrochemical device comprising a negative electrode made of a lithium alloy that absorbs and releases lithium through charging and discharging, a positive electrode made of activated carbon W, and a non-aqueous electrolyte. (The rechargeable electrochemical device according to claim 1, wherein the lithium alloy is an alloy containing at least one selected from the group consisting of bismuth, tin, lead, and cadmium. (3) The lithium alloy 3. A rechargeable electrochemical device according to claim 2, wherein the rechargeable electrochemical device comprises cadmium and at least one member selected from the group consisting of bismuth, tin, and lead.