JPH0451949B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a rechargeable electrochemical device used as a mobile DC power source, a backup power source, or the like. BACKGROUND ART As a rechargeable electrochemical device, a device is known that uses activated carbon mainly using an electric double layer as a positive electrode, and uses a non-aqueous organic solvent in a lithium alloy electrolyte as a negative electrode. The electrolyte in this device contains propylene carbonate (hereinafter referred to as PC) and 1,2-
A lithium salt such as lithium borofluoride was dissolved in a mixed solvent with dimethoxyethane (hereinafter referred to as DME). Problems to be Solved by the Invention Various investigations have been conducted on the known device having this conventional configuration, and it has been found that when long-term discharge is performed in a high temperature or high temperature and humidity atmosphere, and discharge is performed to near OV (over-discharge state), It was found that gas generation and an abnormal increase in internal resistance were observed within the device, which did not recover even after charging, and the electrical capacity significantly decreased. A similar phenomenon was also observed when a device that was in an overdischarged state at room temperature was stored at a high temperature or in a hot and humid atmosphere. The reason for this is not clear, but when discharging in a high-temperature atmosphere, active lithium generated on the surface of the activated carbon of the positive electrode reacts with DME or a small amount of moisture, causing reactions such as decomposition of DME, and this phenomenon occurs. It is thought that this is particularly noticeable in high-temperature atmospheres. Conventionally, PC and γ-butyrolactone (hereinafter referred to as BL) have been investigated as solvents for the electrolyte in this type of electrochemical device, but recently,
PC has excellent strong load discharge characteristics due to its high dielectric constant, but is inferior in its ability to form a protective film on the lithium surface.Also, BL has inferior strong load discharge characteristics compared to PC, but it does It was found that the ability to form a protective film was very excellent. As a result, it has become clear that the addition of BL is very effective in solving the above-mentioned problem. At the same time, the amount of lithium in the negative electrode alloy is also involved in the above-mentioned problem, and if the amount of lithium in the load alloy is excessive, the discharge reaction
It was found that the amount of active lithium generated on the surface of the positive electrode was excessive, and that the above-mentioned problem could not be solved by adding BL alone. It is an object of the present invention to solve the above-mentioned problems and to improve recovery characteristics by charging after overdischarging in a high-temperature atmosphere without impairing conventional performance. Means for Solving the Problem In order to solve this problem, the present invention has investigated the mixing ratio of the aforementioned PC, BL, and DME as the solvent of the electrolytic solution, and as a result, the composition ratio of BL has been changed to 3% of the total solvent.
~50% by volume, and the amount of lithium in the negative electrode lithium alloy is 5wt% or less of the entire alloy. Function According to this configuration, BL acts on the active lithium surface generated on the activated carbon surface of the positive electrode due to the discharge reaction, forming a protective film, and the reaction between lithium and DME or trace moisture in a high temperature atmosphere. can be suppressed. Furthermore, since the amount of lithium in the negative electrode alloy is limited, a large amount of lithium does not move to the positive electrode activated carbon, making it possible to maximize the BL effect. As a result, even after long-term overdischarge to near OV in a high-temperature atmosphere, it can be fully recovered by charging, and there is little capacity deterioration, dramatically improving the recovery characteristics after overdischarge in a high-temperature atmosphere. be able to. In addition, since it is effective if the composition ratio of BL in the electrolyte is at least a few percent of the total solvent in terms of volume ratio, there is almost no need to change the conventional composition ratio, which means that the heavy-load discharge characteristics will also deteriorate. There's nothing to do. PC-BL two-component system excluding DME or
Solvents consisting of PC and BL alone are also effective for the above-mentioned problems, but in that case, the viscosity of the solvent becomes high, and the wettability of the impregnating material, positive electrode activated carbon, etc. becomes very poor, and the number of steps increases. This causes manufacturing problems such as: Embodiments Embodiments of the present invention will be described below with reference to FIGS. 1 to 3. (Example 1) FIG. 1 shows a rechargeable electrochemical device using activated carbon as a positive electrode and a lithium alloy as a negative electrode. 1 in the diagram
is a case that also serves as a positive electrode terminal, 2 is a sealing plate that is punched from the same material as the case and serves as a negative electrode terminal, 3 is a polypropylene gasket that insulates the case and the sealing plate, and 4 is a positive electrode, which is made of activated carbon powder with a weight of 70 kg. 10 parts by weight of acetylene black, which is a conductive material, and 20 parts by weight of an aqueous dispersion of fluororesin (about 60% solid content), which is a binder, are kneaded and passed between two rolls to form a sheet. After molding, it was transferred to a titanium lath plate with a thickness of 0.2 mm, which forms the positive electrode current collector 5. Then under vacuum at 150℃
After drying for 12 hours, the thickness is 0.8mm and the diameter is
A 14.5 mm pellet was punched out, a part of the mixture was peeled off to expose a titanium lath plate, and this was welded to Case 2. 6 is a lithium alloy for the negative electrode, containing 50 parts by weight of lead, 47 parts by weight of cadmium, and 3 parts by weight of lithium.
It was melted and alloyed in an argon atmosphere, rolled to a thickness of 0.1 mm in the same atmosphere, and transferred to Nickel Net 7. Thereafter, it was punched out to a diameter of 15 mm and welded to the back surface of the sealing plate 3. 8 is a separator made of polypropylene nonwoven fabric. The solvent of the electrolyte is PC,
The composition ratios (volume ratios) of BL and DME were varied as shown in the following table, and lithium borofluoride was dissolved as a solute in each of them at a ratio of 1 mol/mol. These electrochemical devices were designated as A-1 to A-10. However, A-1
is the conventional product mentioned above.

[Table] (Example 2) 50 parts by weight of lead and cadmium were used as the negative electrode alloy described above.
An electrochemical device was made using an alloy having a composition ratio of 45 parts by weight, 5 parts by weight of lithium, 50 parts by weight of lead, 40 parts by weight of cadmium, and 10 parts by weight of lithium, and the other configuration was completely the same as in Example 1. Each B-1
- B-10 and C-1 to C-10. In addition, the size of the device in both Examples 1 and 2 is 20 mm in diameter.
mm, thickness 2.0mm. Further, in these examples, lead and cadmium were used for the negative electrode alloy, but other materials such as bismuth and indium having lithium storage capacity may also be used. Furthermore, lithium perchlorate (LiClO ₄ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), lithium aluminum chloride (LiAlCl ₄ ), etc. can be similarly applied as the solute of the electrolytic solution. Using these devices, we connected a 10KΩ constant resistance in an atmosphere of 60°C and discharged for 30 days to reduce the voltage of the device.
After lowering the voltage to around 0V, it was charged at a constant voltage of 3V at room temperature, and discharged with a constant resistance of 10KΩ.The discharge capacity from 3V to 2V was determined and shown in Figure 2. In addition, from 3V of a device that has not been over-discharged under the conditions described above,
The discharge capacity up to 2V is 10KΩ constant resistance, and A is
1.20mAh, B 1.21mAh, and C 1.20mAh (each value is an average of 5 pieces). As can be seen from FIG. 2, compared to the conventional structure of A-1, the one with BL added recovers by charging even after overdischarging in an atmosphere of 60° C., and has less capacity deterioration. However, when the amount of lithium in the negative electrode alloy is excessive like C, the BL effect decreases. Next, in device A, constant resistance discharge of 10KΩ and 1KΩ was performed at room temperature using the device before the above-described overdischarge test, and the discharge capacity from 3V to 2V was determined. The results are shown in FIG. As shown in Figure 3, as the composition ratio of BL increases, there is almost no difference in discharge capacity during low load discharge of 10KΩ, but there is a tendency for it to decrease during heavy load discharge of 1KΩ. Effects of the invention As is clear from the above explanation, by limiting the amount of lithium in the negative electrode alloy and adding BL as a solvent to the electrolyte, it is possible to reduce the OV in a high temperature atmosphere.
It is possible to obtain an excellent electrochemical device that can be sufficiently recovered by charging even if overdischarged to a certain point.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view of a rechargeable electrochemical device according to an embodiment of the present invention, and FIGS. 2 and 3 are diagrams showing discharge capacities of various electrochemical devices according to embodiments of the present invention. DESCRIPTION OF SYMBOLS 1... Case, 2... Sealing plate, 3... Gasket, 4... Positive electrode, 5... Positive electrode current collector, 6... Negative electrode, 8... Separator.

Claims (1)

[Claims] 1. An electrochemical device comprising a positive electrode made of activated carbon, a negative electrode made of a lithium alloy, and an electrolytic solution made of a non-aqueous solvent and a solute, wherein the amount of lithium in the negative electrode alloy is equal to the total amount of the alloy. 5 wt% or less of A rechargeable electrochemical device. 2. The rechargeable electrochemical device according to claim 1, wherein the solute of the electrolytic solution is lithium borofluoride. 3. The rechargeable electrochemical device according to claim 1, wherein the solute of the electrolytic solution is lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ). 4. The rechargeable electrochemical device according to claim 1, wherein the solute of the electrolytic solution is lithium aluminum chloride (LiAlCl ₄ ).