JPH01200565A - Lithium thermo cell - Google Patents

Abstract

PURPOSE:To improve high effective discharge characteristic of the cell in the caption by setting concentration of lithium chloride in electrolyte composed of compound of potassium chloride and the lithium chloride into a specific value. CONSTITUTION:Concentration of lithium chloride in electrolyte composed of potassium chloride and the lithium chloride is set into 58-90mol%. The compound of the potassium chloride and the lithium chloride becomes eutectic compound when the concentration of the lithium chloride is 42mol% and 58mol%. When it exceeds 58mol%, a solid of the lithium chloride is left in the electrolyte even at a high temperature. This causes that strength of an electrolyte layer increases. Larger amount of Li and S as discharge products are melted into the electrolyte with excess lithium chloride than melted into eutectic electrolyte. Accordingly, an internal resistance of positive liquid is prevented from increasing when the concentration of the lithium chloride is more than 58mol%. When the concentration of the lithium chloride increases, the internal resistance of the electrolyte layer also increases. However, this is not a problem when the concentration of the lithium chloride is less than 90mol%.

Description

[Detailed description of the invention] Industrial applications The present invention relates to improving the performance of lithium-based thermal batteries.

Conventional technology Lithium-based thermal batteries use metallic lithium or lithium alloys (for example, lithium-aluminum alloy, lithium-silicon alloy, lithium-boron alloy, etc.) as the negative electrode active material, ferric sulfide (Fe52) as the positive electrode active material, and electrolytic This is a high-temperature molten salt secondary battery using a mixture of lithium chloride and potassium chloride (eutectic salt consisting of 42 mol% potassium chloride and 58 io 1% lithium chloride) as a liquid.

A feature of lithium-based thermal batteries is that they can discharge large currents of 0.1 to IA/am'. However, in recent years, I A/C
A discharge with an even larger current of 112 or more is now required. In such a large current discharge, conventional thermal batteries have a problem in that the utilization rate of the active material decreases significantly and the energy density of the battery decreases significantly.

There are various causes for the decrease in active material utilization, one of which is local concentration of current. This causes the overvoltage of the reaction to rise abnormally, lowering the cell voltage and reducing the available discharge capacity. Such local concentration of current occurs for the following reasons. The electrolyte layer of a thermal battery is composed of an electrolyte and a powdered separator (M2O is often used), so it is relatively weak, and if there is some unevenness in the pressure on the electrode plates, the electrolyte will deteriorate. The distance between the electrodes tends to become uneven as it melts, and the thickness of the electrolyte layer is extremely thin at 0.2 to 2 mm, so even slight unevenness in the distance between the electrodes tends to cause current concentration. This will lead to a decline in utilization rate.

So @, 1! l Methods have been considered such as using a solid separator such as a ceramic nonwoven fabric in the R layer and increasing the strength of the electrolyte layer by increasing the content of powder separator, but these methods increase the internal resistance of the electrolyte layer. There was a drawback to that.

Another cause of the decrease in the utilization rate when a large current is discharged is an increase in the internal resistance of the positive electrode. This is because Li and S, which are discharge products and are electrically insulating substances, are deposited in a solid phase in the positive electrode plate as the discharge reaction shown by the following equation progresses.

Fed, + 4 Li” + 48-+Fe+ 21
i23 This Li2S dissolves in the electrolytic solution at the beginning of discharge, but when the dissolved amount reaches its limit, it precipitates in the electrode plate and increases the internal resistance.

According to the problem to be solved by the invention, in order to improve the large current discharge characteristics of lithium-based thermal batteries, it is necessary to develop a lithium-ion thermal battery that is stronger than current products, less prone to uneven electrode distance, and has a lower internal resistance. It was necessary to develop a new electrolyte layer and to make solid phase precipitation of Li and S less likely to occur.

Means for Solving the Problems The present invention solves the above problems by controlling the lithium chloride concentration of an electrolytic solution consisting of potassium chloride and lithium chloride to 58° 1% or more and 9011° 1% or less.

A mixture of potassium chloride and lithium chloride each has 4
A eutectic mixture is obtained at a concentration of 2n+o 1% and 58 mol%. And at this time, melting M! ! The temperature in the pot is 352°C, which is also low. When lithium chloride is relatively increased, the melting point is 64io lχ at 400℃, 70no 1%
at 450℃, 79noIX at 500℃, 90mol
The temperature rises to 560℃. Therefore, when the lithium chloride concentration is set to 58 ioH or higher, solid lithium chloride remains in the electrolyte even at high temperatures, increasing the strength of the electrolyte layer. At this time, the internal resistance of the electrolyte layer increases to some extent, but experiments have shown that this hardly poses a problem when the lithium chloride concentration is 90 nk1% or less.

The discharge product Li2S melts in a much larger amount in an electrolyte with excess lithium chloride than in a eutectic electrolyte. Therefore, when the concentration of lithium chloride is set to 58 mol % or more, the precipitation of Li2S is delayed, and an increase in the internal resistance of the positive electrode plate can be prevented.

In this way, the lithium chloride concentration was increased to 58 mol% or more, 90
By controlling the content to 1 at % or less, an electrolyte layer with excellent strength and low internal resistance can be obtained, and an increase in internal resistance of the positive electrode plate can be suppressed.

Example @ Lithium-based thermal batteries were manufactured by varying the lithium chloride concentration in the solution. At this time, a lithium-aluminum alloy was used as the negative electrode active material, and FeS was used as the positive electrode active material.
2 was used. These thermal batteries were heated to 0°C, 2 A/cl'
The active material utilization rate was measured. The results are shown in FIG. 1, which shows that the lithium chloride concentration of the present invention is 5111
It can be seen that the battery with no 1% or more and 90no 1% or less has a better utilization rate than the conventional thermal battery with 58 mol% lithium chloride (eutectic composition).

Effects As described above, according to the present invention, a lithium-based thermal battery with excellent high rate discharge characteristics can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the active material utilization rate of lithium-based thermal batteries with various concentrations of lithium chloride in the electrolyte.

Claims (1)

[Claims] The concentration of lithium chloride in the electrolytic solution consisting of a mixture of potassium chloride and lithium chloride is 58 mol% or more, 90 mol
% or less.