JPS6151774A - Sodium-sulfur battery - Google Patents

Abstract

PURPOSE:To obtain a sodium-sulfur battery having high utilization factor of cathode active material and long life by forming a cathode comprising carbon fibers impregnated with molten sulfur so as to have density gradient of carbon fibers in an axis direction. CONSTITUTION:A cathode is formed by accommodating sulfur 3 and carbon fibers 4 such as PAN carbon fibers in a container and heating at about 150 deg.C in an atmosphere of inactive gas. The cathode is divided into three pieces, for example. The piece 4a is formed by stacking n pieces of carbon fiber felts each of which has a specified thickness, and the pieces 4b and 4c are formed by stacking (n-1) pieces and (n-2) pieces of carbon fiber felts respectively. Or density gradient is formed by molding carbon felts having different length, then impregnating molten sulfur in the felts. Thereby, moving down of molten sulfur caused by gravity can be prevented. Therefore, a sodium-sulfur battery having high utilization factor of cathode active material, long life, and high energy density is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to sodium-sulfur batteries, in particular molten sulfur impregnated into carbon fibers and electrolyte in the cathode active material. The present invention relates to a sodium-sulfur battery that uses a solid electrolyte and is suitable for improving the utilization rate of anode active material.

[Background of the invention]

Up until now, attempts have been made to improve the efficiency of IJ Umu sulfur batteries, and in particular, the
In the publication, various improvements to improve the depth of charge and discharge, that is, increase the charging capacity, of the Nato IJ Umu sulfur battery are disclosed. However, even with these improved techniques, the characteristics of this type of battery, which has a high energy density of a theoretical value of 784 wh/Kf, have not been fully brought out.

As a result of intensive research aimed at further improving the efficiency of this sodium-sulfur battery, the present inventor discovered that one of the major causes of decreasing the efficiency of this type of battery is that sulfur, which should act as an anode active material, is absorbed by gravity. It was found that this is because the particles hang down to the bottom of the battery and no longer contribute sufficiently to the reaction. Therefore, in order to fully exploit the excellent characteristics of the sodium-sulfur flc battery, it is necessary to make some kind of improvement to reduce the sagging of molten sulfur due to gravity.

[Purpose of the invention]

An object of the present invention is to provide a sodium-sulfur battery that has a high utilization rate of anode active material and has an improved lifespan.

[Summary of the invention]

The present invention provides a cathode 2 having a cathode active material, an anode 3 having a cathode active material, and a solid electrolyte 1 disposed between them.
In the Chidori and Kumu sulfur batteries, in which molten sodium is used as the cathode active material and molten sulfur impregnated with carbon fiber is used as the anode active material, 7
(With respect to the anode made of molten sulfur, a density gradient is imparted to the carbon fibers in the axial direction. This structure allows sulfur, which is the anode active material, and sodium polysulfide, which is a battery reaction product. NazSx) is prevented from hanging down to the bottom of the battery due to gravity, thereby preventing a decrease in the utilization rate of the active material.Also, in order to prevent the influence of the hanging of the active material, carbon fiber is used in all anode areas. It would be better to increase the density of the active material, but such a method has the disadvantage that the initial amount of photon of the active material decreases, and the theoretical capacity of the battery is already reduced at the time of battery design.

In other words, the theoretical capacity of a battery is determined by the initial filling amount of active material.
Our theoretical capacity is 0.5574Ah.

Tsumu υ, the initial theoretical capacity of the battery is the initial sulfur capacity x 8
'xo, 5574Ah/8' and Naru.

Here, in order to facilitate understanding of the present invention, the operating principle of the sodium-sulfur battery according to the present invention will be explained. A sodium-sulfur battery is equipped with a solid electrolyte 1f that allows only sodium ions to pass through, molten sodium 2 which is the cathode active material on one side, and 3 molten sulfur which is the anode active material on the other side, and is charged at about 300 to 350C. It is a high-temperature secondary battery that exhibits radiation. This charging/discharging reaction is a discharge 2Na+xS Na25X charge, and during discharging, sodium in the cathode active material liberates electrons and becomes sodium ions, which pass through the partition wall of the solid electrolyte and react with the sulfur in the anode active material, forming polysulfides. Sodium Naz
Generate SX. At the time of secondary charging, by applying a negative voltage greater than the open circuit voltage of the battery, sodium polysulfide N
a25X Decomposed into sodium Na and sulfur S. This type of battery is charged and discharged by the above reaction, so if the sulfur involved in the reaction hangs down to the bottom of the stain pond due to gravity and no longer participates in the reaction, the efficiency of the battery will immediately decrease. This will lower the This phenomenon is shown in Figure 1 in the conventional case).
The IJ Umu sulfur battery will be explained. Note that in the battery used in this experiment, the amount of carbon fiber was intentionally reduced to make the effect of drooping of the active material more noticeable.

When we measured the depth of discharge and voltage in this battery, we found that the second
The result will be as shown in the figure.

That is, in this battery with a low carbon content, as shown by curve a, a sudden drop in voltage occurs when the depth of discharge is around 95 inches. The reason for this is that carbon fibers are coarse, so the molten sulfur cannot be sucked up or held by capillary action, and the molten sulfur (due to gravity) hangs down to the bottom of the battery, causing the sulfur that affects the reaction to drop. This is because the amount decreases. The fact that such a phenomenon occurs is further evident from the experiment shown in FIG. 3, in which the amount of drooping of the active material was measured using simulated anodes with different carbon fiber contents.

Based on these experimental results, the present invention has developed a carbon fiber shaft that penetrates through the molten sulfur in order to reduce the gravitational droop of molten sulfur, which is the active material of the anode, and improve the efficiency and life of the battery. This was done based on the discovery that it is effective to fill the tank with a density gradient in the direction and to absorb or retain the molten sulfur through capillary action. Furthermore, the lifespan of the solid electrolyte is cited as a factor that determines the lifespan of a battery.

One of the factors that affects the life of this solid electrolyte is damage due to local concentration or non-uniformity of current density, but in the battery of the present invention, there is no drooping of the active material, so local concentration of current density is prevented. There is no unevenness in the electrocardiogram, and this is also effective in improving the lifespan of the electrocardiogram.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to FIGS. 4 and 5. An anode obtained by impregnating carbon fiber with sulfur is obtained by storing sulfur 3 and, for example, P to N-based carbon fiber 4 in an integrated jig, and heating it to about 150C in an inert gas, for example. It will be done.

As a method of imparting a density gradient to carbon fibers, the anode is divided into three pieces, 4a, 4b, and 4c, as shown in Figure 4, and for piece 4a, carbon fibers of a specified thickness are made into felt. In addition to the method of impregnating molten sulfur by piling up n sheets of 4b, (n-1) pieces of piece 4b, and (n-2J pieces of piece 44C), as shown in Figure 5, An anode with a density gradient can also be obtained by forming felted carbon fibers without dividing them into different lengths and impregnating them with molten sulfur.Aside from these methods, there are various methods for providing a density gradient. Needless to say, this is possible.

In the sodium-sulfur battery obtained in this way, there is almost no sulfur drooping due to gravity, and when we measured the depth of discharge and voltage, we found that the depth of discharge was 10% as shown in Figure 2.
There is hardly any sudden voltage drop down to 0. This is considered to be a significant effect compared to the capacity reduction of about 5 cm in conventional batteries. In other words, it is generally accepted that the capacity of a practical plant is about 100 Mwt, and if the loss due to drooping of the active material in a single cell is about 5 inches, the loss for the entire plant is 5 MWh, which is a considerably large loss. . In order to compensate for this loss of 5 MWh, approximately 5,000 more practical-scale single cells are required (4
'1') The space required and the cost of producing the pond cannot be ignored.

〔Effect of the invention〕

According to the present invention, it is possible to prevent molten sulfur, which is an anode active material, from sagging due to gravity, and as a result, it is possible to realize a high energy density sodium-sulfur battery with a high utilization rate of the anode active material and a long life. It will be done.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of the conventional technology and the present invention's NatoIJumu sulfur battery, FIG. 2 is a charge-discharge characteristic diagram of the sodium-sulfur battery having the structure shown in FIG. 1, and FIG. Figures 4 and 5, which show the relationship between the amount of active material and the drooping of the active material due to gravity, are diagrams showing the anode of the present invention. DESCRIPTION OF SYMBOLS 1... Solid electrolyte, 2a... Cathode, 2... Sodium, 3... Sulfur, 4... Carbon fiber, 5... Anode container, 6... Cathode container, 7... α-Alumina, 8.
...Anode cap.

Claims (1)

[Scope of Claims] 1. A cathode having a cathode active material, an anode having an anode active material, and a solid electrolyte disposed between them, wherein molten sodium is used as the cathode active material, and the anode 1. A sodium-sulfur battery in which molten sulfur impregnated into carbon fibers is used as an active substance, characterized in that the carbon fibers impregnated with the molten sulfur have a density gradient in the axial direction. 2. The sodium-sulfur battery according to claim 1, wherein the carbon fiber impregnated with molten sulfur is a PAN-based fiber fired at a high temperature. 3. The anode according to claim 1, wherein the anode made of carbon fiber impregnated with molten sulfur is divided in the radial direction and molded with different compression ratios. Sodium-sulfur battery. 4. Claim 1, characterized in that the anode made of carbon fiber impregnated with molten sulfur is formed by felting carbon fiber and changing its length. sodium-sulfur battery.