JPH0351066B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a sodium-sulfur battery, and in particular uses molten sodium as a cathode active material, molten sulfur impregnated with carbon fiber as an anode active material, and a solid electrolyte as an electrolyte. This relates to a sodium-sulfur battery.

[Background of the invention]

Sodium-sulfur batteries have molten sodium, the cathode active material, on one side and molten sulfur, the anode active material, on the other side, through a solid electrolyte that allows only sodium ions to pass through, and are charged and discharged at approximately 300 to 350 degrees Celsius. It is a high temperature secondary battery. This charging/discharging reaction is 2Na + xS discharge ---> ← -- Charged Na ₂ Sx. During discharge, sodium Na in the cathode active material liberates electrons and becomes sodium ions, which pass through the partition wall of the solid electrolyte and form sulfur in the anode active material. Reacts with S to produce sodium polysulfide, Na ₂ Sx. Then, during charging, by applying a negative voltage greater than the open circuit voltage of the battery, sodium polysulfide Na ₂ Sx
is separated into sodium Na and sulfur S.

FIG. 1 shows a conventional example of this type of sodium-sulfur battery. As shown in the figure, the sodium-sulfur battery has a tubular solid electrolyte 1 filled with molten sodium 2 as a cathode active material.
The outside is filled with molten sulfur 3, which is an anode active material, but since this molten sulfur 3 has no electronic conductivity, it is impregnated into porous carbon fibers 4 to impart electronic conductivity. . The anode container 5 that seals the molten sulfur 3 and the cathode container 6 that seals the molten sodium 2 each have a function as an electrode. are insulated and connected. The anode cap 8 provided in the anode container 5 and the cathode cap 9 provided in the cathode container 6 both have the function of electrodes in the same way as the cathode containers 6 and 5. The inserted porous metal fibers 10 serve as molten sodium 2 to prevent a rapid exothermic reaction between molten sodium 2 and molten sulfur 3 in the event that the solid electrolyte 1 is damaged.
It has the function of a holding material. The cathode cap 9 is also provided with a sodium injection tube 11 for injecting sodium into the cathode container 6. In the figure, 2a is a cathode having molten sodium 2, metal fibers 10, and the like.

Anode 3 having carbon fiber 4 impregnated with molten sulfur 3 in a sodium-sulfur battery constructed in this way
The result of analyzing the reaction rate at a in a cylindrical coordinate system is the thickness of the anode 3a on the horizontal axis and the thickness of the anode 3a on the vertical axis.
FIG. 2 shows the change characteristics of the reaction rate depending on the thickness of the anode 3a, taking the reaction rate of a. As shown in the figure, the reaction rates on the solid electrolyte side P and the metal anode container side Q were high and non-uniform; however, if the reaction rate on the solid electrolyte side P is high, the following problems occur. During discharge, the anode on the solid electrolyte side P has a high reaction rate, so sodium polysulfide Na ₂ Sx (x≦3), which has a high melting point and has insulating properties, is generated in the anode on the solid electrolyte side P. An insulating layer is formed on the anode to reduce discharge capacity. Sodium polysulfide Na ₂ Sx generated by discharging during charging separates into sodium Na and sulfur S, but this separation phenomenon is noticeable on the solid electrolyte side P where the reaction rate is high, and the anode on the solid electrolyte side P A large amount of molten sulfur S, which has no electronic conductivity, is generated inside the battery, reducing the charging capacity.

These things are true for sodium with a theoretical capacity of 56Ah.
This is also clear from FIG. 3, which examines the relationship between the charge and discharge amount and the terminal voltage for a sulfur battery, with the horizontal axis representing the charge and discharge amount and the vertical axis representing the terminal voltage. In other words, the discharge capacity is determined by the discharge characteristic curve A that shows the discharge characteristics.
The voltage drops rapidly near 50Ah. This is because, as mentioned above, the insulating sodium polysulfide Na ₂ Sx (x≦3) formed during discharge is formed in the anode on the solid electrolyte side, where the reaction rate is high.
This reduces the discharge capacity of the battery. Then, the charging capacity is determined by charging characteristic curve B, which shows the characteristics during charging.
The voltage increases rapidly near 30Ah. This occurs near the point where the single-phase region of sodium polysulfide Na ₂ Sx changes to the two-phase region of sodium polysulfide Na ₂ Sx and molten sulfur S, that is, near 30 Ah, an insulating layer of molten sulfur S is formed on the solid electrolyte side with a high reaction rate. was formed, and the internal resistance in the anode suddenly increased.

[Purpose of the invention]

The present invention has been made in view of the above points,
The object of the present invention is to provide a sodium-sulfur battery that can improve charging and discharging efficiency.

[Summary of the invention]

That is, the present invention includes a cathode having a cathode active material, an anode having an anode active material, and a solid electrolyte disposed between them, in which molten sodium is used as the cathode active material, and carbon fiber is used as the anode active material. A sodium-sulfur battery using molten sulfur impregnated with molten sulfur, characterized in that a carbon fiber having a surface area larger than that of the carbon fiber impregnated with molten sulfur is provided between the anode and the solid electrolyte. As a result, the surface area ratio of the carbon fibers of the solid electrolyte, that is, the surface area ratio of the carbon fibers impregnated with molten sulfur to the carbon fibers having a larger surface area than the carbon fibers becomes smaller.

In order to improve the charging and discharging efficiency of sodium-sulfur batteries, we focused on carbon fibers impregnated with molten sulfur. Next, carbon fibers with a large surface area are provided on the solid electrolyte side of the anode, and the reaction rate on the solid electrolyte side of the anode is examined by changing the ratio of carbon fibers with a small surface area impregnated with molten sulfur, that is, the surface area ratio. did. The results of the study are shown in Figure 4, which plots the reaction rate on the solid electrolyte side of the anode on the vertical axis and the surface area ratio of the carbon fiber on the horizontal axis, and shows the change characteristics of the reaction rate depending on the surface area ratio of the carbon fiber. As shown, the smaller the surface area ratio, the smaller the reaction rate on the solid electrolyte side of the anode. Based on this result, we fabricated a battery by installing carbon fiber with a larger surface area than the carbon fiber impregnated with molten sulfur on the solid electrolyte side of the anode, and analyzed the reaction rate of the anode depending on the thickness of the anode using a cylindrical coordinate system. did. The study results are as shown in Figure 5, where the horizontal axis shows the anode thickness and the vertical axis shows the reaction rate of the anode. Electrolyte side P
It was observed that the reaction rate had decreased and the reaction rate was more uniform than before. For these reasons, by providing carbon fibers with a large surface area on the solid electrolyte side of the anode and reducing the surface area ratio with the carbon fibers impregnated with molten sulfur, the reaction rate on the solid electrolyte side of the anode can be reduced. It was confirmed that uniformity could be achieved. Therefore, in the present invention, carbon fibers having a larger surface area than carbon fibers impregnated with molten sulfur are provided between the anode and the solid electrolyte. By doing so, it is possible to obtain a sodium-sulfur battery with improved charging and discharging efficiency.

[Embodiments of the invention]

The present invention will be explained below based on the illustrated embodiments. FIG. 6 shows an embodiment of the invention. Note that parts that are the same as those in the conventional system are given the same reference numerals, and therefore their explanations will be omitted. In this embodiment, a carbon fiber 1 having a surface area larger than that of the carbon fiber 4 impregnated with molten sulfur 3 is placed between the anode 3a and the solid electrolyte 1.
2 was established. By doing this, the anode 3a
Since the surface area ratio of the carbon fibers 4 and 12 on the solid electrolyte 1 side is reduced, it is possible to obtain a sodium-sulfur battery in which charging and discharging efficiency can be improved.

This is due to the theoretical capacity created by providing the carbon fiber 12 with a large surface area on the solid electrolyte 1 side of the anode 3a.
This is clear from the results of examining the relationship between the charge/discharge amount and terminal voltage for a 56Ah sodium-sulfur battery. The study results are shown in Figure 7, where the vertical axis shows the terminal voltage and the horizontal axis shows the amount of charge and discharge.During discharging, the discharge characteristic curve
As shown in A ₀ , no significant drop in terminal voltage was observed up to 56Ah, and during charging,
As shown in the charging characteristic curve _B0 , even near 30Ah where the liquid becomes a two-phase liquid, no increase in terminal voltage was observed as in the conventional case. The reason why such good charge/discharge characteristics were exhibited is that the reaction rate on the solid electrolyte side of the anode decreased and the reaction rate of the anode became uniform. In other words, sodium polysulfide has a high melting point and insulating properties due to uniform reaction rate during discharge.
The generation time of Na ₂ Sx (x≦3) is delayed, and the discharge capacity is improved. During charging, molten sulfur S having no electron conductivity is locally generated due to uniformity of the reaction rate, and an insulating layer is no longer formed on the anode on the solid electrolyte side, thereby improving the charging capacity.

〔Effect of the invention〕

As mentioned above, in the present invention, the reaction rate of the anode becomes uniform, the charge/discharge capacity improves, and the sodium-containing material improves the charge/discharge efficiency.
You can get sulfur batteries.

[Brief explanation of drawings]

FIG. 1 is a longitudinal side view of a conventional sodium-sulfur battery, and FIG. 2 is a characteristic diagram showing the relationship between the thickness of the anode and the reaction rate of the anode in a conventional sodium-sulfur battery.
Figure 3 is a characteristic diagram showing the relationship between the charge/discharge amount and terminal voltage of a conventional sodium-sulfur battery, and Figure 4 shows the surface area ratio between carbon fibers with a large surface area and carbon fibers with a small surface area in a sodium-sulfur battery. Figure 5 shows the relationship between the thickness of the anode and the reaction rate of the anode when carbon fiber with a large surface area is provided on the solid electrolyte side of the anode of a sodium-sulfur battery. FIG. 6 is a longitudinal cross-sectional side view of one embodiment of the sodium-sulfur battery of the present invention, and FIG. 7 is a characteristic diagram showing the relationship between charge/discharge amount and terminal voltage of the same embodiment. 1... Solid electrolyte, 2... Molten sodium, 2
a... cathode, 3... molten sulfur, 3a... anode, 4
... Carbon fiber impregnated with molten sulfur, 5 ... Anode container, 6 ... Cathode container, 7 ... α-alumina, 10
...Metal fiber, 12...Carbon fiber with a large surface area.

Claims (1)

[Scope of Claims] 1 Comprising a cathode having a cathode active material, an anode having an anode active material, and a solid electrolyte disposed between them, molten sodium is used for the cathode active material, and the anode active material In a sodium-sulfur battery in which molten sulfur impregnated with carbon fiber is used as a material, a carbon fiber having a surface area larger than that of the carbon fiber impregnated with molten sulfur is provided between the anode and the solid electrolyte. A sodium-sulfur battery characterized by comprising: 2. The sodium-sulfur battery according to claim 1, wherein the carbon fiber impregnated with molten sulfur is formed by firing synthetic fiber at about 2000°C. 3. The sodium-sulfur battery according to claim 1, wherein the carbon fiber with a large surface area is formed by firing synthetic fiber at about 800° C. or lower.