JP3477364B2 - Sodium-sulfur battery - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a positive electrode for a sodium-sulfur battery.

[0002]

2. Description of the Related Art A sodium-sulfur battery has conductivity with respect to a positive electrode active material containing sulfur, a conductive auxiliary material for assisting electron conduction of the positive electrode active material, a negative electrode active material containing sodium, and sodium ions. It is composed of a solid electrolyte, a positive electrode outer can which is a positive electrode terminal, and a sealing lid which is a negative electrode terminal.

The charging / discharging reaction in the negative electrode of the sodium-sulfur battery is as shown in the formula (1). During discharging, sodium, which is the negative electrode active material, is separated into sodium ions and electrons, and the sodium ions pass through the solid electrolyte. And enters the positive electrode side, and the electrons flow to the external circuit via the negative electrode terminal. On the other hand, the charging / discharging reaction in the positive electrode is as shown in the formula (2), and at the time of discharging, sodium ions penetrating into the positive electrode side react with sulfur to generate sodium polysulfide (Na ₂ S _x ). When a sodium-sulfur battery is charged, sodium and sulfur are produced.

[0004]

[Equation 1]

[0005]

[Equation 2]

FIG. 2 shows a phase diagram of sodium and sulfur. The horizontal axis of the figure shows the molar composition ratio of sulfur. When the battery is charged, the amount of sodium in the positive electrode decreases and the sulfur molar composition ratio increases, and when the battery is discharged, the sulfur molar composition ratio decreases. When discharging is started while maintaining the battery at 350 ° C., the molar composition ratio of sulfur in the positive electrode decreases,
Up to 73%, there are two phases, that is, simple substance of sulfur (S, liquid phase) and discharge product Na ₂ S _5.2 (liquid phase). N
a ₂ S _5.2 exhibits electronic conductivity and has an effect of reducing the internal resistance of the positive electrode.

When the discharge is further continued, elemental sulfur (S, liquid phase) completely reacts with sodium in the positive electrode to form Na ₂ S.
A single phase of _X (liquid phase) is generated. The X value changes from 5.2 to 3 as the discharge progresses. The molar composition ratio of sulfur in the positive electrode at the time when Na ₂ S ₃ was produced was 60%. When the discharge is further continued, Na ₂ S ₂ (solid phase) is deposited.

[0008] When a sodium-sulfur battery is charged, the molar composition ratio of sulfur in the positive electrode becomes high, and especially, the sulfur content is 88.2%.
In the above case, the amount of Na ₂ S _5.2 exhibiting electron conductivity is reduced and the internal resistance of the positive electrode is increased, so charging becomes impossible. A higher charging voltage is required to continue charging, but increasing the charging voltage will damage the solid electrolyte.

Further, when the sodium-sulfur battery is discharged, the molar composition ratio of sulfur in the positive electrode becomes low, and particularly, the sulfur content is 6%.
When it is 3.8% or less, Na ₂ S ₂ (solid phase) is precipitated.
Since Na ₂ S ₂ is a solid phase even at the operating temperature of the battery (300 to 350 ° C.), it no longer acts as an active material of the battery.

[0010]

From the above, the conventional sodium-sulfur battery can increase the energy density of the sodium-sulfur battery because only a part of the positive electrode active material can be used for the charge / discharge reaction. There was a problem that I could not do it.

The present invention has been made in order to solve the above problems, and an object thereof is to provide a sodium-sulfur battery having a high energy density by increasing the utilization rate of the positive electrode active material.

[0012]

In order to achieve the above object, the present invention has the following constitutions. The sodium-sulfur battery of the present invention comprises a positive electrode having at least a positive electrode active material made of sulfur, a negative electrode having at least a negative electrode active material made of sodium, and a solid electrolyte having conductivity with respect to sodium ions. In a sulfur battery, the positive electrode contains both Se and B, and Se and B
And the ratio of sulfur as the positive electrode active material is Se 0.1
Mol% or more and 6 mol% or less, B is 0.1 mol% or more and 8 mol% or less, and sulfur is 86 mol% or more and 99.8 mol% or less.
It is characterized by being less than 1 % .

Further, the sodium-sulfur battery of the present invention comprises a positive electrode having at least a positive electrode active material made of sulfur, a negative electrode having at least a negative electrode active material made of sodium, and a solid electrolyte having conductivity with respect to sodium ions. In the sodium-sulfur battery, the positive electrode contains B, and the ratio of B to sulfur as the positive electrode active material is such that B is 0.1 mol% or more and 6 mol% or less,
The sulfur is 94 mol% or more and 99.9 mol% or less.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. The sodium-sulfur battery 1 shown in FIG. 1 includes a positive electrode active material 3 containing sulfur, a negative electrode active material 2 containing sodium, a solid electrolyte tube 4 having conductivity with respect to sodium ions, and a positive electrode terminal 6. Positive electrode outer can 6 ′, insulating ring 8, and negative electrode terminal 7 sealing lid 11
And are provided. As the material of the solid electrolyte tube 4,
β-alumina (Na ₂ O · 11Al ₂ O ₃ ) or β ″ -alumina (3Na ₂ O · 16Al ₂ O ₃ ) to which MgO, Li ₂ O or the like is added as a stabilizer is used.

The positive electrode outer can 6'contains the positive electrode active material 3 and the conductive auxiliary material 5 for assisting the electron conduction of the positive electrode active material 3. The conductive auxiliary material 5 is obtained by impregnating a carbon fiber cloth or the like wound in a hollow cylindrical shape with sulfur in a molten state and cooling and solidifying. The solid electrolyte tube 4 is inserted into the hollow portion of the conductive auxiliary material 5, and the solid electrolyte tube 4 is filled with sodium as the negative electrode active material 2. In this way, sodium-
The sulfur battery 1 is provided with a positive electrode 21 composed of the positive electrode active material 3 and the conductive auxiliary material 5, and a negative electrode 22 composed of the negative electrode active material 2.

An insulating ring 8 is joined to the upper open end of the solid electrolyte tube 4 by an adhesive material such as glass solder. The insulating ring 8 is brazed to the flange 9 of the positive electrode outer can 6'through an aluminum alloy or the like. The insulating ring 8 is brazed to the sealing lid 11, which is the negative electrode terminal 7, through an aluminum alloy or the like. The sealing lid 11 has
A filling hole 11 a for the negative electrode active material 2 is provided. The filling hole 11a is sealed by a sealing material 11b.

Further, the sodium-sulfur battery 1 includes
A safety pipe 10 having a bottomed cylindrical pipe is provided.
The safety tube 10 is arranged in the solid electrolyte tube 4 such that its outer peripheral surface and the inner peripheral surface of the solid electrolyte tube 4 are separated from each other by a predetermined distance. The safety tube 10 is provided with a flow hole 10 ′ on the bottom surface thereof for allowing the negative electrode active material 2 to flow therethrough. The safety tube 10 is provided when the solid electrolyte tube 4 is damaged and the negative electrode active material 2 and the positive electrode active material 3 directly contact with each other and react with each other.
It plays a role in preventing a sudden reaction between the two. Also, safety tube 1
0 is also bonded to the sealing lid 11 and also serves as a negative electrode current collector.

The positive electrode 21 contains at least one element selected from Se and B in addition to the positive electrode active material 3. Se and B have the effect of increasing the utilization rate of the positive electrode active material and increasing the energy density of the sodium-sulfur battery. Se is added to the positive electrode 21 in a form in which the conductive auxiliary material 5 is impregnated with sulfur. The melting point of Se is 217 ° C.,
The operating temperature of the sodium-sulfur battery 1 is 300 to 350 ° C.
Therefore, Se dissolves in sulfur. Since Se is a metal, it has electron conductivity, and has a function of imparting electron conductivity to the positive electrode active material 3. Therefore, sodium-
Even if the amount of Na ₂ S ₅ decreases at the end of charging of the sulfur battery 1, Se keeps the internal resistance of the positive electrode 21 low. When Se is not contained in the positive electrode active material 3, charging can be performed only until the molar composition ratio of sulfur in the positive electrode 21 reaches 88.2%. However, when Se is contained in the positive electrode 21, the molar composition ratio of sulfur is increased. Can be charged up to about 90%. The ratio of Se to sulfur as the positive electrode active material 3 is such that Se is 0.1 mol% or more and 6 mol% or less, sulfur is 94 mol% or more and 99.9 mol% or less, and the total amount of Se and sulfur is Is preferably 100 mol%. S
If the ratio of e is less than 0.1 mol%, the amount of Se is too small to lower the internal resistance of the positive electrode 21, which is not preferable. If the ratio of Se exceeds 6 mol%, the weight of the positive electrode active material is increased. Is increased and the energy density is significantly decreased, which is not preferable.

Similar to Se, B is added to the positive electrode 21 in the form of being impregnated with the conductive auxiliary material 5 together with sulfur. For example,
When B is added in powder form, the average particle size of B is 10 μm.
It is preferably m or more and 1000 μm or less. B
Has the effect of suppressing the precipitation of Na ₂ S ₂ generated at the end of discharge. Therefore, at the end of discharge of the sodium-sulfur battery 1, N
It becomes possible to suppress the decrease of the amount of the active material due to the precipitation of a ₂ S ₂ . When B is not included in the positive electrode 21, the positive electrode 21
The discharge can be performed only until the molar composition ratio of sulfur in the inside becomes 63.8%, but since B is contained in the positive electrode 21, even if the molar composition ratio of sulfur reaches about 61%, Na ₂ is discharged.
S ₂ does not precipitate. The ratio of B to sulfur that is the positive electrode active material 3 is such that B is 0.1 mol% or more and 8 mol% or less, and sulfur is 92 mol% or more and 99.9 mol% or less,
The total amount of B and sulfur is preferably 100 mol%. If the ratio of B is less than 0.1 mol%, the amount of B is too small and the effect of suppressing the precipitation of Na ₂ S ₂ is significantly reduced, which is not preferable.
It is not preferable because precipitation of a ₂ S ₂ is promoted.

Further, Se and B may be added to the positive electrode 21 at the same time. In this case, the ratio of sulfur, Se and B is S
e is 0.1 mol% or more and 6 mol% or less, and B is 0.1
It is preferable that the amount of sulfur is 8 mol% or more, the amount of sulfur is 86 mol% or more and 99.9 mol% or less, and the total amount of sulfur, Se, and B is 100 mol%.

The sodium-sulfur battery 1 described above has a positive electrode 2
Since Se having electronic conductivity is added to 1, the increase in the internal resistance of the positive electrode 21 due to the decrease of Na ₂ S ₅ at the end of charging is suppressed, and the charge / discharge capacity is made higher than that of the conventional battery. You can

The sodium-sulfur battery 1 described above is
Since B which suppresses the precipitation of Na ₂ S ₂ is added to the positive electrode 21, it is possible to suppress the precipitation of Na ₂ S ₂ at the end of discharge to prevent the decrease in the amount of the active material and to increase the charge / discharge capacity of the battery. You can

[0023]

(Experimental Example 1) A carbon fiber cloth is wound into a hollow cylindrical shape to form a conductive auxiliary material, and the conductive auxiliary material is impregnated with molten sulfur and Se, and the sulfur and Se are solidified to form a hollow cylindrical shape. The positive electrode of was produced. Next, the obtained positive electrode was treated with an outer diameter of 3
4 mm, inner diameter 30 mm, height 80 mm, inserted into a stainless steel positive electrode cylindrical can with Cr plating on the inner surface, and further, in the hollow portion of the positive electrode, a bottomed cylindrical tubular member made of β ″ -alumina having an outer diameter of 20 mm and an effective length of 60 mm. A solid electrolyte tube was inserted.
Further, by inserting the safety tube joined to the negative electrode sealing lid, joining the insulating ring, the negative electrode sealing lid, and the positive electrode cylindrical can, respectively, and filling sodium from the filling hole.
The sodium-sulfur batteries of Examples 1-5 as shown in FIG.

(Experimental Example 2) A carbon fiber cloth is wound into a hollow cylinder to form a conductive auxiliary material, and the conductive auxiliary material is impregnated with molten sulfur and B powder having an average particle size of 100 μm to solidify the sulfur. Thus, sodium-sulfur batteries of Examples 6 to 11 as shown in FIG. 1 and Table 1 were assembled in the same manner as in Experimental Example 1 except that a hollow cylindrical positive electrode was prepared.

(Experimental Example 3) A carbon fiber cloth was wound into a hollow cylinder to form a conductive auxiliary material, and sulfur and Se melted in the conductive auxiliary material.
And an example as shown in FIG. 1 and Table 1 in the same manner as in Experimental Example 1 except that a hollow cylindrical positive electrode was prepared by impregnating a powder of B having an average particle diameter of 100 μm and solidifying sulfur. A 12-14 sodium-sulfur battery was assembled.

(Experimental Example 4) A carbon fiber cloth was wound into a hollow cylindrical shape to form a conductive auxiliary material, and the conductive auxiliary material was impregnated with molten sulfur to solidify the sulfur to prepare a hollow cylindrical positive electrode. A sodium-sulfur battery of Comparative Example 1 as shown in FIG. 1 and Table 1 was assembled in the same manner as in Experimental Example 1 except for the above.

The discharge curves of the sodium-sulfur battery obtained as described above at the 5th cycle are shown in FIGS.
Table 1 shows the average discharge voltage of the battery, the ratio of the discharge capacity to the theoretical capacity of the positive electrode active material (utilization rate), and the energy density of the positive electrode at this time. The charging rate in FIGS. 3 to 7 is represented by the following formula (3). The theoretical capacity here means the electric capacity that can be taken out before S, which is the positive electrode active material in the battery, is reduced to Na ₂ S ₃ , and the cell capacity is the theoretical capacity minus the actual discharge capacity. I mean something.

[0028] Charge rate (%) = cell capacity ÷ theoretical capacity × 100 (3)

Further, the sulfur content with respect to the total amount of sodium and sulfur in the positive electrode at the end of charging and the end of discharging was determined by the following formula (4). The results are shown in Table 2. Here, Smol is the molar amount of sulfur in the positive electrode.

[0030]   Sulfur content (mol%) = Smol / (Smol + (discharge capacity / 26.8)) x 100 (4)

[0031]

[Table 1]

[0032]

[Table 2]

As is apparent from Table 1, Examples 1 to 14
It can be seen that the battery of 1 has a higher utilization rate of the positive electrode active material and a higher energy density than the battery of Comparative Example 1. Also, Table 2
As is apparent from Comparative Example 1, the batteries of Examples 1 to 5 were
It can be seen that the amount of sulfur in the positive electrode at the end of charging is higher than that of the battery of No. It is presumed that this is because the addition of Se imparted electron conductivity to the positive electrode and lowered the internal resistance of the positive electrode, so that even if the amount of Na ₂ S ₅ as a discharge product was reduced, charging was possible. . Further, as is clear from Table 2, the batteries of Examples 6 to 7 have a smaller amount of sulfur in the positive electrode at the end of discharge than the battery of Comparative Example 1. This is presumably because the addition of B prevented the precipitation of Na ₂ S ₂ and thus enabled deeper discharge. Furthermore, it is understood that the batteries of Examples 12 to 14 have a larger amount of sulfur in the positive electrode at the end of charging and a smaller amount of sulfur in the positive electrode at the end of discharging than those of the battery of Comparative Example 1.

The technical scope of the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, the sodium-sulfur battery shown in FIG. 1 described so far has an inside-out structure in which the negative electrode active material is housed inside the solid electrolyte and the positive electrode active material is arranged outside the solid electrolyte. However, the present invention is not limited to this, and the positive electrode active material may be housed in the solid electrolyte and the negative electrode active material may be arranged outside the solid electrolyte. Furthermore, the structure of the sodium-sulfur battery is not limited to the inside-out structure, and for example, a flat plate-shaped solid electrolyte is arranged inside the electrolytic cell, and the positive electrode active material and the negative electrode active material face each other through this solid electrolyte. It may be a structure in which they are arranged. Further, the form of the sodium-sulfur battery is not limited to a cylindrical shape, and may be a prismatic shape or another shape.

[0035]

As described above in detail, in the sodium-sulfur battery of the present invention, the positive electrode contains S having electron conductivity.
Since e is added, the increase in the internal resistance of the positive electrode due to the decrease in Na ₂ S ₅ at the end of charging is suppressed, the charging / discharging capacity is higher than that of the conventional battery, and the utilization rate of the positive electrode active material is high. It is possible to increase the energy density. Further, in the above-mentioned sodium-sulfur battery, B which suppresses the precipitation of Na ₂ S ₂ is added to the positive electrode,
It is possible to suppress the precipitation of Na ₂ S ₂ at the end of discharge to prevent a decrease in the amount of the active material, increase the charge / discharge capacity, and increase the utilization rate of the positive electrode active material, which can increase the energy density. .

[Brief description of drawings]

FIG. 1 is a front sectional view of a sodium-sulfur battery according to an embodiment of the present invention.

FIG. 2 is a phase diagram of sodium and sulfur.

FIG. 3 is a view showing a discharge curve at the 5th cycle of the sodium-sulfur battery of Example 1.

FIG. 4 is a diagram showing a discharge curve of a sodium-sulfur battery of Example 2 at the 5th cycle.

FIG. 5 is a diagram showing a discharge curve at the 5th cycle of the sodium-sulfur battery of Example 6.

FIG. 6 is a view showing a discharge curve at the 5th cycle of the sodium-sulfur battery of Example 7.

FIG. 7 is a diagram showing a discharge curve in a fifth cycle of the sodium-sulfur battery of Example 8.

[Explanation of symbols]

1 Sodium-sulfur battery 2 Negative electrode active material 3 Positive electrode active material 4 Solid electrolyte tube 21 Positive electrode 22 Negative electrode

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-5-67474 (JP, A) JP-A-7-335251 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01M 10/39

Claims (2)

(57) [Claims] 1. A sodium-sulfur battery comprising a positive electrode having at least a positive electrode active material made of sulfur, a negative electrode having at least a negative electrode active material made of sodium, and a solid electrolyte having conductivity with respect to sodium ions. The positive electrode contains both Se and B, and the ratio of Se and B to sulfur as the positive electrode active material is such that Se is 0.1 mol% or more and 6 mol% or less and B is 0.1 mol% or less. 8% by mol or more
It is below, and sulfur is 86 mol% or more and 99.8 mol% or less, The sodium-sulfur battery characterized by the above-mentioned. 2. A sodium-sulfur battery including a positive electrode having at least a positive electrode active material made of sulfur, a negative electrode having at least a negative electrode active material made of sodium, and a solid electrolyte having conductivity with respect to sodium ions. The positive electrode contains B, and the ratio of B to sulfur as the positive electrode active material is such that B is 0.1 mol% or more and 6 mol% or less and sulfur is 94 mol% or more and 99.9 mol%. A sodium-sulfur battery characterized by being: