JP2013114824A - Sodium-sulfur battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sodium-sulfur battery having higher durability than conventional ones.SOLUTION: In a sodium-sulfur battery, contents of minor constituents in sodium housed in a sodium container 5 are as follows: Si≤200 ppm, Fe≤1,000 ppm, Ca≤100 ppm and K≤200 ppm. Further, contents of minor constituents in β alumina of a β alumina solid electrolyte forming a solid electrolyte tube 4 are preferably as follows: Si≤200 ppm, Fe≤1,000 ppm, Ca≤100 ppm and K≤200 ppm.

Description

  The present invention relates to a sodium-sulfur battery used as a secondary battery for power storage and the like, and particularly relates to impurities contained in sodium or the like.

  Sodium-sulfur batteries are used in power storage systems for realizing functions such as power leveling and peak cut. In general, an assembled battery of sodium-sulfur batteries has a string formed by connecting a predetermined number of cells in series, a block is formed by connecting a predetermined number of strings in parallel, and a predetermined number of blocks using the block as a basic unit. It is configured by being connected in series. And when charging this assembled battery, it charges until it reaches a certain voltage.

  In order to increase the discharge capacity or increase the average discharge voltage, it is better to increase the charge amount of the battery as much as possible. However, in the conventional method for charging a sodium-sulfur battery, when the charge termination voltage is increased to increase the amount of charge, the voltage is concentrated on a specific unit cell and the unit cell may be damaged due to an overvoltage. On the other hand, when the charge cutoff voltage is set low, there is a problem that the uncharged amount increases and the battery deteriorates with time. Therefore, studies have been made on a method for charging a sodium-sulfur battery (see Patent Documents 1 and 2).

  On the other hand, β-alumina tubes used in sodium-sulfur batteries are required to have a mechanical strength and sodium ion conductivity of a certain value or more so that they can withstand long-term use. Thus, studies have been made on the amount of impurities in the β alumina tube (see Patent Documents 3 to 4).

  Further, a sodium / sulfur battery having a structure in which the Na supply hole is not blocked by paying attention to the supersaturated oxide in Na of the negative electrode active material so that the charging / discharging efficiency of the battery is not lowered is also disclosed. (See Patent Document 5).

Japanese Patent No. 2878584 Japanese Patent No. 3505116 JP-A-6-72762 Japanese Patent Publication No. 8-21426 JP-A-7-249430

  However, there has been a demand for a sodium-sulfur battery that has durability that can withstand long-term use, can be efficiently charged and discharged, and can be stably operated for a long time.

  The subject of this invention is providing the sodium-sulfur battery which has durability rather than before.

  The present inventors have found that the above problem can be solved by defining the amount of impurities contained in sodium or the like, which is a negative electrode active material housed in a sodium container. That is, according to the present invention, the following sodium-sulfur battery is provided.

[1] A negative electrode chamber and a positive electrode chamber are formed inside and outside of a bottomed cylindrical solid electrolyte tube disposed in the positive electrode container, respectively, and the positive electrode chamber is cylindrically impregnated with sulfur, which is a positive electrode active material. A cylindrical partition with a bottom is disposed inside the solid electrolyte tube that houses the positive electrode mold and serves as a negative electrode chamber, with a predetermined gap between the solid electrolyte tube and a negative electrode inside the solid electrolyte tube. A sodium-sulfur battery in which a sodium container containing sodium as an active material is disposed with a predetermined gap between the partition walls, and a predetermined number of single cells of the sodium-sulfur battery are connected in series. And a predetermined number of strings connected in parallel to form a block, and a predetermined number of the blocks connected in series with the block as a basic unit. The content of the trace components in sodium, which is, Si ≦ 200ppm, Fe ≦ 1000ppm, Ca ≦ 100ppm, sodium was K ≦ 200 ppm - sulfur battery.

[2] The content of a trace component in β alumina of β alumina solid electrolyte forming the solid electrolyte tube is Si ≦ 200 ppm, Fe ≦ 1000 ppm, Ca ≦ 100 ppm, and K ≦ 200 ppm. Sodium-sulfur battery.

[3] When the battery is charged, the terminal voltage is constant current I ₀ and the terminal voltage is set to a constant in the range of 0.05 V or less in the following formula (1), and the battery is charged until it reaches V ₀ [1] ] Or the sodium-sulfur battery according to [2].
V ₀ = (2.075 + α) × m + R × I ₀ (1)
(Where V ₀ is the voltage cutoff voltage (volt), 2.075 is the open-circuit voltage of the single cell at the operating temperature of the sodium-sulfur battery, m is the total series number of single cells in the string, and I ₀ is the charging current (ampere). And R is the internal resistance (ohms) of the assembled battery.)

  The durability can be improved by setting the content of trace components in the sodium contained in the sodium container of the sodium-sulfur battery to a predetermined value or less. Furthermore, durability can be improved more by making content of the trace component in beta alumina of beta alumina of beta alumina solid electrolyte which forms a solid electrolyte pipe below into predetermined value.

It is sectional drawing which shows an example of the basic structure of a sodium-sulfur battery. It is a circuit diagram which shows an assembled battery.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the following embodiments, and changes, modifications, and improvements can be added without departing from the scope of the invention.

  FIG. 1 shows an embodiment of the sodium-sulfur battery of the present invention. In the sodium-sulfur battery of the present invention, a negative electrode chamber R1 is formed inside a bottomed cylindrical solid electrolyte tube 4 disposed in the positive electrode container 1, and a positive electrode chamber R2 is formed outside. A cylindrical positive electrode mold 7 impregnated with sulfur S as a positive electrode active material is accommodated in the positive electrode chamber R2, and a bottomed cylindrical partition wall 8 is provided inside the solid electrolyte tube 4 serving as the negative electrode chamber R1. A predetermined gap is provided between the solid electrolyte tube 4 and the solid electrolyte tube 4. Further, a sodium container 5 containing sodium, which is a negative electrode active material, is disposed inside the partition wall 8 with a predetermined gap between the partition wall 8.

  FIG. 2 is a circuit diagram showing the assembled battery. In the sodium-sulfur battery of the present invention, a predetermined number of sodium-sulfur battery cells 10 are connected in series to form a string 12, and a predetermined number of strings 12 are connected in parallel to form a block 13. Is used as a collective battery 14 connected in series in a predetermined number.

  And the content of the trace component in the sodium accommodated in the sodium container 5 is set to Si <= 200 ppm, Fe <= 1000 ppm, Ca <= 100 ppm, K <= 200 ppm. Thus, durability of a sodium-sulfur battery can be improved by making the amount of impurities in sodium below a predetermined value.

  Furthermore, in the sodium-sulfur battery of the present invention, the content of trace components in the β alumina of the β alumina solid electrolyte forming the solid electrolyte tube 4 is as follows: Si ≦ 200 ppm, Fe ≦ 1000 ppm, Ca ≦ 100 ppm, K ≦ 200 ppm. It is preferable to do. Thus, the durability of the sodium-sulfur battery can be further improved by setting the amount of impurities in β alumina to a predetermined value or less.

In addition, the sodium-sulfur battery of the present invention is charged at a constant current I ₀ when charging the assembled battery 14 until the terminal voltage becomes V _{0 represented} by the following formula (1).
V ₀ = (2.075 + α) × m + R × I ₀ (1)
(Where V ₀ is the voltage cutoff voltage (volt), 2.075 is the open-circuit voltage of the cell at the operating temperature of the sodium-sulfur battery, α is the end-of-charge polarization voltage, preferably in the range of 0.05 or less. Constant, m is the total number of single cells in the string, I ₀ is the charging current (unit is ampere), R is the internal resistance of the assembled battery (unit is ohm), and the open circuit voltage V at the end of charging ₍₁₎ and the maximum voltage V ₂ during constant current discharge are obtained by the following equation: R = (V ₁ −V ₂ ) / discharge current.

  The configuration of the unit cell 10 of the sodium-sulfur battery of the present invention will be described more specifically with reference to FIG. A solid electrolyte tube 4 having a bottomed cylindrical shape having a function of selectively permeating sodium ions is disposed in the positive electrode container 1, and a positive electrode chamber R <b> 2 is formed outside the solid electrolyte tube 4. The positive electrode container 1 is made of aluminum, stainless steel, or the like, and a constriction 9 is formed in the vicinity of the upper end of the positive electrode container 1 for absorbing and relaxing expansion / contraction due to the heat change of the container. The solid electrolyte tube 4 is made of β alumina, β ″ alumina, or the like. A cylindrical positive electrode mold 7 impregnated with sulfur S, which is a positive electrode active material, is accommodated in the positive electrode chamber R2.

  On the other hand, a negative electrode chamber R1 is formed inside the solid electrolyte tube 4, and a bottomed cylindrical partition wall (safety tube) 8 made of a metal material having excellent corrosion resistance against sodium such as aluminum, aluminum alloy, and stainless steel, It is arranged with a predetermined gap between it and the solid electrolyte tube 4. Further, a covered / bottomed cylindrical sodium container 5 containing molten metal sodium Na, which is a negative electrode active material, is disposed inside the partition wall 8 with a predetermined gap between the partition wall 8.

  The positive electrode container 1 and the solid electrolyte tube 4 are coupled via an insulating ring 2, and a negative electrode lid 3 is joined to the upper end surface of the insulating ring 2. The insulating ring 2 is preferably made of an insulating ceramic because it is necessary to maintain electrical insulation between the positive electrode chamber R2 and the negative electrode chamber R1, and α-alumina or the like is preferably used in view of strength, cost, and the like. The A small hole 6 is formed at the bottom of the sodium container 5, and an inert gas G such as nitrogen gas or argon gas is sealed in the upper space of the sodium container 5 at a predetermined pressure. The active gas G pressurizes the sodium Na in the sodium container 5 in the direction of flowing out from the small holes 6.

  During the discharge of the sodium-sulfur battery, the sodium Na flowing out from the small hole 6 at the bottom of the sodium container 5 moves upward in the gap between the sodium container 5 and the partition wall 8 and further climbs over the upper end edge of the partition wall 8, thereby solid electrolyte. The gap between the tube 4 and the partition wall 8 moves downward and stays in this gap.

  Here, sodium Na emits electrons to become sodium ions, passes through the solid electrolyte tube 4 and moves to the positive electrode chamber R2, and reacts with the sulfur S in the positive electrode chamber R2 and the electrons that have passed through the external circuit. Generates sodium sulfide and generates a voltage. In addition, during charging, a reaction of generating sodium Na and sulfur S occurs in contrast to discharging.

  Further, the partition wall 8 is thermally expanded by heat generated by the direct reaction when the solid electrolyte tube 4 is damaged while the unit cell is used and the sulfur S in the positive electrode chamber R2 comes into direct contact with the sodium Na of the negative electrode. In addition, the gap between the partition wall 8 and the solid electrolyte tube 4 is reduced, thereby reducing the amount of direct reaction. Thereby, the heat generated by the direct reaction accelerates the temperature of the unit cell and protects the failed unit cell from dangers such as melting the positive electrode container 1.

  In the sodium-sulfur battery of the present invention, the contents of trace components in the sodium contained in the sodium container 5 are set to Si ≦ 200 ppm, Fe ≦ 1000 ppm, Ca ≦ 100 ppm, and K ≦ 200 ppm. The reason for limiting the numerical value of each trace component is as follows. First, when Si exceeds 200 ppm, the crushing strength and Na ion conductivity after long-term energization decrease, and this tendency becomes remarkable particularly when the energization amount is increased. Next, when Fe exceeds 1000 ppm, not only does the pressure ring strength and Na ion conductivity after long-term energization decrease, but also the pressure ring strength tends to decrease even before the current supply. Further, when Ca exceeds 100 ppm and when K exceeds 200 ppm, the Na ion conductivity is lowered, and the basic characteristics as a solid electrolyte are lowered.

  Furthermore, in the sodium-sulfur battery of the present invention, it is more preferable that the content of trace components in β-alumina is Si ≦ 200 ppm, Fe ≦ 1000 ppm, Ca ≦ 100 ppm, and K ≦ 200 ppm. The reason for limiting the numerical value of each trace component is as follows. First, when Si exceeds 200 ppm, the crushing strength and Na ion conductivity after long-term energization decrease, and this tendency becomes remarkable particularly when the energization amount is increased. Next, when Fe exceeds 1000 ppm, not only does the pressure ring strength and Na ion conductivity after long-term energization decrease, but also the pressure ring strength tends to decrease even before the current supply. Further, when Ca exceeds 100 ppm and when K exceeds 200 ppm, the Na ion conductivity is lowered, and the basic characteristics as a solid electrolyte are lowered.

  Next, an assembled battery of sodium-sulfur battery and its charging will be described with reference to FIG. As shown in FIG. 2, a string 12 is formed by connecting a unit cell 10 made of a sodium-sulfur battery in series by a predetermined number m in series, and this string 12 is connected in parallel by a predetermined number n in parallel. 13 is configured. A collective battery 14 is configured by connecting a predetermined number in series with the block 13 as a basic unit. The number m in series, the number n in parallel, and the number of series connections of the blocks 13 are appropriately set according to the output voltage, battery capacity, etc. of the target assembled battery 14.

Next, charging / discharging of the assembled battery 14 will be described. The operating temperature of the unit cell 10 is 300 to 360 ° C., and charging / discharging is performed at this operating temperature. For example, when discharging is performed for 8 hours with the current I ₀ , the voltage is maintained at a constant value for a predetermined time, and then gradually decreases at a constant slope. Subsequently, when a predetermined time has elapsed after the end of discharging, a charging operation is performed. That is, charging is performed for 7.5 hours, for example, until the terminal voltage reaches V _{0 represented} by the following equation at a constant current I ₀ .

V ₀ = (2.075 + α) × m + R × I ₀ (1)
(However, V ₀ is the charge cutoff voltage (volt), 2.075 is the open-circuit voltage of the unit cell at the operating temperature of the sodium-sulfur battery, α is the end-of-charge polarization voltage, preferably in the range of 0.05 or less. Constant, m is the total number of single cells in the string, I ₀ is the charging current (unit is ampere), R is the internal resistance of the assembled battery (unit is ohm), and the open circuit voltage V at the end of charging ₍₁₎ and the maximum voltage V ₂ during constant current discharge are obtained by the following equation: R = (V ₁ −V ₂ ) / discharge current.

  From the aspect of charge / discharge efficiency, it is preferable to operate the sodium-sulfur battery at a high temperature of 280 ° C. or higher for reasons such as temperature characteristics of sodium ion conductivity with respect to β alumina. On the other hand, the operating temperature of the sodium-sulfur battery is limited due to restrictions such as heat resistance of various members constituting the battery. Therefore, in a sodium-sulfur battery, it is preferable to perform charging and discharging while controlling the operating temperature within a predetermined range (300 ° C. to 360 ° C.).

As described above, in the sodium-sulfur battery of the present invention, the content of trace components in the sodium contained in the sodium container 5 is set to Si ≦ 200 ppm, Fe ≦ 1000 ppm, Ca ≦ 100 ppm, K ≦ 200 ppm. ing. Furthermore, it is preferable that the content of trace components in β alumina of the β alumina solid electrolyte forming the solid electrolyte tube 4 is Si ≦ 200 ppm, Fe ≦ 1000 ppm, Ca ≦ 100 ppm, and K ≦ 200 ppm. The sodium-sulfur battery of the present invention is charged until the terminal voltage becomes V ₀ (α is preferably 0.05 or less) at a constant current I ₀ during charging. Thus, by defining the amount of impurities in sodium, the amount of impurities in β-alumina, and the α value, it is possible to improve the properties of the sintered body after long-term energization.

  EXAMPLES Hereinafter, although this invention is demonstrated further in detail based on an Example, this invention is not limited to these Examples.

[Production of sodium]
Sodium with different amounts of impurities was produced. The manufactured samples are shown in Table 1.

[Manufacture of β alumina tube]
Using various alumina raw materials, β-alumina tubes (solid electrolyte tubes 4) having different trace components were produced by the following method. Three types of raw materials were prepared as basic alumina raw materials, and SiO ₂ , Fe ₂ O ₃ , CaCO ₃ , and K ₂ O were added to obtain a predetermined amount of trace components. In this example, Na ₂ CO ₃ was used as the Na ₂ O source, and a predetermined amount of MgO was measured as the MgO source, and wet mixing was performed in a ball mill at a moisture of 50% for 30 minutes. The mixed slurry was dried and then packed in an alumina brick crucible and calcined in an air atmosphere at a maximum temperature of 1400 ° C. using an electric furnace. The powder after calcination was wet pulverized and mixed in a ball mill, and 2% ethylene glycol was added as a binder and granulated with a spray dryer. The obtained granulated powder was filled into a rubber mold having a predetermined shape and pressed with a rubber press pressure of ² ton / cm ² to obtain a molded body. This molded body was fired to obtain a sintered body. The amounts of trace components in the β-alumina tube (solid electrolyte tube 4) thus obtained are as shown in Table 1.

[Durability test method]
A sodium-sulfur battery was assembled, and discharging and charging were repeated, and durability was examined. Specifically, the sodium-sulfur battery was started from discharging for assembly at the end of charging, and subsequently charged to the end-of-charge cut voltage V ₀ obtained by the following formula (1). The number of cycles until failure was investigated with one cycle of discharge / charge. The α value in Table 1 is α in the following formula (1).
V ₀ = (2.075 + α) × m + R × I ₀ (1)
(Where V ₀ is the end-of-charge cutoff voltage (volt), 2.075 is the open-circuit voltage of the single cell at the operating temperature of the sodium-sulfur battery, α is the end-of-charge polarization voltage and a constant, and m is the unit voltage in the string. The total number of batteries in series, I ₀ is the charging current (unit is ampere), and R is the internal resistance (unit is ohms) of the assembled battery, which is the open-circuit voltage V ₁ at the end of charging and the maximum during constant current discharge. it is determined by the following equation from the voltage _{V 2} Prefecture. That is, _{R = (V 1 -V 2)} / discharge current.)

  As shown in Table 1, when the α value was 0.1 V, in Comparative Example 1, the number of cycles until failure was 500. In contrast, in Examples 1 and 2 in which the amount of impurities in sodium was suppressed, the number of cycles until failure increased compared to Comparative Example 1. In particular, in Example 2 in which the amount of impurities in the β alumina tube was also suppressed, the number of cycles until failure was 1200.

  When the α value is 0.04 V, in Comparative Example 2, the number of cycles until failure was 700, whereas in Example 3 in which the amount of impurities in sodium was suppressed, the number of cycles until failure was 2000. It was times. Furthermore, when the α value was 0.05 V, Example 4 in which the amount of impurities in sodium and the amount of impurities in the β alumina tube was suppressed had more than 3000 cycles to failure.

  The sodium-sulfur battery of the present invention can be used as a sodium-sulfur battery having further improved durability.

1: positive electrode container, 2: insulating ring, 3: negative electrode cover, 4: solid electrolyte tube, 5: sodium container, 6: small hole, 7: positive electrode mold, 8: partition wall (safety tube), 9: constriction, 10: Single cell, 12: string, 13: block, 14: assembled battery.

Claims (3)

A negative electrode chamber is formed inside and a positive electrode chamber is formed outside the bottomed cylindrical solid electrolyte tube disposed in the positive electrode container, and a cylindrical positive electrode mold impregnated with sulfur as a positive electrode active material is formed in the positive electrode chamber. A cylindrical wall with a bottom is disposed inside the solid electrolyte tube to be housed and serving as a negative electrode chamber, with a predetermined gap between the solid electrolyte tube and a negative electrode active material inside the partition. A sodium-sulfur battery in which a sodium container containing a certain sodium is disposed with a predetermined gap between the sodium container,
As a collective battery in which a predetermined number of cells of the sodium-sulfur battery are connected in series to form a string, a predetermined number of the strings are connected in parallel to form a block, and a predetermined number of the blocks are connected in series as a basic unit. Is used,
The sodium-sulfur battery which made content of the trace component in the sodium accommodated in the said sodium container into Si <= 200ppm, Fe <= 1000ppm, Ca <= 100ppm, K <= 200ppm.   2. The sodium-sulfur battery according to claim 1, wherein the content of trace components in β alumina of the β alumina solid electrolyte forming the solid electrolyte tube is Si ≦ 200 ppm, Fe ≦ 1000 ppm, Ca ≦ 100 ppm, and K ≦ 200 ppm. . 3. The battery according to claim 1, wherein when the battery pack is charged, the terminal voltage is a constant current I ₀ and the terminal voltage is a constant in the range of 0.05 V or less in the following formula (1), and charging is performed until V ₀ is reached. Sodium-sulfur battery as described.
V ₀ = (2.075 + α) × m + R × I ₀ (1)
(Where V ₀ is the voltage cutoff voltage (volts), 2.075 is the open circuit voltage of the cells at the operating temperature of the sodium-sulfur battery, m is the total number of cells in series in the string, and I ₀ is the charging current ( And R is the internal resistance (ohms) of the assembled battery.)

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