Sodium-sulfur secondary battery and method for manufacturing sodium-sulfur secondary battery

Abstract

This sodium-sulfur secondary battery is provided with: a positive electrode container in which an inner surface thereof is an anti-corrosion layer comprising an Fe-Cr alloy; a solid electrolyte tube disposed inside the positive electrode container so as to be separated from the positive electrode container; a positive electrode chamber which is surrounded by the positive electrode container and the solid electrolyte tube and in which a positive electrode current collector containing sulfur, which is a positive electrode active material, is disposed; and a negative electrode chamber which is the interior of the solid electrolyte tube separated from the positive electrode chamber by the solid electrolyte tube and in which metal sodium, which is a negative electrode active material, is disposed, wherein a high resistance layer containing glass fibers is provided integrally with the positive electrode current collector on the solid electrolyte tube side of the positive electrode current collector, the high resistance layer is in contact with the solid electrolyte tube, and a carbon sheet is interposed between the positive electrode current collector and the inner surface of the positive electrode container.

Description

Sodium-sulfur secondary battery and method of manufacturing the sodium-sulfur secondary battery

[0001] The present invention relates to a sodium-sulfur secondary battery, and in particular to the structure of a single cell thereof.

[0002] A sodium-sulfur secondary battery (NaS battery) is already widely known, having a configuration in which a solid electrolyte tube having sodium ion conductivity is disposed within a positive electrode container, thereby separating a positive electrode chamber surrounded by the positive electrode container and the solid electrolyte tube from an anode chamber inside the solid electrolyte tube by the solid electrolyte tube, in which sodium, which is a negative electrode active material, is accommodated in the anode chamber, and a positive electrode current collector containing sulfur, which is a positive electrode active material, and sodium polysulfide, is accommodated in the positive electrode chamber.

[0003] Measures for preventing corrosion on the inner surface of the positive electrode container in such NaS batteries have already been known (see, for example, Patent Documents 1 to 6).

[0004] Patent Document 1 discloses a method of providing a chrome plating layer on the inner surface of a positive electrode container.

[0005] Patent Documents 2 to 4 disclose embodiments in which a corrosion-resistant coating is formed on the inner surface of a positive electrode container by plasma spraying a powder material made of a corrosion-resistant alloy in an inert gas atmosphere or at atmospheric pressure.

[0006] Patent Document 5 discloses an embodiment in which a corrosion-resistant film is formed on a positive electrode container without defects by gradually reducing the Cr content in multiple corrosion-resistant coating layers, each made of a Cr—Fe alloy, from the surface side.

[0007] Patent Document 6 discloses an embodiment in which a dense carbon sheet or the like made of carbon powder and carbon fiber is provided on the inner surface of a positive electrode container in order to impart corrosion resistance to the positive electrode container.

[0008] Also, an embodiment in which a high resistance layer made of an insulating material is provided on the surface of the positive electrode current collector that contacts the solid electrolyte tube is already known (see, for example, Patent Document 7).

[0009] Patent Document 7 discloses an embodiment in which a high resistance layer is formed by needle-punching glass fibers with a fiber diameter of 5 μm to 15 μm into one surface of a felt-like substrate made of carbon fiber or graphite fiber.

[0010] NaS batteries operate at high temperatures of 300°C to 350°C. Therefore, during operation of a NaS battery, the positive electrode container is constantly exposed to molten sulfur, which has a strong corrosive effect, and molten sodium polysulfide, which is generated during discharge and decomposes into sulfur during charge. Even if a corrosion-resistant coating such as that disclosed in Patent Documents 3 and 4 is provided, the battery resistance increases with repeated charge and discharge, and the effect of the corrosion-resistant coating in suppressing the decrease in battery capacity is weakened.

[0011] In addition, in the case of the embodiment disclosed in Patent Document 6 in which a carbon sheet is laid on the inner surface of the positive electrode container, even if the carbon sheet is dense, it is not possible to completely prevent molten sulfur and molten sodium polysulfide from penetrating between the carbon sheet and the positive electrode container. If such penetration occurs, the positive electrode container will be corroded, causing a sudden increase in battery resistance and making it impossible to charge and discharge.

[0012] JP 2-142066 JP 4-349343 JP 6-231802 Patent 2520986 JP 8-78049 JP 2000-323168 JP 2003-100304

[0013] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to realize a sodium-sulfur secondary battery in which deterioration of performance with use is suppressed compared to conventional batteries.

[0014] In order to solve the above-mentioned problems, a first aspect of the present invention is a sodium-sulfur secondary battery comprising: a cathode can whose inner surface is a corrosion protection layer made of an Fe—Cr alloy; a solid electrolyte tube disposed within the cathode can and spaced apart from the cathode can; a cathode chamber surrounded by the cathode can and the solid electrolyte tube and containing a cathode current collector containing sulfur as a cathode active material; and an anode chamber inside the solid electrolyte tube separated from the cathode chamber by the solid electrolyte tube and containing metallic sodium as an anode active material, wherein a high-resistance layer containing glass fiber is provided integrally with the cathode current collector and in contact with the solid electrolyte tube, and a carbon sheet is interposed between the cathode current collector and the corrosion protection layer.

[0015] A second aspect of the present invention is the sodium-sulfur secondary battery according to the first aspect, wherein the glass density in the high resistance layer is 0.02 mg / mm ３ ~0.05mg / mm ３ The present invention is characterized in that:

[0016] A third aspect of the present invention is the sodium-sulfur secondary battery according to the second aspect, wherein the glass density in the high-resistance layer is two to four times the glass density in the vicinity of the positive electrode can of the positive electrode current collector.

[0017] A fourth aspect of the present invention is a method for manufacturing a sodium-sulfur secondary battery, the method comprising the steps of: (a) preparing a cathode container having an inner surface provided with an anticorrosion layer made of an Fe—Cr alloy; (b) arranging a solid electrolyte tube inside the cathode container at a distance from the cathode container to provide a cathode chamber surrounded by the cathode container and the solid electrolyte tube, and an anode chamber inside the solid electrolyte tube separated from the cathode chamber by the solid electrolyte tube; and (c) placing glass felt on one side of graphite felt and needle-punching from the glass felt side to remove the glass felt. d) providing a high-resistance layer on the one side of the graphite felt by driving lath fibers into the graphite felt; d) impregnating the graphite felt after the high-resistance layer has been provided with sulfur, which is a positive electrode active material, to obtain a positive electrode current collector integrated with the high-resistance layer; e) placing the positive electrode current collector in the positive electrode chamber while bringing the high-resistance layer into contact with the solid electrolyte tube and interposing a carbon sheet between the high-resistance layer and the corrosion protection layer; and f) placing metallic sodium, which is a negative electrode active material, in the negative electrode chamber.

[0018] A fifth aspect of the present invention is the method for producing the sodium-sulfur secondary battery according to the fourth aspect, wherein in the step c), the graphite felt is prepared, the thickness of which is greater than a width of the positive electrode chamber before being placed in the positive electrode chamber in the step e), and in the step e), the positive electrode current collector and the carbon sheet are fixed in the positive electrode chamber by a biasing force generated in the positive electrode current collector compressed to the width of the positive electrode chamber.

[0019] A sixth aspect of the present invention is a method for producing a sodium-sulfur secondary battery according to the fourth or fifth aspect, wherein in the step c), a glass density of the high resistance layer after being placed in the positive electrode chamber together with the positive electrode current collector in the step e) is 0.02 mg / mm ３ ~0.05mg / mm ３ The high resistance layer is provided so as to

[0020] A seventh aspect of the present invention is the method for producing the sodium-sulfur secondary battery according to the sixth aspect, wherein in the step c), the glass density of the high-resistance layer after being placed in the positive electrode chamber together with the positive electrode current collector in the step e) is set to be two to four times the glass density of the positive electrode current collector in the vicinity of the positive electrode case.

[0021] An eighth aspect of the present invention is the method for manufacturing the sodium-sulfur secondary battery according to the fourth or fifth aspect, characterized in that a carbon sheet from which a moisture-adsorbing element has been removed is used as the carbon sheet.

[0022] A ninth aspect of the present invention is a method for producing a sodium-sulfur secondary battery according to the fourth or fifth aspect, characterized in that, prior to the step e), a step g) of drying the carbon sheet is performed.

[0023] According to the first to ninth aspects of the present invention, even when a sodium-sulfur secondary battery is continuously used, it is possible to obtain the effect of suppressing an increase in battery resistance and an increase in residual capacity due to adhesion of sulfur to a solid electrolyte tube.

[0024] In particular, according to the second, third, sixth, and seventh aspects, the effect of the carbon sheet interposed therebetween caused by excessive compression of the high-resistance layer is offset, and deterioration of the charging performance of the NaS battery over time is suitably suppressed.

[0025] 1 is a cross-sectional view along the longitudinal direction of the NaS battery 1. It is a schematic enlarged view of part A in FIG. 1. It is a schematic cross-sectional view of the positive electrode container 2 perpendicular to the longitudinal direction of the NaS battery 1. It is a diagram showing the appearance of the vicinity of the positive electrode chamber 5 in an NaS battery 1α not provided with a carbon sheet 8 at the beginning of use and after deterioration of the anticorrosion layer 2b. It is a diagram showing the appearance of the vicinity of the positive electrode chamber 5 in an NaS battery 1 provided with a carbon sheet 8 at the beginning of use and after deterioration of the anticorrosion layer 2b. It is a graph showing the relationship between the number of charge-discharge cycles and the amount of elution in the positive electrode converted into film thickness for the NaS battery 1α and the NaS battery 1. It is a graph plotting the battery resistance values ​​measured at appropriate times while repeating charge-discharge cycles in the NaS battery 1 against the number of charge-discharge cycles. It is a graph showing the distribution of glass density in the positive electrode current collector 7 of each NaS battery 1 when the density of the glass felt when integrating the high-resistance layer 15 with the positive electrode current collector 7 is changed in four ways (conditions 1 to 4). 1 is a graph showing the change in charging resistance with respect to the number of charge-discharge cycles when charge-discharge cycles are repeated in four NaS batteries 1 each provided with a high-resistance layer 15 based on each of conditions 1 to 4.

[0026] <Configuration of Single Cell> A sodium-sulfur secondary battery (hereinafter referred to as NaS battery) 1 according to this embodiment is generally a high-temperature operating secondary battery that uses metallic sodium (Na) as a negative electrode active material and sulfur (S) as a positive electrode active material and operates at 300°C to 350°C.

[0027] Generally, a module battery is constructed by housing a plurality of NaS batteries 1 as single cells connected in series and parallel in a container. A battery unit consisting of one or more module batteries is used for various purposes, such as a standby power source or for storing power generated by distributed power sources such as solar cells.

[0028] Fig. 1 is a cross-sectional view along the longitudinal direction (vertical direction as viewed in the drawing) of a NaS battery 1 serving as a single cell according to this embodiment. As shown in Fig. 1, the NaS battery 1 mainly comprises a cylindrical cathode container 2 with a bottom and a cylindrical solid electrolyte tube 3 with a bottom and a curved bottom 3a, which has sodium ion conductivity. The cathode container 2 is made of aluminum (Al), for example, and the solid electrolyte tube 3 is made of beta-alumina, for example.

[0029] The solid electrolyte tube 3 is disposed inside the cathode container 2 so as to be coaxial with the cathode container 2 and so that its bottom 3a is located on the bottom surface (lower part in FIG. 1 ) 2a side of the cathode container 2. However, the solid electrolyte tube 3, including the bottom 3a, is separated from the cathode container 2 by being laterally supported by an insulating ring 4 fitted between the cathode container 2 and the solid electrolyte tube 3 near its tip end (upper part in FIG. 1 ) 3b. Hereinafter, the direction from the central axis C of the cathode container 2 and the solid electrolyte tube 3 toward the outside of the cathode container 2 will also be referred to as the radial direction.

[0030] With this arrangement, in the NaS battery 1, a cathode chamber 5 surrounded by the cathode container 2 and the solid electrolyte tube 3 is separated from an anode chamber 6 inside the solid electrolyte tube 3 by the solid electrolyte tube 3.

[0031] The positive electrode chamber 5 contains a positive electrode current collector 7. The positive electrode current collector 7 contains sulfur (S) 7a, which is a positive electrode active material. However, after the NaS battery 1 starts operating, sodium polysulfide may also be present in the positive electrode current collector 7. A carbon sheet 8 having a thickness of approximately 0.2 mm to 0.4 mm (e.g., 0.3 mm) is disposed in the positive electrode chamber 5 so as to cover the inner surface of the positive electrode case 2. In other words, the carbon sheet 8 is interposed between the positive electrode current collector 7 and the positive electrode case 2 in the radial direction of the NaS battery 1. The carbon sheet 8 may also be provided so as to cover the bottom surface 2a of the positive electrode case 2. The detailed internal configuration of the positive electrode chamber 5 and the effects of the carbon sheet 8 will be described later.

[0032] The anode chamber 6 contains metallic sodium (Na) 9, which is the anode active material. Therefore, the solid electrolyte tube 3 serves as a separator that separates the metallic sodium 9, which is the anode active material, from the sulfur 7a, which is the cathode active material. The anode chamber 6 also includes a storage container 10 used when placing the metallic sodium 9 in the anode chamber 6, and an aluminum safety tube 11 that is disposed around the storage container 10. The storage container 10 and the safety tube 11 serve to regulate the amount of metallic sodium 9 that reacts with the sulfur, which is the cathode active material, in the unlikely event that the solid electrolyte tube 3 is broken, thereby ensuring safety.

[0033] A positive electrode terminal 1p electrically connected to the positive electrode chamber 5 protrudes from the peripheral edge of the front end (top in FIG. 1 ) of the positive electrode container 2. On the other hand, a negative electrode terminal 1n electrically connected to the negative electrode chamber 6 protrudes from the center of the front end of the positive electrode container 2. The positive electrode terminal 1p and the negative electrode terminal 1n are also insulated from each other by the ring 4.

[0034] A metallic (e.g., stainless steel) sleeve tube 12 is fitted around the outside of the positive electrode container 2. The sleeve tube 12 is provided to prevent deformation of the NaS battery 1, which becomes hot during use and standby (to restrict deformation of the positive electrode container 2). The sleeve tube 12 may have a thickness of approximately 0.2 mm to 0.4 mm.

[0035] Furthermore, an insulating sheet 13 made of an insulator (e.g., mica) is fitted around the outside of the sleeve tube 12. The insulating sheet 13 is provided for the purpose of ensuring safety when the battery module is constructed. The insulating sheet 13 may be provided to a thickness of 0.2 mm or more. Alternatively, in consideration of breakage, the insulating sheet 13 may be made of multiple layers.

[0036] The NaS battery 1 having the above-described configuration is heated to 300°C to 350°C by a heater (not shown) provided outside (for example, in the housing of the module battery) during operation, and the sulfur 7a serving as the positive electrode active material and the metallic sodium 9 serving as the negative electrode active material are both in a molten state. During discharge, the metallic sodium 9 serving as the negative electrode active material emits electrons to an external circuit to form sodium ions (Na ＋The sodium ions pass through the solid electrolyte tube 3 and move to the positive electrode chamber 5, where they react with sulfur 7a, which is the positive electrode active material, and electrons supplied from an external circuit to produce sodium polysulfide.

[0037] On the other hand, during charging, application of voltage from an external circuit causes sodium polysulfide present in the positive electrode chamber 5 to release electrons to the external circuit, generating sulfur and sodium ions. The generated sodium ions pass through the solid electrolyte tube 3 and move to the negative electrode chamber 6, where they react with electrons supplied from the external circuit and are electrically neutralized. This converts electrical energy into chemical energy.

[0038] <Detailed Structure of the Inside of the Positive Electrode Chamber> Next, a more detailed structure of the inside of the positive electrode chamber 5 will be described. Fig. 2 is a schematic enlarged view of part A in Fig. 1 .

[0039] 1, the illustration is simplified, but as shown in Fig. 2, in the positive electrode current collector 7, sulfur 7a, which is a positive electrode active material, is present in a state of being impregnated into graphite felt (also referred to as graphite mat) 7b having a predetermined thickness. The graphite felt 7b is a felt-like member made of graphite fiber, and functions as a conductive auxiliary material for the sulfur 7a, which is an insulator.

[0040] Furthermore, a high resistance layer 15 is provided on the side of the positive electrode current collector 7 that contacts the surface 3 c of the solid electrolyte tube 3 .

[0041] The high-resistance layer 15 is provided for the purpose of suppressing the generation and adhesion of sulfur on the surface 3c during charging by reducing the conductivity of the positive electrode current collector 7 near the surface 3c of the solid electrolyte tube 3. If sulfur is generated on the surface 3c during charging, the internal resistance of the NaS battery 1 increases, and the charging reaction does not proceed even though sodium polysulfide remains, resulting in a phenomenon in which charging is not completed, and charge recovery is reduced, which is undesirable.

[0042] As disclosed in Patent Document 7, for example, the high-resistance layer 15 is formed integrally with the graphite felt 7b constituting the positive electrode current collector 7 by a process in which a graphite felt 7b (first substrate) and a glass felt (second substrate, not shown), which is a felt-like member made of glass fiber, are overlapped, the glass fibers of the glass felt are needle-punched from the second substrate side, and the glass felt is then cut to a desired size. Therefore, although an explicit boundary is provided between the high-resistance layer 15 and the positive electrode current collector 7 for convenience of illustration in Figure 2, in reality, the abundance ratio of the glass fiber constituting the high-resistance layer 15 gradually decreases in the direction from the solid electrolyte tube 3 toward the positive electrode container 2, and a clear boundary does not necessarily exist.

[0043] More specifically, in the present embodiment, the high resistance layer 15 is a region that originates from one surface side of the positive electrode current collector 7, is integrated with the positive electrode current collector 7, and has a relatively high content of glass fibers compared to the vicinity of the end of the positive electrode current collector 7 on the positive electrode container 2 side. Therefore, it is not necessarily easy to specify and evaluate the presence width (thickness), which is a numerical value that directly indicates the presence range of the high resistance layer 15 in the radial direction.

[0044] The positive electrode current collector 7 is formed by placing the graphite felt 7b integrated with the high-resistance layer 15 in a predetermined mold (not shown) and impregnating (injecting) sulfur 7a into the mold. Therefore, hereinafter, the high-resistance layer 15 is considered to be a part of the positive electrode current collector 7.

[0045] More specifically, the positive electrode current collector 7 is elastic and is molded so that its thickness, including the high-resistance layer 15, before being placed in the positive electrode chamber 5 is greater than the radial width of the positive electrode chamber 5. Therefore, the positive electrode current collector 7 is compressed when placed in the positive electrode chamber 5, and is fixed in the positive electrode chamber 5 by the action of a restoring force (biasing force) generated by such compression. As a result, the high-resistance layer 15 is pressed against the surface 3c of the solid electrolyte tube 3, and the carbon sheet 8 is also pressed toward the positive electrode container 2 by the biasing force from the positive electrode current collector 7.

[0046] For example, a positive electrode current collector 7 having a thickness of 14.8 mm to 16.5 mm including the high-resistance layer 15 before placement and a carbon sheet 8 having a thickness of 0.3 mm are placed in a positive electrode chamber 5 having a radial size of 12.8 mm. In this case, the width of the high-resistance layer 15 is, at most, 3 mm or less from the contact portion with the surface 3 c of the solid electrolyte tube 3.

[0047] 3 is a schematic cross-sectional view of the cathode container 2 perpendicular to the longitudinal direction of the NaS battery 1, illustrating one embodiment of the arrangement of the cathode current collector 7. However, the solid electrolyte tube 3 and the inside of the anode chamber 6 are not shown.

[0048] The positive electrode current collector 7 is disposed on the outer periphery of the solid electrolyte tube 3, and therefore needs to have a circular ring shape in cross section. However, from the viewpoint of ease of molding and arrangement, it may be divided into three partial current collectors 7p, 7q, and 7r, each of which forms a third of the circular ring, as shown in Fig. 3. These three partial current collectors 7p, 7q, and 7r form a single circular ring as a whole when disposed in the positive electrode chamber 5. Alternatively, the positive electrode current collector 7 may be divided into two or four or more partial current collectors.

[0049] Although not shown in FIG. 1, as shown in FIG. 2, a corrosion-resistant layer 2b is provided on the inner surface of the cathode casing 2 that forms the cathode chamber 5. The corrosion-resistant layer 2b is a corrosion-resistant coating made of an Fe—Cr alloy and having a thickness of about 20 μm to 80 μm. The corrosion-resistant layer 2b is provided to prevent corrosion of the cathode casing 2 due to contact with molten sulfur and molten sodium polysulfide present in the cathode chamber 5. The corrosion-resistant layer 2b is formed by a known method, such as plasma spraying.

[0050] <Carbon Sheet> As described above, in the NaS battery 1 according to this embodiment, the inner surface of the positive electrode case 2 is made into the anticorrosion layer 2b, and further, the carbon sheet 8 is arranged so as to be in contact with the anticorrosion layer 2b. More specifically, the carbon sheet 8 is pressed on one side by the biasing force from the adjacent positive electrode current collector 7, and is fixed to the anticorrosion layer 2b, which is the inner surface of the positive electrode case 2, at an appropriate position on the opposite side with the double-sided tape 14.

[0051] The carbon sheet 8 is provided generally for the purpose of suppressing a decrease in battery performance caused by sulfides (anticorrosion layer sulfides) generated by the reaction of the anticorrosion layer 2b with sulfur during use of the NaS battery 1. More specifically, the provision of the anticorrosion layer 2b prevents molten sulfur and molten sodium polysulfide from immediately contacting and corroding the cathode container 2 in the NaS battery 1, but the generation of anticorrosion layer sulfides due to reaction with the anticorrosion layer 2b may proceed gradually. The carbon sheet 8 is provided to suppress the elution and diffusion of such anticorrosion layer sulfides into the cathode chamber 5.

[0052] The carbon sheet 8 has a density of about 0.7 to 1.3 g / cc, preferably about 1.0 g / cc, for example.

[0053] As shown in FIG. 3, the carbon sheet 8 may be divided into three partial sheets 8p, 8q, and 8r, similar to the positive electrode current collector 7, from the viewpoint of ease of fabrication and arrangement.

[0054] 4 and 5 are diagrams for explaining the effect of the carbon sheet 8. Fig. 4(a) and Fig. 4(b) respectively show the state of the vicinity of the positive electrode chamber 5 at the beginning of use and after deterioration of the anticorrosion layer 2b in an NaS battery 1α not provided with the carbon sheet 8. Fig. 5(a) and Fig. 5(b) respectively show the state of the vicinity of the positive electrode chamber 5 at the beginning of use and after deterioration of the anticorrosion layer 2b in an NaS battery 1 according to the present embodiment provided with the carbon sheet 8.

[0055] 6 is a graph showing the relationship between the number of charge / discharge cycles and the value obtained by converting the amount of elution of the anticorrosion layer 2b into the positive electrode chamber 5 into the film thickness of the anticorrosion layer 2b reduced by elution (positive electrode elution amount converted into film thickness) for the NaS battery 1α and the NaS battery 1. The larger the positive electrode elution amount converted into film thickness in FIG. 6 , the more the elution of the anticorrosion layer (generation of sulfides in the anticorrosion layer) progresses and the thickness of the anticorrosion layer is reduced.

[0056] The NaS battery 1α and the NaS battery 1 had the same configuration except for the presence or absence of the carbon sheet 8. That is, the size of the positive electrode chamber 5 was the same in both batteries, and the sizes of the positive electrode current collector 7 and the high-resistance layer 15 before being placed in the positive electrode chamber 5 were also the same in both batteries. The thickness of the carbon sheet 8 was 0.3 μm. The charge / discharge cycle was counted as one cycle (the same applies hereinafter).

[0057] The amount of elution from the positive electrode converted into the film thickness was calculated by the following procedure.

[0058] (1) Dismantle the cell after operation and remove the positive electrode current collector 7; (2) Drill holes in the removed positive electrode current collector 7 and collect shavings; (3) Quantitatively analyze (chemically analyze) the collected shavings to identify the content of the anticorrosion layer 2b component in the shavings; (4) Calculate the content of the anticorrosion layer 2b component contained in the entire positive electrode current collector 7 based on the content of the anticorrosion layer 2b component in the identified shavings and the ratio of the volume of the perforated portion of the positive electrode current collector 7 to the total volume of the positive electrode current collector 7; (5) Calculate the film thickness equivalent of the amount of elution in the positive electrode based on the calculated content, the density of the anticorrosion layer 2b, and the area of ​​the inner surface of the positive electrode container 2.

[0059] In FIG. 6, small white circles indicate the results for NaS battery 1α, and large black circles indicate the results for NaS battery 1.

[0060] In both the NaS battery 1α and the NaS battery 1 in the early stage of use, the anticorrosion layer 2b is sufficiently provided on the inner surface of the positive electrode case 2, and the high resistance layer 15 is also maintained at a sufficiently high resistance. Therefore, as shown in Figures 4(a) and 5(a), the presence of the high resistance layer 15 suitably prevents sulfur (S) generated from sodium polysulfide during charging from being generated on the surface 3c of the solid electrolyte tube 3 and adhering to the surface 3c. Therefore, sodium ions (Na ＋ ) passes smoothly through the solid electrolyte tube 3 and moves to the anode chamber 6.

[0061] However, as use continues (as charge / discharge cycles are repeated), the corrosion protection layer 2b reacts with sulfur to produce corrosion protection layer sulfides 16, and the corrosion protection layer 2b decreases (becomes thinner) as a result.

[0062] In the case of the NaS battery 1α not provided with the carbon sheet 8, as shown in FIG. 4(b), the generated anticorrosion layer sulfide 16 easily moves into the positive electrode chamber 5. The results shown in FIG. 6 also generally follow this trend. That is, as shown by the dashed line, it can be seen that as the charge / discharge cycles increase, the anticorrosion layer sulfide 16 increases and the anticorrosion layer 2b decreases.

[0063] The corrosion protection layer sulfides 16 that have moved into the positive electrode chamber 5 pass through the high resistance layer 15, reach the surface 3c of the solid electrolyte tube 3, and adhere to the surface 3c. Because the corrosion protection layer sulfides 16 have a lower resistance than the high resistance layer 15, sulfur (S) generated from sodium polysulfide tends to selectively adhere to the corrosion protection layer sulfides 16 that have adhered to the surface 3c of the solid electrolyte tube 3.

[0064] As a result, on the surface 3c of the solid electrolyte tube 3, the adhesion of sulfur (S) via the anticorrosion layer sulfide 16 progresses, and sodium ions (Na ＋ ) is inhibited from moving to the negative electrode chamber 6. This is undesirable because it increases the battery resistance and the residual capacity (reduces the chargeable capacity).

[0065] 5(b), the generation of anticorrosion layer sulfides 16 and the accompanying reduction in the anticorrosion layer 2b also progress because the carbon sheet 8 does not necessarily provide an airtight seal between the anticorrosion layer 2b and the positive electrode current collector 7.

[0066] However, in the case of the NaS battery 1 according to the present embodiment, since the carbon sheet 8 is interposed between the anticorrosion layer 2b and the positive electrode current collector 7, most of the generated anticorrosion layer sulfides 16 remain near the positive electrode container 2. The amount of anticorrosion layer sulfides 16 that moves through the positive electrode chamber 5, passes through the high resistance layer 15, and reaches the surface 3c of the solid electrolyte tube 3 is suppressed compared to the NaS battery 1α. The results shown in FIG. 6 can be said to be in line with this.

[0067] That is, in the case of the NaS battery 1 according to this embodiment, even if use is continued, adhesion of the anticorrosion layer sulfide 16 and further sulfur to the surface 3c of the solid electrolyte tube 3 is suppressed, and therefore the migration of sodium ions generated from sodium polysulfide to the anode chamber 6 is suitably maintained. This means that the presence of the carbon sheet 8 is effective in suppressing an increase in battery resistance and an increase in residual capacity due to sulfur adhesion.

[0068] Figure 7 is a graph plotting the battery resistance values ​​measured at appropriate intervals while repeatedly charging and discharging NaS battery 1 against the number of charge-discharge cycles. The thickness of the carbon sheet 8 was 0.3 μm. In Figure 7, a predicted change in electrical resistance when the number of charge-discharge cycles is further increased based on the plotted results is extrapolated as curve L1. In addition, the relationship between the number of charge-discharge cycles and battery resistance for NaS battery 1α, which has the same configuration as NaS battery 1 except for not including the carbon sheet 8, is also shown as curve L0.

[0069] FIG. 7 suggests that the increase in battery resistance during continued use of the NaS battery 1 is suppressed compared to the increase in battery resistance during continued use of the NaS battery 1α.

[0070] Preferably, the carbon sheet 8 is one that has been made as free from moisture absorption as possible. Carbon sheets are generally known to have the property of absorbing moisture. If a carbon sheet 8 that has absorbed moisture is placed in the cathode case 2, the moisture will volatilize during battery operation, increasing the pressure in the cathode chamber 5 and changing the pressure balance between the cathode chamber 5 and the anode chamber 6 from what was assumed at the time of design, which is undesirable from a safety standpoint.

[0071] Methods for preventing the carbon sheet 8 from absorbing moisture include, for example, (1) a method of subjecting the carbon sheet 8 to a high-purity treatment to remove elements that absorb moisture, and (2) a method of drying the carbon sheet 8 prior to assembling the NaS battery 1.

[0072] An example of method (1) is a method in which the carbon sheet 8 is heated to a high temperature in a vacuum, and then a highly reactive gas is passed through the carbon sheet 8 to highly purify it by reacting the impurity components with the gas, thereby reducing the ash content to approximately 50 ppm or less.

[0073] As an example of method (2), prior to assembling the NaS battery 1, the carbon sheet 8 is heated and dried in a vacuum at a temperature of 100°C or higher to remove only the moisture from the carbon sheet 8, and then the assembly is performed before the moisture originating from the atmosphere during the assembly process is re-adsorbed.

[0074] In such a case, if the moisture content of the atmosphere during the assembly process is not at an acceptable level, simple drying may be performed by temporarily storing the product in a container through which dry air or the like is circulated after vacuum heating, and assembly may be performed in a state in which the amount of moisture adsorbed has been reduced.

[0075] For example, it has been confirmed that the internal battery pressure after heating is approximately the same for a NaS battery 1 that is assembled immediately after drying using a carbon sheet 8 that has been dried in a vacuum at a heating temperature of 200°C for approximately 1 hour, and a NaS battery 1 that is assembled immediately after storage using a carbon sheet 8 that has been dried under the same conditions and then stored in dry air with a dew point of approximately -60°C for approximately 15 to 30 minutes.

[0076] At first glance, the arrangement of the carbon sheet 8 in the NaS battery 1 according to this embodiment may be said to resemble the arrangement of the carbon sheet in the sodium-sulfur secondary battery disclosed in Patent Document 6.

[0077] However, the carbon sheet disclosed in Patent Document 6 is very dense and provided in contact with the cathode can 2 in order to significantly reduce the penetration of sulfur and / or sodium polysulfide, which corrode the cathode can 2, through the film to the cathode can wall. In contrast, in the NaS battery 1 according to the present embodiment, corrosion of the cathode can 2 is essentially suppressed by the formation of the anticorrosion layer 2b. The carbon sheet 8 is provided solely for the purpose of preventing the migration of anticorrosion layer sulfides 16, which are generated by the reaction between the anticorrosion layer 2b and sulfur, within the cathode chamber 5. Furthermore, as can be seen from the fact that the carbon sheet 8 is fixed to the cathode can 2 with double-sided tape 14 as described above, it is acceptable for there to be some gap between the carbon sheet 8 and the cathode can 2 (anticorrosion layer 2b). Therefore, the carbon sheet 8 does not necessarily need to be dense.

[0078] Therefore, the carbon sheet 8 in the NaS battery 1 according to this embodiment and the carbon sheet disclosed in Patent Document 6 are completely different in purpose, arrangement including adjacent components, and action and effect.

[0079] <Measures against excessive compression of high-resistance layer> As described above, in the NaS battery 1 according to this embodiment, in which the carbon sheet 8 is disposed adjacent to the anticorrosion layer 2b forming the inner surface of the positive electrode container 2 in the positive electrode chamber 5, adhesion of sulfides and even sulfur to the surface 3c of the anticorrosion layer 2b is suppressed.

[0080] However, in both NaS battery 1α and NaS battery 1, the positive electrode current collector 7 is fixed in a compressed state in the positive electrode chamber 5, and if the shapes and sizes of the positive electrode container 2 and the solid electrolyte tube 3 of both batteries are the same, the positive electrode current collector 7 in NaS battery 1 is compressed more by the amount of the carbon sheet 8. In other words, the repulsive force generated in the positive electrode current collector 7 due to such compression is greater in NaS battery 1 equipped with carbon sheet 8 than in NaS battery 1α not equipped with carbon sheet 8.

[0081] The difference in the magnitude of the repulsive force appears as a difference in the pressing force that brings the high-resistance layer 15 into contact with the solid electrolyte tube 3. Therefore, in the NaS battery 1 according to this embodiment, the high-resistance layer 15 receives a larger pressing force than the high-resistance layer 15 provided in the NaS battery 1α, and may be in an excessively compressed state.

[0082] Such excessive compression of the high-resistance layer 15 weakens the effect of the high-resistance layer 15 in suppressing the adhesion of sulfur to the surface 3c of the solid electrolyte tube 3, and further leads to an increase in the battery resistance during charging. This is undesirable because it cancels out the effect of the carbon sheet 8 in suppressing the increase in residual capacity described above.

[0083] In view of this point, the NaS battery 1 according to this embodiment is designed to deal with excessive compression of the high resistance layer 15 and to ensure the effect of interposing the carbon sheet 8.

[0084] Generally speaking, the countermeasure against such excessive compression is achieved by increasing the glass density (glass weight per unit volume) of the high resistance layer 15 compared to an NaS battery not provided with the carbon sheet 8, and increasing the proportion of glass fibers present at least near the surface 15a of the high resistance layer 15. This can be achieved, for example, by increasing the surface density of the glass felt when the high resistance layer 15 is integrated with the positive electrode current collector 7.

[0085] In addition, the degree of increase in the proportion of glass fibers near the surface 15a of the high-resistance layer 15 can be evaluated by using as an index the normalized value (normalized glass reflectivity Rn) obtained by dividing the light reflectivity (glass reflectivity) value at the surface 15a by the glass reflectivity value in the NaS battery 1 provided with the high-resistance layer 15 under the same conditions as the NaS battery 1α without the carbon sheet 8.

[0086] As described above, it is not easy to clearly specify the numerical value of the width of the high-resistance layer 15 integrated with the positive electrode current collector 7 by needle punching. However, it is believed that an increase in the glass density in the high-resistance layer 15 also contributes to increasing the radial extent of the high-resistance layer 15 relative to that of the NaS battery 1α (making the high-resistance layer 15 relatively thicker). However, an excessive increase in the width of the high-resistance layer 15 may impair the function of the positive electrode current collector 7. As long as the proportion of glass fibers near the surface 15a is suitably increased, the increase in the width of the high-resistance layer 15 due to excessive compression may be limited.

[0087] FIG. 8 shows the surface density of the glass felt when the high resistance layer 15 is integrated with the positive electrode current collector 7, at 220 g / m ２ , 300 g / m ２ , 400 g / m ２ , 350 g / m ２ 1 is a graph showing the distribution of glass density in the positive electrode current collector 7 of each NaS battery 1 when the conditions are changed in four ways (conditions 1 to 4).

[0088] Condition 1 is the same as the condition when the high resistance layer 15 is provided in the NaS battery 1α without the carbon sheet 8. For the NaS batteries 1 under conditions 1 to 4, evaluation was performed on three samples.

[0089] Alternatively, one NaS battery 1 was selected for each condition, and the normalized glass reflectance Rn of the surface 15a of the high-resistance layer 15 was determined. In this case, the glass reflectance was measured under light from a fluorescent lamp before the positive electrode current collector 7 was placed in the positive electrode chamber 5. Note that an LED may also be used as the light source.

[0090] The normalized glass reflectances Rn under conditions 1, 2, 3, and 4 were 1.0, 1.4, 1.5, and 1.9, respectively.

[0091] The glass weight density was measured at three different locations on the positive electrode current collector 7: a predetermined range from the surface 15a of the high-resistance layer 15 (near the solid electrolyte tube 3), a predetermined range in the center, and a predetermined range near the positive electrode container 2. Specifically, a first boundary was defined as a boundary between the vicinity of the solid electrolyte tube 3 and the center of the positive electrode current collector, and a second boundary was defined as a boundary between the vicinity of the solid electrolyte tube 3 and the center of the positive electrode current collector, and a second boundary was defined as a boundary between the vicinity of the center and the vicinity of the positive electrode container 2, and the positive electrode current collector was sliced ​​at these first and second boundaries to obtain test specimens at each location. The test specimens were heated in air to 800°C or higher to burn off the carbon, and the remaining glass weight was measured. The measured value was divided by the volume of the test specimen to obtain the glass density at each location.

[0092] 8 confirms that the normalized glass reflectance Rn increases as the glass density in the high-resistance layer 15 increases, but the difference in glass density between conditions is smaller in the central portion and in the vicinity of the positive electrode case 2 than in the high-resistance layer 15. In the cases of Conditions 3 and 4, the glass density in the high-resistance layer 15 is approximately two to four times the glass density in the vicinity of the positive electrode case 2.

[0093] The results of Conditions 3 and 4 mean that the high resistance layer 15 , which has a higher proportion of glass fibers than the inside of the positive electrode current collector 7 , is suitably formed in the contact portion with the solid electrolyte tube 3 .

[0094] Considering the glass density in the high resistance layer 15 under conditions 3 and 4 in FIG. 8, the glass density is at least 0.02 mg / mm ３ ~0.05mg / mm ３ By providing the high resistance layer 15 so as to satisfy the above condition, even when the carbon sheet 8 is provided, it can be said that the high resistance layer 15 has a large proportion of glass fibers and therefore a large normalized glass reflectance Rn.

[0095] On the other hand, for condition 1, the glass density was approximately 0.008 mg / mm ３ ~0.015mg / mm３ This suggests that when the high-resistance layer 15 is provided under the same conditions as the NaS battery 1α without the carbon sheet 8, as in condition 1, the high-resistance layer 15 may not be sufficiently formed. This is thought to be because the high-resistance layer 15 is excessively compressed, reducing the width of the high-resistance layer 15.

[0096] 9 is a graph showing the change in charging resistance versus the number of charge-discharge cycles when charge-discharge cycles were repeated for four NaS batteries 1 each provided with a high-resistance layer 15 under conditions 1 to 4. For each NaS battery 1, the normalized glass reflectance Rn measured for the positive electrode current collector 7 before placement in the positive electrode chamber 5 was 1.0, 1.3, 1.6, and 1.9 under conditions 1 to 4, respectively.

[0097] 9 further shows the approximate slope (corresponding to the rate of change) of the charge resistance versus the number of charge / discharge cycles for each NaS battery 1, indicated by arrows AR1, AR2, AR3, and AR4. The magnitude of the slope of these arrows AR1 to AR4 relative to the horizontal direction indicates the rate of increase in the charge resistance with continued use of the NaS battery 1.

[0098] The charging resistance at 55 cycles was sufficiently small, at around 1.2 mΩ, for all NaS batteries 1. However, as can be seen from arrows AR1 to AR4, the gradient of the charging resistance tends to be smaller for the NaS batteries 1 under conditions 3 and 4, where the normalized glass reflectance Rn is 1.6 and 1.9, respectively, than for the NaS batteries 1 under conditions 1 and 2, where the normalized glass reflectance Rn is 1.0 and 1.3, respectively.

[0099] These results indicate that increasing the proportion of glass fibers in the high-resistance layer 15 has the effect of offsetting the effect of the carbon sheet being inserted due to over-compression of the high-resistance layer 15, thereby suppressing deterioration of charging performance over time.

[0100] The glass density of the high resistance layer 15 is 0.05 mg / mm ３ Although the same effect can be obtained at more than 0.05 mg / mm ３It is not preferable to exceed this value because it increases the initial resistance of the NaS battery 1.

[0101] As described above, according to the present embodiment, in an NaS battery, a positive electrode current collector containing sulfur as a positive electrode active material is disposed in a positive electrode chamber surrounded by a positive electrode container having an anticorrosion layer made of an Fe—Cr alloy on its inner surface and a solid electrolyte tube, and metallic sodium as a negative electrode active material is disposed in an anode chamber inside the solid electrolyte tube. By interposing a carbon sheet between the positive electrode current collector and the anticorrosion layer, an effect of suppressing an increase in battery resistance and an increase in residual capacity due to adhesion of sulfur to the solid electrolyte tube can be obtained even when the NaS battery is used continuously.

[0102] Preferably, the glass density of the high resistance layer provided on the surface of the positive electrode current collector facing the solid electrolyte tube is 0.02 mg / mm ３ ~0.05mg / mm ３ This suitably prevents the effect of the carbon sheet from being offset by excessive compression of the high resistance layer, thereby preventing the charging performance of the NaS battery from deteriorating over time.

Claims

1. A sodium-sulfur secondary battery comprising: a positive electrode can, the inner surface of which is a corrosion-resistant layer made of an Fe—Cr alloy; a solid electrolyte tube disposed within the positive electrode can and spaced apart from the positive electrode can; a positive electrode chamber surrounded by the positive electrode can and the solid electrolyte tube, in which a positive electrode current collector containing sulfur as a positive electrode active material is disposed; and an anode chamber within the solid electrolyte tube separated from the positive electrode chamber by the solid electrolyte tube, in which metallic sodium as an anode active material is disposed; wherein a high-resistance layer containing glass fiber is provided integrally with the positive electrode current collector and in contact with the solid electrolyte tube; and a carbon sheet is interposed between the positive electrode current collector and the corrosion-resistant layer.

2. The sodium-sulfur secondary battery according to claim 1, wherein the glass density in the high resistance layer is 0.02 mg / mm ３ ~0.05mg / mm ３ The sodium-sulfur secondary battery is characterized by:

3. A sodium-sulfur secondary battery according to claim 2, characterized in that the glass density in the high resistance layer is two to four times the glass density in the vicinity of the positive electrode container of the positive electrode current collector.

4. A method for manufacturing a sodium-sulfur secondary battery, comprising: a) preparing a cathode container having an anticorrosion layer made of an Fe—Cr alloy on its inner surface; b) arranging a solid electrolyte tube inside the cathode container at a distance from the cathode container to provide a cathode chamber surrounded by the cathode container and the solid electrolyte tube, and a cathode chamber inside the solid electrolyte tube separated from the cathode chamber by the solid electrolyte tube; c) providing a high-resistance layer on one side of graphite felt by overlapping glass felt on the glass felt side and needle-punching from the glass felt side to drive the glass fibers of the glass felt into the graphite felt; and d) impregnating the graphite felt after the high-resistance layer has been provided with sulfur, which is a cathode active material, to obtain a cathode current collector integrated with the high-resistance layer. e) placing the positive electrode current collector in the positive electrode chamber while bringing the high-resistance layer into contact with the solid electrolyte tube and while interposing a carbon sheet between the high-resistance layer and the corrosion protection layer; and f) placing metallic sodium, which is a negative electrode active material, in the negative electrode chamber.

5. A method for producing a sodium-sulfur secondary battery according to claim 4, wherein in step c), the graphite felt is prepared having a thickness greater than the width of the positive electrode chamber before being placed in the positive electrode chamber in step e), and in step e), the positive electrode current collector and the carbon sheet are fixed in the positive electrode chamber by a biasing force generated in the positive electrode current collector compressed to the width of the positive electrode chamber.

6. A method for producing a sodium-sulfur secondary battery according to claim 4 or 5, wherein in step c), the glass density of the high resistance layer after being placed in the positive electrode chamber together with the positive electrode current collector in step e) is 0.02 mg / mm ３ ~0.05mg / mm ３ and providing the high resistance layer so that the high resistance layer satisfies the above condition.

7. A method for producing a sodium-sulfur secondary battery according to claim 6, wherein in step c), the glass density of the high-resistance layer after being placed in the positive electrode chamber together with the positive electrode current collector in step e) is set to be two to four times the glass density of the positive electrode current collector in the vicinity of the positive electrode container.

8. A method for producing a sodium-sulfur secondary battery according to claim 4 or 5, characterized in that the carbon sheet used is a carbon sheet from which elements that adsorb moisture have been removed.

9. A method for producing a sodium-sulfur secondary battery according to claim 4 or 5, characterized in that, prior to step e), the following step g) is carried out: drying the carbon sheet.

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