JPS62110270A - Sodium-sulphur cell - Google Patents

Abstract

PURPOSE:To make it possible to enhance the charge/discharge depth at the maximum degree and reduce the inner resistance, by arranging a thin material with a high electronic conductivity, allowing a fluidity of active substance, parallel to the surface of a solid electrolyte. CONSTITUTION:A sodium 2 which is an anode active substance enclosed and held in an anode container 1 is separated from the cathode by a solid electrolyte tube 4. In order to feed a melted metallic sodium to all the area of inner wall of a solid electrolyte tube 4, sodium is filled in the area between the sodium holding tube 3 and the solid electrolyte tube 4 with a surface tension. The cathode 5 is a porous electronic conductive material impregnated with sulphur as a cathode active substance, and held in the area composed of a cathode container 6 and a solid electrode outer wall. By furnishing a highly conductive area 7 close to the surface of the solid electrolyte tube, the conductive rate in the direction parallel to the surface of the solid electrolyte tube can be increased, and a reaction in the direction parallel to the surface of the solid electrolyte tube can be made even.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Applications] The present invention relates to a sodium-sulfur battery, and particularly to a structure of a battery anode portion suitable for improving the internal resistance, charge/discharge depth, and life of the battery.

[Prior art and its problems]

The anode structure of a sodium-sulfur battery has a large effect on the battery's internal resistance and depth of charge and discharge. The performance of these two batteries is best achieved by uniformly reacting the anode active material present in the anode.

Generally, in order to achieve uniformity of this reaction, an anode structure is used in which a porous electron conductive material is impregnated with sulfur, which is an anode active material, inside the anode.

In many cases, the battery reaction inside the anode cannot be made sufficiently uniform by simply placing a porous conductive material inside the anode. A structure has been used in which the longitudinal direction of the fibers of a mat or felt-like porous electron conductive material is arranged so as to alleviate the heterogeneity of the reaction. However, in the case of high-speed charging and discharging of batteries, or in the case of large-capacity single cells, the conventional structure could not completely homogenize the reaction, and sufficient charging and discharging capacity could not be obtained. This is caused by both the non-uniform reaction occurring in the direction perpendicular to the solid electrolyte tube surface and the reaction non-uniformity occurring in the direction parallel to the solid electrolyte tube surface. Non-uniformity can be alleviated by reducing the conductivity of the porous electronically conductive material near the surface of the solid electrolyte tube. The reaction nonuniformity that occurs in the direction parallel to the solid electrolyte tube surface is thought to be due to nonuniformity in the volume density of the active material caused by the influence of gravity, and is especially noticeable when large batteries are charged and discharged at high speed. . This reaction non-uniformity that occurs in the direction parallel to the solid electrolyte tube surface cannot be resolved by the conventional method of aligning the fiber direction of the porous electron conductive material, and the reaction ends in the area where the reaction has progressed inside the anode. There is a problem in that the internal resistance of the battery increases, and therefore, charging and discharging of the battery must be terminated even though unreacted active material remains in the anode.

[Purpose of the invention]

An object of the present invention is to maximize the depth of charge and discharge of a sodium-sulfur battery and provide an anode structure with low internal resistance.

[Means for solving problems]

The present invention makes active use of the basic property that the electromotive force of a sodium-sulfur battery decreases as the reactivity of the anode active material increases, and the electromotive force decreases due to local differences in electromotive force due to non-uniform reaction. It generates a forward current flowing inside the anode, and the negative feedback effect of this current eliminates localization of battery reactions. Even in the structure of the conventional porous electron conductive material in which the fiber directions are aligned, the reaction becomes uniform within the anode due to this effect. In other words, if the conductivity of the electron-conducting material is large in the direction where the reaction is non-uniform, electron current will flow more easily in this direction, and the reduction current will flow to the areas where the discharge has progressed, and the reduction current will flow to the areas where the discharge has been delayed. An oxidation current is supplied from the electron conductive material to the anode active material, resulting in uniform distribution of the active material inside the anode.

The rate at which this reaction is made uniform is thought to be approximately proportional to the conductivity of the electron conductive material in the direction in which the reaction becomes non-uniform, so in order to further make the reaction uniform, the conductivity of the porous electron conductive material should be increased. Just do it. However, in contrast to the direction in which the sodium ion current generated from the solid electrolyte flows, increasing the conductivity of the porous electron conductive material causes the battery reaction to concentrate near the solid electrolyte tube surface, resulting in poor battery performance. It is known that Therefore, the porous electronically conductive material used in the anode part of sodium-sulfur batteries has high conductivity in the direction parallel to the surface of the solid electrolyte.
In the vertical direction, anisotropy of conductivity is required, such that the conductivity is less than a few mho per unit of senna meter. For porous current collectors with uniform fiber directions, this anisotropy is small, and the electrical conductivity in the fiber direction is at most several times the electrical conductivity in the direction perpendicular to the fibers, so that the anisotropy occurs in the direction parallel to the solid electrolyte tube. It is difficult to suppress the localization of the reaction distribution, and is almost impossible, especially for large batteries.

Looking at the entire sodium-sulfur battery, the conductivity of the metallic sodium in the cathode and the collector electrode connected to the porous electron-conducting material in the anode part is extremely high at several thousand mho per senna meter. The electrical conductivity is approximately 0.2 mo per centimeter. Therefore, a large potential gradient can be generated near the solid electrolyte surface in a direction parallel to the solid electrolyte tube surface compared to other parts.
Reaction non-uniformity is likely to occur in this area. Therefore, the reaction can be most effectively uniformized by installing a thin material having high electronic conductivity and not preventing the flow of the active material in this portion parallel to the solid electrolyte surface.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIG. Sodium 2, which is a cathode active material, is hermetically held in a cathode container l, and is isolated from the anode part by a solid electrolyte tube 3.

In order to supply molten metal sodium to the entire inner wall of the solid electrolyte tube 4, sodium fills the area between the sodium holding tube 3 and the solid electrolyte tube 40 due to surface tension. The anode section 5 is made of a porous electron conductive material impregnated with sulfur, which is an anode active material, and is held within a region formed by the anode container 6 and the outer wall of the solid electrolyte tube.

In the initial state of discharge when the anode is filled with sulfur, a voltage of about 2.08 V, which corresponds to the electromotive force between sodium and atomic sulfur, is generated between the metal container 6, which is the anode electrode, and the metal container L, which is the cathode electrode. When an external load is connected between these metal containers, a current is driven. When a current is applied to the outside, electrons are supplied to the anode portion from an external circuit, and these electrons can be absorbed by sulfur atoms present in the anode to form negative sulfur ions. This negative charge is neutralized by combining with the positively charged sodium ion, but the negative space potential that is formed before being neutralized moves in the direction from the sodium cathode to the anode.
An electric field is generated inside the solid electrolyte, and as a result, movable sodium ions inside the solid electrolyte are supplied to the anode side. This movement of sodium ions ionizes the sodium atoms on the solid electrolyte tube surface of the sodium inside the cathode,
Electrons generated as a result of this reaction are supplied from the cathode container to an external circuit. Since the above reactions occur continuously, current is constantly supplied to the external load.

As the discharge progresses, the amount of sodium inside the anode increases, and when the amount of sodium becomes approximately 0.4 mol per 1 mol of sulfur, the electromotive force of the battery gradually decreases as the discharge progresses. This decrease in electromotive force is the composition at which the active material within the anode begins to solidify. The discharge continues until the depth of discharge becomes 0.7 mol of sodium per 1 mol of sulfur, and with this composition an electromotive force of about 1.78 V is obtained from the battery. Discharging the battery to a composition where the active material begins to solidify not only leads to a decrease in energy efficiency due to an increase in the internal resistance of the battery, but also leads to the formation of acicular Na2S in the active material.

Since solids accelerate the deterioration of the solid electrolyte and other anode components and have a negative impact on battery life, in normal battery operation, discharging is terminated when the concentration of sodium is 0.67 mol per 1 mol of sulfur. ing.

When charging a battery, the opposite reaction occurs when discharging.

In other words, negative sulfur ions dissociated from sodium in the anode are neutralized by releasing reduction electrons by the electric current supplied from the outside, and the positive space potential due to the remaining positive sodium ions moves the sodium ions to the cathode through the solid electrolyte. By the electrons supplied from the external circuit inside the cathode,
With the continuous charging reaction of 0 or more in which sodium ions are neutralized, the electromotive force of the battery increases reversibly with the discharging reaction, and at the end of charging, almost all the sodium inside the anode returns to the cathode, reaching 2.08V again. Become. During charging, the sodium density near the surface of the solid electrolyte tube gradually decreases, but when the sodium in this area completely disappears and a layer of sulfur atoms is formed on the solid electrolyte surface, sodium transport from the anode to the cathode occurs. become unable to do so. As a result, the internal resistance of the battery increases, energy efficiency decreases as in the case of the discharge path, and local concentration of current occurs, causing deterioration of the solid electrolyte, which causes a reduction in battery life. Therefore, in normal battery operation, charging is not performed until the amount of sodium is 0.05 mol or less per 1 mol of sulfur.

During actual battery operation, the above battery reactions occur non-uniformly at the anode portion. Although this non-uniformity is three-dimensional,
When the sodium ion current and the electron current necessary to oxidize or reduce the active material flow in the direction from the solid electrolyte surface toward the anode container, the difference in potential gradient generated by both types of flow becomes non-uniform. At this time, the reaction distribution becomes non-uniform. In order to eliminate this non-uniformity, the porous electron conductive material inside the anode is arranged in layers so that the conductivity increases in the direction from the solid electrolyte surface toward the anode container. In this way, in order to equalize the reaction in the direction perpendicular to the solid electrolyte tube surface by adjusting the conductivity of the porous electron conductive material, it is necessary to adjust the conductivity of the porous electron conductive material at the solid electrolyte tube surface. It must be less than 1 mho per centimeter. The conductivity of the solid electrolyte tube is also less than 1 mho per centimeter, so a small current generates a large potential difference at the anode near the solid electrolyte.

In other words, there is a high possibility that non-uniform reaction will occur in the direction parallel to the solid electrolyte tube surface.Especially when a large battery is placed vertically, the density of the active material will be lower than that of the anode due to the influence of gravity. Because it is sparse in the upper part, the reaction distribution in this direction becomes non-uniform.In other words, discharge progresses faster in the upper part where there is less active material than in the lower part. Since it is proportional to , it becomes even more noticeable during high-speed charging and discharging.

As shown in FIG. 1, providing a region 7 with high conductivity near the solid electrolyte tube surface can increase the conductivity in the direction parallel to the solid electrolyte tube surface near the solid electrolyte tube surface. The reaction in the direction parallel to can be made uniform. For such areas with high conductivity, use a material with high conductivity that is highly resistant to corrosion against sulfur and sodium polysulfide, such as a thin film made of metal molybdenum with holes or a thin wire network. By doing so, it can be easily formed. When a mesh of metal molybdenum wire is used, the effect of homogenizing the reaction by this high electron conductivity material layer is about several thousand times greater than the effect of a structure without a mesh, and this effect is particularly noticeable when the anode is long.

What is necessary to form a layer of deceptively high electronic conductivity material is that the flow of the active material through this layer is unobstructed, and for this reason only when using a mesoche structure is the diameter of the metal wires adjusted to the distance between the wires. is preferably about 18 or less.

Further, it is not preferable to bring the high electron conductivity material into direct contact with the solid electrolyte tube surface because it may cause a sulfur layer to be formed between the high electron conductivity material and the solid electrolyte during charging. If the gap between the solid electrolyte tube and the electron conductive material is several millimeters or less, a sufficient effect of homogenizing the reaction can be obtained. It is preferable to insert a porous material having a resistivity necessary to uniformize the reaction in both directions. To manufacture an anode body with such a laminated structure, each member is stacked in a molded body that has a space divided into several parts, and after being impregnated with sulfur, it is taken out from the molded body and inserted into the battery anode part. Good.

Another important point in forming an anode having a high electronic conductivity material layer is that the high electronic conductivity material placed near the solid electrolyte tube does not touch the anode container. This is because when the above two components are electrically connected, the battery reaction progresses only between the high electron conductivity material and the solid electrolyte, and the reaction of the active material between the high electron conductivity material and the metal container is suppressed. This is because the depth of charge and discharge of the battery is significantly reduced.

In order to avoid such electrical contact without reducing the amount of active material in the anode, it is necessary to add sulfur to the anode end on the tubular end side of the solid electrolyte bag in a porous material with an electrical conductivity of less than a few moles per centimeter. A plate-like material impregnated with the anode may be inserted from the anode end, and then the anode container may be sealed by welding or the like.

The internal resistance of the battery having the above structure remains constant for almost all the amount of discharged charge during the charging and discharging period. On the other hand, the internal resistance of a battery without a high electronic conductivity material layer increases with the progress of discharging and charging, and as a result, the depth of charge and discharge may be 40% or less of the theoretical capacity. It was confirmed by measuring the current density at the battery anode that the cause of this was non-uniform reaction in the direction parallel to the solid electrolyte tube surface. Makes the distribution of ion current non-uniform. Localization of the sodium ion current in the solid electrolyte due to this effect results in a significant shortening of the life of the solid electrolyte tube, and the life of a single cell with a markedly non-uniform reaction is 10-100% less than that of a cell with a uniform reaction. It is about 1/10th of that.

Furthermore, since it is possible to place vertically even for large batteries, Fig.
β”-alumina tube, which is a solid electrolyte in 4 of 1, and α- in 8
The reliability of the battery is improved because the glass solder material used to connect the alumina rings is no longer immersed in corrosive sodium polysulfide, reducing battery damage due to corrosion in this area.

〔Effect of the invention〕

By using the present invention, the reaction inside the anode of a sodium-sulfur battery can be made uniform, so it is possible to manufacture a large-sized battery with a large charge/discharge depth, low internal resistance, and a long life.

[Brief explanation of drawings]

FIG. 1 is a longitudinal cross-sectional view of a sodium-sulfur battery according to the invention. FIG. 2 is an enlarged view of the vicinity of the solid electrolyte tube in FIG. 1. 1... Cathode metal container, 2... Metallic sodium. 3... Sodium holding tube, 4... Solid electrolyte tube 5.
・Anode material made of porous electron conductive material impregnated with sulfur. 6... Anode metal container, 7... High electron conductivity material layer.

Claims (4)

[Claims] (1) A cathode active material consisting of molten sodium, an anode active material consisting of molten sulfur, a solid electrolyte tube separating both active materials,
In a sodium-sulfur battery, a porous electron conductive material is placed around a solid electrolyte tube to impregnate and hold the anode active material in order to extract the current generated by the battery reaction from the anode active material. A plate-shaped high electron conductive material having a structure that does not hinder movement of the anode active material in the vertical direction is arranged in the porous electron conductive material with its surface parallel to the surface of the solid electrolyte tube. Sodium-sulfur battery. (2) The plate-shaped high electron conductivity material is made of a highly conductive metal or ceramic that is resistant to corrosion against molten sulfur and sodium polysulfide and is formed into a mesh or perforated plate shape. A sodium-sulfur battery according to claim 1. (3) The sodium-sulfur battery according to claim 1 or 2, characterized in that the plate-shaped high electron conductivity material is arranged so as not to be in direct electrical contact with the anode of the battery. (4) A sodium-sulfur battery according to any one of claims 1 to 3, characterized in that a plate-shaped high electron conductivity material is arranged close to the surface of a solid electrolyte tube. .