1. Field of the Invention
This invention relates to a method and apparatus for diagnosing a sodium-sulfur cell for the presence or absence of any abnormal state, and more particularly to a method and apparatus of the kind described above which can detect progress of degradation of a sodium-sulfur cell and predict possibility of catastrophic breakdown of the cell in a very near future before such a final breakdown of the cell actually occurs.
2. Description of the Prior Art
A sodium-sulfur cell (which will be often abbreviated hereinafter as an Na-S cell) is a secondary cell of a type operating at a high temperature. In the sodium-sulfur cell, molten sodium which is a reactant of a negative electrode is disposed on one side of an electrolyte tube permeable to sodium ions only, while molten sulfur which is a reactant of a positive electrode is disposed on the other side of the electrolyte tube. The sodium-sulfur cell is charged and discharged at a temperature as high as about 300.degree. to 350.degree. C. The following cell reaction occurs when the sodium-sulfur cell is charged and discharged: ##STR1##
That is, in the discharge mode, sodium turns into sodium ions by freeing electrons, and the sodium ions permeate the electrolyte tube, which is the separator, and react with sulfur to produce sodium polysulfide, Na.sub.2 Sx, as a discharged product. On the other hand, in the charge mode, a voltage higher than the cell voltage is applied across the cell to cause a reaction reverse to that occurring in the discharge mode.
FIG. 1 shows a practical structure of a prior art sodium-sulfur cell 9.
Referring to FIG. 1, a bladder-like electrolyte tube 1 made of a material such as .beta.-alumina having an Na-ion conductivity is coaxially disposed in a positive electrode container (an outer electrode) 2 in the form of a cylindrical metal member having a closed bottom, in such a relation that a predetermined spacing a is maintained between the electrolyte tube 1 and the positive electrode container 2. The electrolyte tube 1 is joined or fixed at the outer periphery of its upper end opening 3 to a member 4 of an electrical insulator such as .alpha.-alumina ring in a cantilever fashion by a jointing member 5 such as glass solder. A negative electrode container 6 in the form of a cylindrical member having a closed top is disposed opposite to and fixed to the positive electrode container 2 through the insulator member 4 to constitute the cell body.
In the space b enclosed by the electrolyte tube 1 and negative electrode container 6, sodium acting as a negative reactant is filled together with porous metal fibers to constitute a negative electrode 7.
On the other hand, in the space c enclosed by the electrolyte tube 1 and positive electrode container 2, porous carbon impregnated with sulfur acting as a positive reactant is filled to constitute a positive electrode 8. The metal fibers filled in the space b to constitute the negative electrode 7 together with the sodium have a function of holding the sodium so as to prevent occurrence of a violent exothermic reaction between the sodium and the sulfur which direct reaction occurs when the electrolyte tube 1 is ruptured. Further, the carbon filled in the space c to constitute the positive electrode 8 acts to give an electron conductivity to the sulfur. That is, a graphite (carbon) mat is impregnated with sulfur.
In order that the cell reaction can easily take place in the Na/S cell constructed in the manner described above, the wall thickness of the partitioning electrolyte tube 1 permeable to sodium ions is advantageously as small as possible.
However, minute cracks tend to occur in the electrolyte tube 1 as the Na/S cell is repeatedly charged and discharged, and the cracks will develop to such an extent that the electrolyte tube 1 is finally ruptured. In such an event, the sodium directly reacts with the sulfur thereby destroying the function of the cell.
Primarily, many sodium-sulfur cells are connected for the purpose of storage of electric power. Accordingly, impossibility of electric power supply or storage due to breakdown of any one of such sodium-sulfur cells greatly adversely affects the social life as a result of, for example, a sudden stoppage of electric power supply.
JP-A-47-28431 (Japanese patent laid-open in 1972) is a newest publication relating to an Na/S cell. However, this known publication does not disclose any especial technique for diagnosing an abnormal state of the Na/S cell. That is, the known publication merely discloses the structure of an Na/S cell in which a conductive composite having a porosity of 50 to 98% and a pore diameter of 10 to 1,000 .mu. is used to constitute its positive electrode.