Sodium sulfur cell and process of manufacturing the same

Abstract: A sodium sulfur cell comprises a cathode active material (5) of metallic sodium, and anode active material (3) of sulfur or sodium polysulfide, and a beta alumina solid electrolyte separator (1). At least one of the above active materials containing, as an impurity, calcium of at most 20 ppm and/or potassium of at most 200 ppm, by weight. The cell of the invention has excellent characteristics, particularly a prolonged electromotive life.

Description: The present invention relates to secondary batteries to be used for example for electric power load leveling or electric vehicles, and particularly to sodium sulfur cells and a process for manufacturing the same. Sodium sulfur cells are high temperature type secondary cells which operate at 300°C-350°C and are composed of molten sodium as a cathode active material, molten sulfur and/or sodium polysulfide as an anode active material, a sodium ion conductive ceramic as a solid electrolyte, and a metallic container. The structure of a typical sodium sulfur cell is shown in Fig. 1. In Fig. 1, the numeral 1 indicates a sodium ion conductive beta alumina tube having a closed tip end, 2 is a metallic container functioning as an anode, 3 is sulfur or sodium polysulfide, 4 is a metallic container functioning as a cathode, 5 is sodium, 6 is a ring insulator such as an α-alumina, and 7 is a metallic lid. As the beta alumina 1, there are β″-alumina, β-alumina, and mixture of the both, or the like. Processes for manufacturing the above-described sodium sulfur cells generally comprise the steps of: bonding the open end periphery of the prepared β-alumina tube 1 with the ring insulator 6 of α-alumina or the like, by means of glass-soldering or the like; further bonding the ring insulator 6 supporting the β-alumina tube 1, with the metallic containers 2 and 4, by a solid phase reaction or the like at a high temperature under pressure, to form upper and lower spaces; then loading the metallic containers 4 and 2 with sodium 5 and sulfur or sodium polysulfide 3, respectively; and hermetically closing the metallic containers 2 and 4 with the lids 7,7′, by means of welding 8,8′ or the like, to provide a cell. In the sodium sulfur cells having the above-described structure, if a current concentration, an overvoltage on the end of charging or discharging, or impurity inclusion in the active materials, is caused by a certain reason, there have arisen problems of fracturing of the beta alumina tube and impairment of charging or discharging efficiency as the resistance increases. In particular, it has been found that impurity inclusion in the active materials is a serious problem. Reference is now made to some prior art. The Report "Electrode-induced beta battery degradation" prepared for New York State Energy Research and Development Authority (NYSERDA Report 84-9) of November 1983 reports some testing of the effects of K and Ca impurities in the sodium of such cells and concludes (i) that 270 ppm K in Na is acceptable, (ii) that 50 ppm Ca in Na does not affect cycling performance or cell life and (iii) that use of unpurified S did not affect performance. Further testing is suggested. In US-A-4220691 it is suggested that 200 ppm K in Na of a cell can cause adverse effects and that purification of sodium to below such a level would be unduly expensive. A proposal is made of providing gettering means for unwanted ions in the sodium. An object of the present invention is to solve the above-mentioned problems. By defining contents of specified impurities in the active materials, the invention can provide sodium sulfur cells with various characteristics improved, particularly cell life. A further object of the present invention is to provide a process for manufacturing such a sodium sulfur cell. A sodium sulfur cell according to the present invention is set out in claim 1. Furthermore, the process according to the present invention for manufacturing a sodium sulfur cell is set out in claim 4. It is found that various characteristics, specifically electromotive life, are improved according to the present invention, when, among impurities expected to be included in the active materials, the calcium content is restricted to at most 20 ppm, preferably at most 10 ppm, and the potassium content is restricted to at least 20 and at most 200 ppm, preferably at most 100 ppm, by weight. The reason why the calcium content should be limited to at most 20 ppm by weight is because, if it exceeds 20 ppm by weight, the electromotive life will deteriorate very much, as will be clear from the below-described Examples. This is caused by a resistance increase due to accumulation of Ca²⁺ ion-exchanged for Na⁺, on the surface as well as in the grain boundary of the beta alumina. Additionally, the reason why the potassium content should be limited also to at most 200 ppm by weight is because, if it exceeds 200 ppm by weight, the electromotive life will deteriorate extremely, as will be clear from the below-described Examples. This is caused by deterioration of the beta alumina by ion-exchange of K⁺ for Na⁺ in the beta alumina. Such an ion-exchange readily occurs as K⁺ has an ion radius of 1.4 times that of Na⁺. The present invention will be explained in more detail hereinafter by way of example with reference to the appended drawings wherein: Fig. 1 is a vertical sectional view of an embodiment of the sodium sulfur cell according to the present invention; Fig. 2 is a vertical sectional view showing an electromotion test of the cell of the present invention; and Figs. 3 and 4 are graphs showing relations of inclusion amounts, respectively of calcium and potassium, with electromotive life. Example 1 A beta alumina tube of a predetermined shape was prepared. Using an anode active material containing restricted impurities and a cathode active material i.e. sodium, an electromotion test was conducted with an Na/Na electromotion testing apparatus as shown in Fig. 2. Referring to Fig. 2, the Na/Na electromotion testing apparatus was composed of a beta alumina tube 15 to be tested, α-alumina insulative supports 16, 17, a stainless steel container 18, a stainless steel electrode 19 and electrode terminals 20, 21. The container 18 and beta alumina tube 15 were charged with molten sodium 22. By passing a constant electric current between the terminals 20 and 21, the electromotive life of the beta alumina tube 15 to be tested was determined. The reason why the electromotion test was conducted in the state of Na/Na instead of Na/S was to determine the pure electromotive force in the state of beta alumina tube alone by eliminating other factors, such as difference in contact resistance caused by S, and the like. Furthermore, in Fig. 2, the molten sodium 22 was fed into the stainless steel container 18 so that the sodium liquid level 23 might reach 40 mm below the top end of the beta alumina tube 15. The beta alumina tube 15 had an inside diameter of 17.6 mm, an outside diameter of 20.0 mm and a length of 140 mm. The current was passed at a temperature of 350°C with a current density of 1 A/cm², reversing the positive pole and negative pole every 120 seconds to minimize fluctuation of the sodium liquid level. An initial polarization value immediately after commencement of passage of the current was denoted by V₁ . When the polarization value during current passage increased to 1.5 times or more the initial polarization value V₁ or when it decreased to half or less of V₁, the life was regarded as exhausted and the test was stopped. An electromotive life J h (Ah/cm²) was found from the period of time from commencement of the current flow to the stop of the test. After stopping the test, Na was removed with ethanol and the outer appearance of the beta alumina tube was visually inspected. Here, the initial polarization value V₁ is found from the equation: V₁=V 1a -V 1b , where V 1a is the voltage between the electrode terminals 20 and 21 at a constant current density, and V 1b is the voltage between the electrode terminals 20 and 21 when the beta alumina tube 15 has been removed and the loaded sodium short-circuits. In Table 1 and Fig. 3, there is shown the result of the electromotive life when the potassium content was made constant at 20 ppm by weight and the calcium content was regulated. In Table 2 and Fig. 4, shown is the result of the electromotive life when the calcium content was made constant at 2 ppm by weight and the potassium content was regulated. The concentration of the impurities was found by a chemical analysis of Na. Further, other impurities included in the employed Na were 3 ppm Fe and less than 1 ppm Cr, by weight. It can be understood from the results shown in Tables 1 and 2 that when, as in the present invention, the calcium content is 20 ppm or less, and the potassium content is 200 ppm or less, by weight, there is obtained a prolonged electromotive life. Example 2 A sodium sulfur cell as shown in Fig. 1 was actually assembled. As a cathode active material, sodium containing K and Ca in regulated amounts and, as an anode active material, sulfur containing K and Ca also in regulated amounts, were prepared. These active materials were charged into the cell and a charge-discharge test was conducted at a temperature of 350°C with a current density of 100 mA/cm², in an 8 hour-cycle. The concentration of the impurities was found by a chemical analysis. Further, other impurities included in the employed Na were 6 ppm Fe and 2 ppm Cr, by weight, and those included in the employed S were less than 3 ppm Fe and less than 3 ppm Li, by weight. In the discharge test, when a mean value V BC over the period of charging or a mean value V BD over the period of discharging, of the voltage drop V B due to the resistance of the cell, increased to 1.5 times or more, or decreased to half or less, of the value in the first charge-discharge cycle, the life was regarded as exhausted. Then, an electromotive life J h (Ah/cm²) was found from the period of time from commencement of passage of the current to the stop of the test. Here established is an equation, V B =|V I -V OCV |, where V I represents a voltage between cell electrodes at the above-described constant current density, and V OCV represents an electromotive force when the cell electrodes circuit is opened. The results are shown in Table 3. It can be understood from the results shown in Table 3 that a satisfactory life of cell is obtained according to the present invention wherein the K and Ca contents in sodium as a cathode active material and in sulfur as an anode active material are restricted to not more than the defined amounts required by the invention, as compared with the Comparative Examples. As is clear from the above explanation, according to the process of the invention for manufacturing sodium sulfur cells wherein, among the impurities expected to be included in the active materials, tolerances of K and Ca are limited, sodium sulfur cells having excellent characteristics, particularly a prolonged electromotive life, can be obtained.

Claims: A sodium sulfur cell comprising a cathode active material, an anode active material and a beta alumina solid electrolyte separating said active materials from each other, wherein each of said active materials contains, as an impurity, calcium in an amount of at most 20 ppm by weight, and each of said active materials contains additionally, as an impurity, potassium in an amount of 20 - 200 ppm by weight. A sodium sulfur cell according to claim 1, wherein the calcium is contained in each of said active materials in an amount of at most 10 ppm by weight. A sodium sulfur cell according to claim 1 or claim 2 wherein the potassium is contained in each of said active materials in an amount of at most 100 ppm by weight. A process for manufacturing a sodium sulfur cell comprising a cathode active material, an anode active material and a beta alumina solid electrolyte separator separating the above active materials from each other, which process comprises, using as each of said active materials a material containing, as an impurity, calcium in an amount of at most 20 ppm by weight and contains, as an impurity, potassium in an amount of 20 - 200 ppm by weight. A process according to claim 4, wherein the calcium and potassium are contained in each of said active materials in amounts of at most 10 ppm and at most 100 ppm, by weight, respectively.

Inventors: KOBAYASHI AKIRA C O TOKYO ELEC, OHSHIMA MASAAKI C O TOKYO ELEC, YOSHIDA AKIHIKO, KOBAYASHI, AKIRA, C/O TOKYO ELECTRIC POWER CO.INC., OHSHIMA, MASAAKI, C/O TOKYO ELECTRIC POWER CO.INC., YOSHIDA, AKIHIKO
Applicants: NGK INSULATORS LTD, TOKYO ELECTRIC POWER CO, NGK INSULATORS, LTD., THE TOKYO ELECTRIC POWER CO., INC.
Application year: 1990
Main classification: H01M10
IPC codes: H01M10