The Na-S System

In 1967 Kummer and Weber of the Ford Motor Company described the sodium/ 

In the Na/S system the sulfur can react with sodium yielding various reaction products, i.e. sodium polysulfides with a composition ranging from Na2S to NUjSj. Because of the violent chemical reaction between sodium and sulfur, the two reactants have to be separated by a solid electrolyte which must be a sodium-ion conductor, yff -Alumina is used at present as the electrolyte material because of its high sodium-ion conductivity. 

The theoretical specific energy based on this reaction is calculated as 760 Wh kg .  

In 1967 Kummer and Weber of the Ford Motor Company described the sodium/sulfur system as a new secondary battery [11]. At almost the same time Levine and Brown published a paper on a similar system, differing only in using 

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The Na—S system is expected to provide significant iacreases ia energy density for sateUite battery systems (69). In-house testing of Na—S cells designed to simulate midaltitude (MAO) and geosynchronous orbits (GEO) demonstrated over 6450 and over 1400 cycles, respectively. 

Difficulties with the Na—S system arise ia part from the ceramic nature of the alumiaa separator the specific P-alumiaa is expeasive to prepare and the material is brittie and quite fragile. Separator failure is the leading cause of early cell failure. Cell failure may also be related to performance problems caused by polarization at the sodium/soHd electrolyte iaterface. Lastiy, seal leakage can be a determiaant of cycle life. In spite of these problems, however, the safety and rehabiUty of the Na—S system has progressed to the poiat where pilot plant production of these batteries is anticipated for EV and aerospace apphcations. 

Like the Li/FeSx system, which is presently the most advanced rechargeable battery system based on a molten salt electrolyte, the Na/S system is presently the most advanced rechargeable battery system based on a solid electrolyte (beta-alumina) It operates at about 300 C. 

Another, exploratory version of the Na-S system (aimed at present at stationary applications) is under development at Dow Chemical Company. It operates at 300°C and utilizes hollow sodium borate glass fibers as electrolyte in the form of thousands of thin hollow fibers sealed at one end and open at the other to a common reservoir of molten sodium. The fibers are wrapped in Mo-coated aluminum foil which serves as a positive electrode for the polysulfide melt, which fills the space between the fiber wall and the Al. 

Figure 1 Phase diagram of the Na-S system between the compositions NagSg and 100% S...

The Na—S battery couple is a strong candidate for appHcations ia both EVs and aerospace. Projected performance for a sodium—sulfur-powered EV van is shown ia Table 4 for batteries having three different energies (68). The advantages gained from usiag a Na—S system rather than the conventional sealed lead—acid batteries are evident. 

A battery system closely related to Na—S is the Na—metal chloride cell (70). The cell design is similar to Na—S however, ia additioa to the P-alumiaa electrolyte, the cell also employs a sodium chloroalumiaate [7784-16-9J, NaAlCl, molten salt electrolyte. The positive electrode active material coasists of a transitioa metal chloride such as iroa(Il) chloride [7758-94-3] EeQ.25 or nickel chloride [7791-20-0J, NiQ.25 (71,72) in Heu of molten sulfur. This technology is in a younger state of development than the Na—S. 

Conventional batteries consist of a liquid electrolyte separating two solid electrodes. In the Na/S battery this is inverted a solid electrolyte separates two liquid electrodes a ceramic tube made from the solid electrolyte sodium /5-alumina (p. 249) separates an inner pool of molten. sodium (mp 98°) from an outer bath of molten sulfur (mp 119°) and allows Na" " ions to pass through. The whole system is sealed and is encased in a stainless steel canister which also serves as the sulfur-electrode current collector. Within the battery, the current is passed by Na+ ions which pass through the solid electrolyte and react with the sulfur. The cell reaction can be written formally as... 

A prerequisite of long-life sodium/sulfur batteries is that the cells contain suitable corrosion-resistant materials which withstand the aggressively corrosive environment of this high—temperature system. Stackpool and Maclachlan have reported on investigations in this field [17], The components in an Na/S cell are required to be corrosion-resistant towards sodium, sulfur and especially sodium polysulphides. Four cell components suffer particularly in the Na/S environment the glass seal, the anode seal, the cathode seal, and the current collector (in central sodium arrangements, the cell case). 

Several relatively common disorders result in aldosterone secretion abnormalities and aberrations of electrolyte status. In Addison s disease, the adrenal cortex is often destroyed through autoimmune processes. One of the effects is a lack of aldosterone secretion and decreased Na+ retention by the patient. In a typical Addison s disease patient, serum [Na+] and [CL] are 128 and 96 meq/L, respectively (see Table 16.2 for normal values). Potassium levels are elevated, 6 meq/L or higher, because the Na+ reabsorption system of the kidney, which is under aldosterone control, moves K+ into the urine just as it moves Na+ back into plasma. Thus, if more Na+ is excreted, more K+ is reabsorbed. Bicarbonate remains relatively normal. The opposite situation prevails in Cushing s disease, however, in which an overproduction of adrenocorticosteroids, especially cortisol, is present. Glucocorticoids have mild mineralocorticoid activities, but ACTH also increases aldosterone secretion. This may be caused by an oversecretion of ACTH by a tumor or by adrenal hyperplasia or tumors. Serum sodium in Cushing s disease is slightly elevated, [K+] is below normal (hypokalemia), and metabolic alkalosis is present. The patient is usually hypertensive. A more severe electrolyte abnormality is seen in Conn s syndrome or primary aldosteronism, usually caused by an adrenal tumor. Increased blood aldosterone levels result in the urinary loss of K+ and H+, retention of Na+ (hypernatremia), alkalosis, and profound hypertension. 

The NiCl2 positive electrode technology is presently under field testing and the Na/S technology has been set aside. The Na/NiCl2 system offers several potential advantages compared with Na/S ... 

Much of the effort to develop the Na/S battery was aimed at its use in electric vehicles. Current applications of this advanced battery system are now mainly in the stationary battery area, but feasibility studies were done on the recycling of this system before the EV development efforts were suspended. Sodium/sulfur batteries contain reactive and corrosive materials, but not toxic ones. By treatment of the battery waste, the reactivity problems can be removed. 

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