Na—S system

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. 

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. 

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 Na2S5. 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. / " -Alumina is used at present as the electrolyte material because of its high sodium-ion conductivity. 

Na-S system/battery 571,574 nail penetration, safety tests 354 naphthalene 428... 

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...

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