A new rechargeable cell Na/Na+ ion normalconductor/SCl3+ in molten AlCl3‐normalNaCl is described. This cell operates at temperatures in the range of 180°–250°C and has an open‐circuit voltage of 4.2V. The discharge process involves the reduction of tetravalent sulfur to the elemental state; sulfur can be further reduced to sulfide. The preferred sodium ion conductor is β″‐alumina. High energy density values, large percent utilization of the active material, and good energy efficiency have been demonstrated. The performance of cells prepared in the discharged and charged states were found to be the same. The number of deep charge/discharge cycles has exceeded 400.

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The solubility of alumina was measured by rotating an alumina cylinder (~500 rpm) in a high-purity melt for~3 to 6 hours, crushing and sampling the frozen melt, and determining the oxygen content in a Leco analyzer. The alumina solubilities determined were as follows:(1) 3.2 ± 0.3 wt pct in NaF-AlF 3 eutectic at 1023 K (750°C); (2) 3.0 ± 0.3 wt pct in NaF-AlF 3 -CaF 2 (5 wt pct) at 1023 K (750°C); and (3) 5.2 ± 0.5 wt pct in a KF-AlF 3 eutectic at 1003 K (730°C). The alumina solubility in the KF-AlF 3 eutectic was 2 wt pct more than in the sodium analogue, offering the possibility of operating a low-temperature aluminum smelting cell without the need for an alumina slurry. The diffusion coefficient of the dissolved alumina species was determined in the NaF-AlF 3 eutectic at 1023 K (750°C) using the rotating disc method and applying the Levich equation. Through a limited range of rotation rates, the system seemed to be mass-transfer controlled, and the diffusion coefficient was estimated to be in the range 1.8 to 2.2 9 10 À6 cm 2 s À1 . This value is about five times lower than the values encountered at traditional aluminum smelting temperatures (~1233 K (960°C)) and would result in relatively low mass transfer coefficients.

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