In this study, we reported an intermediate-temperature (∼150 °C) sodium–sulfur (Na–S) battery. With a relatively low operating temperature, this novel battery could reduce the cost and safety issues associated with the conventional high-temperature (300–350 °C) Na–S battery. A dense β′′-Al2O3 solid membrane and tetraglyme were utilized as the electrolyte separator and catholyte solvent in this battery. Solubility tests indicated that a cathode mixture of Na2S4 and S exhibited extremely high solubility in tetraglyme (e.g., >4.1 M for Na2S4 + 4 S). CV scans of Na2S4 in tetraglyme revealed two pairs of redox couples with peaks at around 2.22 and 1.75 V, corresponding to the redox reactions of polysulfide species. The discharge/charge profiles of the Na–S battery showed a slope region and a plateau, indicating multiple steps and cell reactions. In situ Raman measurements during battery operation suggested that polysulfide species were formed in the sequence of Na2S5 + S → Na2S5 + Na2S4 → Na2S4 + Na2S2 during discharge and in a reverse order during charge. This battery showed dramatic improvement in rate capacity and cycling stability over room-temperature Na–S batteries, which makes it more attractive for renewable energy integration and other grid related applications.
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