Thermodynamics and cell chemistry of room temperature sodium/sulfur cells with liquid and liquid/solid electrolyte

The cell chemistry of sodium/sulfur cells operating at room temperature (RT-Na/S cells) is being studied electrochemically and structurally. We show by means of X-ray photoelectron spectroscopy that the cell reaction is incomplete but prove that the end members of the cell reaction (S and Na2S) form among the expected polysulfide species Na2Sx. The sulfur utilization can be improved by employing a solid electrolyte membrane (beta-alumina) that prevents the diffusion of the soluble polysulfide species toward the sodium side. As an important finding, the Na+ conduction within the solid electrolyte phase and across the two liquid/solid interfaces results in only small overpotentials. Nevertheless the utilization of sulfur in the present RT-Na/S (475 mAh g(-1)) cells is lower than the theoretical value (1675 mAh g(-1)). One probable reason is the chemical instability of the widely used PVDF binder. Also, the thermodynamic properties of RT-Na/S cells operating at room temperature are discussed and compared with the currently much more studied RT-Li/S cells. (C) 2013 Elsevier B.V. All rights reserved.