The present disclosure relates to an electrolyte comprising: a) a sodium salt; b) an additive comprising at least one additional metallic/metalloid cation having a standard reduction potential which is at least 2.5V more positive than that of sodium cation; wherein said sodium salt and said additive are dispersed in a solvent comprising at least one alkyl carbonate, and wherein the concentration of said metallic/metalloid cation in the electrolyte is 15 mM to 250 mM. The present disclosure also relates to a sodium-sulfur cell comprising a sodium anode, a microporous sulfur cathode, and the electrolyte as described herein. The present disclosure further provides a method of improving cycling life of a sodium-sulfur cell, wherein the sodium-sulfur cell comprising a sodium anode, a sulfur cathode, and an electrolyte containing a sodium salt dispersed in an alkyl carbonate solvent.

The present disclosure relates to an electrolyte comprising: a) a sodium salt; b) an additive comprising at least one additional metallic/metalloid cation having a standard reduction potential which is at least 2.5V more positive than that of sodium cation; wherein said sodium salt and said additive are dispersed in a solvent comprising at least one alkyl carbonate, and wherein the concentration of said metallic/metalloid cation in the electrolyte is 15 mM to 250 mM. The present disclosure also relates to a sodium-sulfur cell comprising a sodium anode, a microporous sulfur cathode, and the electrolyte as described herein. The present disclosure further provides a method of improving cycling life of a sodium-sulfur cell, wherein the sodium-sulfur cell comprising a sodium anode, a sulfur cathode, and an electrolyte containing a sodium salt dispersed in an alkyl carbonate solvent.

An electrode unit for an electrochemical device, comprising (i) a solid electrolyte which divides a space for molten cathode material, selected from the group consisting of elemental sulfur and polysulfide of the alkali metal anode material, and a space for molten alkali metal anode material, and (ii) a porous solid state electrode directly adjacent to the solid electrolyte within the space for the cathode material, with a non-electron-conducting intermediate layer S present between the solid state electrode and the solid electrolyte, wherein this intermediate layer S has a thickness in the range from 0.5 to 5 mm and, before the first charge of the electrochemical device, has been impregnated fully with a polysulfide composition, comprising (A) pure polysulfides Met.sub.2S.sub.x with Met=alkali metal of the alkali metal anode material selected from lithium, sodium, potassium, and x is dependent on the alkali metal and is 2, 3, 4 or 5 for Na and is 2, 3, 4, 5, 6, 7, 8 for Li and is 2, 3, 4, 5, 6 for K, or (B) mixtures of the polysulfides of one and the same alkali metal from (A) with one another.

The invention relates to sodium-ion-conducting elements for use in electrochemical cells, more particularly as solid electrolyte/separator in high-temperature batteries. In these elements, a surface of a porous substrate bears a coating which is obtained by sintering at a temperature of not more than 1100 C. and which is formed with the system Na.sub.2OSiO.sub.2R.sub.2O.sub.5R1.sub.2O.sub.3, in which R1=Sc, Y, La and/or B and R2=P, Sb, Bi, Sn, Te, Zn and/or Ge.

A process for making a solid electrolyte for an electrochemical cell. The process includes providing a multilayer material having a porous layer and a nonporous layer, the nonporous layer containing a first oxide selected from alpha-alumina, gamma-alumina, alpha-gallium oxide, and/or combinations thereof. In addition, an alkali-metal oxide vapor is provided and the nonporous layer is exposed to the alkali-metal oxide vapor at an elevated temperature such that the nonporous layer is converted to a solid second oxide electrolyte layer that is conductive to alkali metal ions. The second oxide is an alkali-metal-beta-alumina, alkali-metal-beta-alumina, alkali-metal-beta-gallate, and/or alkali-metal-beta-gallate.