Electrolyte for use in sodium-sulfur batteries

A sodium-sulfur cell comprising:

• a. a sodium anode;
• b. a microporous sulfur cathode, and
• c. an electrolyte comprising:
• i. a sodium salt;
• ii. 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;
• iii. wherein said sodium salt and said additive are dispersed in a solvent comprising at least one alkyl carbonate, and
• iv. wherein the concentration of said metallic/metalloid cation in the electrolyte is 20 mM to 100 mM.

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 method comprising:

• introducing one or more additives into said electrolyte, each additive independently capable of forming a sodium alloy interphase on a surface on the sodium anode;
• and
• wherein the one or more additives independently comprise at least one metal or metalloid cation having a standard reduction potential which is at least 2.5V more positive than that of sodium cation;
• and wherein the at least one metal or metallic cation is provided in a concentration of 20 mM to 100 mM.