Hard carbon anodes and novel electrolytes for long-cycle-life room temperature sodium-sulfur full cell batteries
A novel combination of hard carbon anode sodium pre-loading and a tailored electrolyte is used to prepare room temperature sodium-sulfur full cell batteries. The electrochemical loading with sodium ions is realized in a specific mixture of diethyl carbonate, ethylene carbonate, and fluoroethylene carbonate electrolyte in order to create a first solid electrolyte interface (SEI) on the anode surface. Combining such anodes with a porous carbon/sulfur composite cathode results in full cells with a significantly decreased polysulfide shuttle when compared to half cells combined with metallic sodium anodes. Further optimization involves the use of Na2S/P2S5 doped tetraethylene glycol dimethyl ether based electrolyte in the full cell for the formation of a second SEI, reducing polysulfide shuttle even further. More importantly, the electrochemical discharge processes in the cell are improved by adding this dissolved complexation agent to the electrolyte. As a result of this combination sodium-sulfur cells with tailored cathode materials and electrolytes can achieve high discharge capacities up to 980 mAh g-1 sulfur and 1000 cycles with 200 mAh g-1 sulfur remaining capacity, at room temperature.