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Enabling Ambient Sodium Sulfur Batteries

Sodium and Sulfur are attractive energy storage materials because of their reactivity with each other and global abundance. Therefore, the Na-S battery is one of the oldest conceptualized batteries (Ford 1960s). However, the complex, multistep electrochemical reaction between the two has prevented commercial realization. We couple novel ultrafast spectroscopy with optical microscopy to bridge length and time scales of the reaction, to obtain the fundamental information necessary to facilitate a functioning battery (separator, electrolyte, geometry, etc.). With in-situ optical microscopy, we observe polysulfide anions ranging from S8^2- to S3^.- via color changes in the electrolyte during electrochemcial cycling on the order of hours. The observed, soluble species contribute to the reaction complexity and are most commonly blamed for low cell capacity or poor capacity retention. To better understand their interaction with the electrolyte solvent, novel pump/probe, ultrafast lasing is used to observe transient signals indicative of chemical recombination. This allows comparison of the strength of polysulfide interaction with solvent molecules at specific cell voltages and in various electrolyte cocktails. Similar analysis is also used in a symmetric sodium system to isolate plating and stripping phenomenon from the sulfur reaction.