Tailoring Lithium Polysulfide Coordination and Clustering Behavior through Cationic Electrostatic Competition.

Abstract

The materials chemistry underlying lithium-sulfur (Li-S) batteries is uniquely dependent on the behavior of soluble lithium polysulfide intermediates, which form during operation and mediate the charge-transfer process in solution. The manner by which lithium polysulfides are solvated by surrounding solvent and salt compounds is a critical factor with regards to electrochemical utilization and reversibility of the sulfur active material. Particularly at low-temperature and lean electrolyte conditions, lithium polysulfides tend to coordinate with other polysulfide units in solution, forming large, aggregated clusters that stymie the electrochemical conversion process. However, the tendency to cluster is known to be influenced by the presence of strongly binding anionic species in solution, which present electrostatic competing interactions with Li+. The heightened electrostatic competition in turn can dissuade the formation of clustered Li+-Sx 2- bond networks. Here, we extend that understanding to the influence of distinct cationic species in solution, which can present analogous competing interactions with Sx 2- dianions to stymie polysulfide cluster formation. We find that introducing NH4 + cations into solution through an ammonium trifluoroacetate additive positively tailors the polysulfide coordination shell. This improves the electrochemical conversion kinetics at challenging lean electrolyte and subzero low-temperature conditions, and provides a more holistic understanding of polysulfide coordination behavior.