Deciphering the Impact of Polysulfide Solvation Structure on Electrical Double Layer Chemistry in Sodium-Sulfur Batteries.

Room-temperature sodium-sulfur (RT Na-S) batteries are garnering attention for large-scale energy storage. However, their practical application is hindered by challenges, such as the shuttle effect of sodium polysulfides (NaPS) and dendrite growth. The high solubility of NaPS in the electrolyte is particularly problematic. It disrupts electron transfer and obstructs mass transport in the electrical double layer (EDL) region. The EDL plays a pivotal role governing the interfacial chemistry between the electrode and electrolyte, significantly impacting the overall electrochemical performance. Through simulations and experimental screening of various solvents, including 1,2-dimethoxyethane (DME), tetrahydrofuran (THF), and tetrahydropyran (THP), along with ethyl 1,1,2,2-tetrafluoroethyl ether (ETFE) as a diluent, the relationship between NaPS solvation structure and EDL chemistry has been elucidated. Our findings reveal that THP-based localized high-concentration electrolyte (LHCE) not only reduces the solubility of NaPS by altering its solvation structure, but also promotes the formation of a stable inorganic solid-electrolyte interphase (SEI) and improves compatibility with sodium metal. Consequently, Na-S batteries with LHCE-THP/ETFE exhibit long-term stability over 500 cycles at 1C rate with a capacity decay rate of only 0.07% per cycle. This study provides a methodology for designing electrolytes based on molecular structure, polysulfides solvation properties, and EDL interfacial chemistry.