Effect of Electrolytes on the Na/S Electrode Stability

Abstract

Elemental sulfur (S) with the advantages of a multi-electron-transfer reaction mechanism and earth abundances is often paired with an alkaline metal (lithium or sodium) for higher energy densities. Sulfur conversion reactions in a metal-sulfur battery exhibit different electrochemical reduction mechanism depending on the initial states of the cathode and as well as the type of electrolyte present. Previous research showed that a series of intermediate redox species, formed during the cell operation, largely depends on the types of salts and solvents present in the system, and in some electrolytes, sodiation reaction is incomplete yielding lower achievable capacity. To alleviate early shutdown of the sodiation process, dissolution of polysulfides, and the shuttle phenomenon, identifying the optimal electrolyte composition is imperative. In this work, NaClO4 and NaPF6 salts with different types of ether-based solvents (mono-, di-, and tetra-glyme) were used to explore the interfacial stability and electrochemical responses of the Na-S system in different liquid electrolyte environment. The electrolyte composition has a perceptible effect on the evolution of both Na metal and sulfur cathode, and it was observed that in some electrolytes, the charge curve (de-sodiation) was extended to random lengths (more than theoretical capacity) of voltage plateaus indicating soft shorts in the cell. These new challenges introduce higher demands for directed control strategies such as partial sodiation/ de-sodiation, forced discharge, etc. Herein, the dynamic changes in the reaction steps for Na metal and sulfur electrodes were examined through the use of both two-electrode and three-electrode cell analysis. The interfacial impedance and morphological implications in sulfur cathodes due to sodiation/de-sodiation were studied for a range of electrolytes. In-depth investigation of general charge-discharge patterns, different cell reaction steps in different electrolytes, identified in this work presents fresh insights for future research direction in room temperature sulfur-based batteries.