Revealing the Hidden Polysulfides in Solid-State Na-S Batteries: How Pressure and Electrical Transport Control...

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• Room temperature sodium-sulfur (Na-S) batteries face challenges with polysulfide solubility and shuttle effects in liquid electrolytes.
• Inorganic solid electrolytes are explored as a solution to block polysulfide migration and reduce capacity fading.
• Understanding the behavior of cycling products in solid-state Na-S batteries at room temperature is crucial but poorly understood.

Purpose of the Study:

• To investigate the sulfur conversion mechanisms in Na-S cells utilizing inorganic solid electrolytes at room temperature.
• To elucidate the formation, stability, and transient behavior of polysulfide species within these solid-state systems.
• To identify key factors governing the electrochemical reactions and performance of solid-state Na-S batteries.

Main Methods:

• Operando scanning microbeam X-ray diffraction (XRD).
• Operando X-ray photoelectron spectroscopy (XPS).
• Ex-situ X-ray absorption spectroscopy (XAS).

Main Results:

• Observed formation of crystalline and amorphous polysulfides, including Na2S5, Na2S4, Na2S2, and Na2S, consistent with the Na-S phase diagram.
• Identified high-order polysulfides (Na2Sx, x=6-8) and Na2S3, phases typically stable under extreme conditions.
• Demonstrated that transitions are controlled by diffusion-limited kinetics and localized stress, with pressure playing a critical role.

Conclusions:

• Pressure is a key thermodynamic variable and design parameter for optimizing solid-state Na-S battery performance.
• The complex polysulfide chemistry underpins the performance limitations and opportunities for these batteries.
• Further research into pressure management and kinetics is essential for advancing solid-state Na-S battery commercialization.