Understanding Sulfur Redox Mechanisms in Different Electrolytes for Room-Temperature Na–S Batteries

Abstract

Highlights

A ‘solid–liquid’ conversion for increasing the sulfur content from ~ 50 to 72% for RT Na–S batteries is developed.

The redox mechanisms of two types of sulfur: sulfur on the surface of a cathode host (155S) and sulfur in the pores of the host (300S) in ether and carbonate ester electrolytes are studied.

The function of NaNO₃ additive on modifying Na anode and confining the shuttle effect of dissolving polysulfides during ‘solid–liquid’ conversion is visualized.

This work reports influence of two different electrolytes, carbonate ester and ether electrolytes, on the sulfur redox reactions in room-temperature Na–S batteries. Two sulfur cathodes with different S loading ratio and status are investigated. A sulfur-rich composite with most sulfur dispersed on the surface of a carbon host can realize a high loading ratio (72% S). In contrast, a confined sulfur sample can encapsulate S into the pores of the carbon host with a low loading ratio (44% S). In carbonate ester electrolyte, only the sulfur trapped in porous structures is active via ‘solid–solid’ behavior during cycling. The S cathode with high surface sulfur shows poor reversible capacity because of the severe side reactions between the surface polysulfides and the carbonate ester solvents. To improve the capacity of the sulfur-rich cathode, ether electrolyte with NaNO₃ additive is explored to realize a ‘solid–liquid’ sulfur redox process and confine the shuttle effect of the dissolved polysulfides. As a result, the sulfur-rich cathode achieved high reversible capacity (483 mAh g⁻¹), corresponding to a specific energy of 362 Wh kg⁻¹ after 200 cycles, shedding light on the use of ether electrolyte for high-loading sulfur cathode.