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

Abstract

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.

Streamline Sulfur Redox Reactions to Achieve Efficient Room‐Temperature Sodium–Sulfur Batteries

“…Typically, the S cathode undergoes multi‐step conversion reactions involving formation of long‐chain polysulfide intermediates and subsequent reduction into short‐chain sodium sulfide (Figure 1a) [17] . Unfortunately, the long‐chain NaPSs are reactive towards the commonly used carbonate electrolyte, leading to rapid capacity decay and battery failure [10c, 18] . It is ideal to realise a streamlined redox process via the assistance of both electron transfer and electrocatalysis.…”

Nanostructure Engineering Strategies of Cathode Materials for Room-Temperature Na–S Batteries

“…A recent study indicates that the S confined within the nanopores reinforces the solid−solid conversion in carbonate or etherbased electrolytes, while most of S on the surface suffers from severe nucleophilic reactions between the nucleophilic polysulfide anions and carbonate ester solvents when using carbonate electrolytes and severe shuttle effect when using ether-based electrolytes. 40 Regardless the nucleophilic reactions or shuttle effect, the S cathode has to experience irreversible loss of the active materials, resulting in rapid capacity fading. In order to prolong the cycling lifespan of RT Na−S batteries, NaNO 3 is added into ether-based electrolytes to induce a robust Na−O-rich solid electrolyte interphase (SEI) film on Na surface enabling suppressing deposition of Na 2 S, while fluoroethylene carbonate (FEC) is utilized in carbonate-based electrolytes to assist in generation of a protective layer constituted by SEI on the Na anode.…”

“…Compared to ether-based electrolytes, RT Na−S batteries with carbonate-based electrolytes often exhibit higher cycling stability due to the suppression of the polysulfide shuttling. 20,40 In summary, the methods to inhibit the shuttle effect of NaPSs include three types of mechanisms: physical blocking, covalent bonding, and polar adsorption (Figure 4). The physical blocking mechanism mainly refers to the spatial confinement of nanovoids of porous/hollow materials, particularly carbons.…”

Growing Mechanism of Polysulfides and Elemental Sulfur Formation: Implications to Hindered Chalcopyrite Dissolution

“…Polysulfides, S n 2– , are a class of structures that are involved in many different processes and subject to controversies. Thirty years ago, polysulfides were an important subject of research because of their role in processes such as desulfurization of fossil fuels, hydrogen sulfide oxidation, environmentally important processes, and as promising electrolytes for photoelectrochemical solar cells. − However, in the last years, the interest for the polysulfides increased again because of their importance in the emerging field of sodium–sulfur batteries , and also due to other technological processes concerning the leaching of sulfide minerals to extract noble metals and their environmental impacts. − …”