Research Progress of Electrolyte for Room-Temperature Sodium-Sulfur Batteries

Room-temperature sodium-sulfur (RT Na–S) batteries are a potential candidate for next generation of large-scale energy storage systems due to their high energy density and low cost. Nevertheless, their practical application is seriously hindered by the "shuttle effect" of sodium polysulfides, sluggish conversion kinetics, sodium dendrite growth, and electrolyte instability. In view of the problems above, extensive strategies are proposed to enhance the battery performance, such as cathode material engineering, anode interface modification, and electrolyte optimization. The modification of the RT Na–S batteries electrolyte has attracted recent attention. However, there is a lack of summary regarding the optimization mechanisms and design principles of different electrolytes. This review represented the research progress and optimization mechanism of electrolyte designs (i.e., the selection of solvent molecules, sodium salts, and functional additives) as well as the development of high-concentration/locally high-concentration electrolytes and flame-retardant electrolytes based on the working principle and challenges of RT Na–S batteries. In addition, some future research directions for electrolyte regulation toward practical RT Na–S batteries were also summarized.

In summary, the rational design and optimization of electrolytes is an effective approach to enhancing the performance of RT Na–S batteries. This review outlines the working principles and major challenges faced by RT Na–S batteries and systematically discusses the research advancements in carbonate-based, ether-based electrolytes, and electrolyte additives for RT Na–S batteries. In RT Na–S batteries, carbonate, and ether-based electrolytes exhibit different properties. The solubility of NaPSs is lower in carbonate-based electrolytes, but they can undergo side reactions with the carbonate solvent, thus leading to the loss of active sulfur and consumption of the electrolyte. The design of carbonate electrolytes focuses on avoiding side reactions between the carbonate solvent and NaPSs, such as selecting appropriate solvents or additives to induce the formation of a stable CEI layer, preventing direct contact between the carbonate solvent and NaPSs. In contrast, NaPSs are more stable in ether-based electrolytes, but their high solubility in ether electrolytes results in severe shuttle effects and corrosion of the sodium metal anode. The design of ether-based electrolytes emphasizes the suppression of NaPSs shuttle effect and the interface modification of the sodium metal anode. This can be achieved via selecting specific solvents and additives that form coordination bonds with NaPSs and a stable SEI to inhibit NaPSs shuttle and stabilize the sodium metal anode. Some strategies involving high-concentration or localized high-concentration electrolytes have some positive effects in the design of both carbonate and ether-based electrolytes. In addition, the introduction of flame-retardant solvents into the electrolyte can also enhance the safety of the battery to some extent.

Despite significant research progress on RT Na-S battery electrolytes, there are still some scientific problems to be addressed to promote the commercial application of RT Na-S batteries. Future research should be as follows:

1) Theoretical calculations can be employed to reveal the dissolution and diffusion mechanisms of NaPSs in electrolytes, as well as the general interactions between solvent molecules, additives, and NaPSs. This can then enable the efficient screening of suitable solvents and additives for sodium-sulfur battery electrolytes through machine learning.

2) The sodium metal used in the anode of sodium-sulfur batteries is excessive, which increases manufacturing costs and reduces energy density. Developing sodium-sulfur batteries with Na₂S as a cathode and a current collector as an anode (i.e., anode-free sodium-sulfur batteries) represents a promising avenue for future development, as it can maximize the energy density of sodium-sulfur batteries. However, the sodium in anode-free sodium-sulfur batteries is limited as it is sourced from Na₂S. It is thus necessary to develop electrolytes with a high coulombic efficiency to minimize sodium loss and enhance the cycling stability of the battery.

3) Although the operating temperature of sodium-sulfur batteries reduces from high temperatures (~350 ℃) to room temperature, a future research should focus on the development of low-melting-point solvents or ionic liquids for the design of low-temperature electrolytes, to accommodate more extreme operating environments. It is of great significance to promote the practical application of sodium-sulfur batteries.

4) Solid-state electrolytes can effectively suppress the shuttle effect of NaPSs and the growth of sodium dendrites, while also avoiding the risks associated with electrolyte leakage. Therefore, the development of novel solid-state electrolytes shows a great promise for enabling higher performance and safety sodium-sulfur batteries.