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.

Room-temperature sodium-sulfur (RT Na-S) batteries, as one of effective candidates for next-generation high-energy-density battery systems, have the advantages of high theoretical energy density (i.e., 1274 W·h·kg^–1), high elemental abundance (i.e., S and Na) and low cost. However, the practical application of RT Na-S batteries is restricted due to the poor electronic conductivity of sulfur, sluggish reaction kinetics, and sodium polysulfides (NaPSs) shuttle effect. The related studies are performed on cathode materials of RT Na-S batteries. Among various materials, two-dimensional layered materials have attracted extensive attention due to their unique structures and physicochemical properties, showing a substantial promise for applications. Compared to conventional experimental research methods, first-principles computational techniques can assist in designing novel high-performance electrode materials in atomic and electronic scales. This paper was thus to investigate g-C₃N₄ as a catalyst for RT Na-S batteries based on the first-principles calculation methods. In addition, the chemical interactions between g-C₃N₄ and NaPSs, electronic structure, and reaction energy barriers were also analyzed.

Density functional theory (DFT) calculations were carried out by a software named Vienna ab initio simulation package (VASP). The exchange-correlation energy was described using the Perdew-Burke-Ernzerhof (PBE) functional within the framework of the generalized gradient approximation (GGA). A plane-wave cutoff energy of 500 eV was chosen. A vacuum layer larger than 20 Å was used in the calculations to avoid the interlayer interactions. The convergence of energy and force criteria on the atoms were set to be 10^–5 eV and 0.02 eV·Å^–1, respectively. The DFT-D3 method was used to calculate the long-range van der Waals interactions. The 3×3×1 and 4×4×1 Monkhorst-Pack K-points were set in the first Brillouin zone for geometric optimization and calculation of density of states (DOS), respectively. The adsorption energy of NaPSs adsorbed on g-C₃N₄ monolayer was calculated by

(1) $$E_{\mathrm{a}\mathrm{d}\mathrm{s}}=E_{\mathrm{g}-{\mathrm{C}}_3{\mathrm{N}}_4/\mathrm{N}\mathrm{a}\mathrm{P}\mathrm{S}\mathrm{s}}-E_{\mathrm{g}-{\mathrm{C}}_3{\mathrm{N}}_4}-E_{\mathrm{N}\mathrm{a}\mathrm{P}\mathrm{S}\mathrm{s}}$$

where E_g-C3N4/NaPSs, E_g-C3N4 and E_g-C3N4 are the calculated total energies of g-C₃N₄ monolayer with adsorption of NaPSs, g-C₃N₄ before adsorption, and isolated NaPSs. The differential charge density was calculated by

(2) $$\mathrm{\Delta }\rho ={\rho }_{\mathrm{g}-{\mathrm{C}}_3{\mathrm{N}}_4/\mathrm{N}\mathrm{a}\mathrm{P}\mathrm{S}\mathrm{s}}-{\rho }_{\mathrm{g}-{\mathrm{C}}_3{\mathrm{N}}_4}-{\rho }_{\mathrm{N}\mathrm{a}\mathrm{P}\mathrm{S}\mathrm{s}}$$

where ρ_g-C3N4/NaPSs, ρ_g-C3N4 and ρ_NaPSs are the calculated electron densities of g-C₃N₄ monolayer with adsorption of NaPSs, g-C₃N₄ monolayer, and isolated NaPSs.

The g-C₃N₄ studied belongs to a hexagonal crystal system, where C and N atoms are bonded in sp² hybridized configuration, forming a conjugated π-electron structure. The density of states (DOS) of g-C₃N₄ monolayer shows semiconductor characteristics with a band gap of 1.12 eV. The adsorption energy is a fundamental criterion for assessing whether a material can anchor polysulfides. The adsorption energies of NaPSs and S₈ on the g-C₃N₄ surface were calculated, and all the values range from –1.0 eV to –5.0 eV, indicating that g-C₃N₄ can be ideal candidates for anchoring NaPSs. The analysis of DOS and differential charge density of these adsorption systems shows that electron transfer occurs through Na-S and Na-N bonds and the band gap decreases, compared to the pristine g-C₃N₄ (except S₈ adsorption system), facilitating electron transfer and providing electrons for the redox processes of NaPSs. The Gibbs free energy calculation for the entire discharge process reveals that the energy barrier of the rate-determining step is only 0.70 eV. These results emphasize a pivotal role played by the g-C₃N₄ in accelerating the conversion of NaPSs.

Based on first-principles calculations, we systematically investigated the anchoring and catalytic behavior of g-C₃N₄ toward NaPSs. The results showed that g-C₃N₄ could have a great potential as a sulfur host and catalytic material for RT Na-S batteries. g-C₃N₄ had an anchoring effect on NaPSs, contributing to improved battery cycle life. g-C₃N₄ could effectively capture NaPSs from the electrolytes, suppressing the shuttle effect. And g-C₃N₄ accelerated the conversion kinetics of NaPSs, enhancing sulfur utilization. All these findings could underscore an immense potential of g-C₃N₄ in the design of RT Na-S battery cathodes.