Theoretical study on the role of magnesium chloride complexes induced by different magnesium-to-chlorine ratios in magnesium-sulfur batteries.

In magnesium-sulfur batteries, electrolyte exploration is vital for developing high-energy-density, safe, and reliable batteries. This study focused on cyclic THF and chain DME, representative solvents in ether electrolytes. MgCl2, an ideal anionic salt, forms mono-nuclear (MgCl2(DME)2), bi-nuclear ([Mg2(µ-Cl)2(DME)4]2+), and tri-nuclear ([Mg3(µ-Cl)4(DME)5]2+) complexes in DME. With increasing salt concentration, these complexes sequentially form. Under lower salt concentrations, THF and MgCl2 form mono-nuclear complexes ([MgCl2(THF)4]) and continue to form bi-nuclear complexes ([Mg2(µ-Cl)3(THF)6]+). However, at higher salt concentrations, bi-nuclear complexes ([Mg2(µ-Cl)3(THF)6]+) directly form in THF. Comparing HOMO-LUMO values, [Mg(DME)3]2+ is easily oxidized. Energy gaps decrease with Cl- ion addition, enhancing solution conductivity. Ratios of Mg2+ and Cl- in S-reduction complexes differ, suggesting DME is better at a low Mg/Cl ratio, and THF at a high Mg/Cl ratio. This study contributes to understanding complexes and enhancing Mg-S battery performance.

First-principles investigation on multi-magnesium sulfide and magnesium sulfide clusters in magnesium-sulfide batteries.

Because of the abundance of magnesium and sulfur and their low cost, the development of magnesium sulfur batteries is very promising. In particular, the battery performance of nanoscale (MgS)n clusters is much better than that of bulk sized MgS. However, the structures, stability, and properties of MgxSy and (MgS)n clusters, which are very important to improve the performance of Mg-S batteries, are still unexplored. Herein, the most stable structures of MgxSy (x = 1-8, y = 1-8) and (MgS)n (n = 1-10) are reliably determined using the structure search method and density functional theory to calculate. According to calculation results, MgS3 and Mg6S8 may not exist in the actual charging and discharging products of magnesium sulfide batteries. The (MgS)n (n ≥ 5) clusters exhibit intriguing cage-like structures, which are favorable for eliminating dangling bonds and enhancing structural stability. Compared to the MgS monomer, each sulfur atom in the clusters is coordinated with more magnesium atoms, thus lengthening the Mg-S bond length and decreasing the Mg-S bond activation energy. Notably, with the increase of dielectric constant of electrolyte solvent, compared to the DME (ε = 7.2), THF (ε = 7.6) and C2H4Cl2 (ε = 10.0), MgxSy and (MgS)n clusters are most stable in the environment of C3H6O (ε = 20.7). It can delay the transformation of magnesium polysulfide to the final product MgS, which is conducive to improving the performance of Mg-S batteries. The predicted characteristic peaks of infrared and Raman spectra provide useful information for in situ experimental investigation. Our work represents a significant step towards understanding (MgS)n clusters and improving the performance of Mg-S batteries.

First-Principles-Based Optimized Design of Fluoride Electrolytes for Sodium-Ion Batteries.

Because of the abundance and low cost of sodium, sodium-ion batteries (SIBs) are next-generation energy storage mediums. Furthermore, SIBs have become an alternative option for large-scale energy storage systems. Because the electrolyte is a critical component of SIBs, fluorination is performed to improve the cycling performance of electrolytes. Based on the first-principles study, we investigated the effects of the type, quantity, and relative position relationships of three fluorinated units, namely -CF1, -CF2, and -CF3, on the cyclic ester molecule ethylene carbonate (EC) and the linear ether molecule 1,2-dimethoxylethane (DME). The optimal fluorination was proposed for EC and DME by studying the bond length, highest occupied molecular orbital, lowest unoccupied lowest orbital, and other relevant parameters. The results revealed that for EC, the optimal fluorination is 4 F fluorination based on four -CF1 units; for DME, CF3CF1CF1-, CF3CF2CF2-, CF3CF1CF2CF3, and CF3CF2CF2CF3, four combinations of three -CF1, -CF2, and -CF3 units are optimal. The designed fluorinated EC and DME exhibited a wide electrochemical stability window and high ionic solvation ability, which overcomes the drawback of conventional solvents and can improve SIB cycling performance.