Solvating magnesium polysulfides enables low–barrier speciation for magnesium sulfur batteries

Domain: physics
Published: 2026-03-10
Editorial outcome: minor_revision

Abstract
Magnesium–sulfur batteries offer high theoretical energy density but suffer from low sulfur utilization and rapid capacity fading. This work identifies the strong Mg2+–polysulfide Coulombic interaction as responsible for sluggish kinetics. We demonstrate that the solvation of magnesium polysulfides fundamentally governs their behavior, relaxing Mg2+–polysulfide coupling to facilitate polysulfide speciation and conversion kinetics. Our findings reveal that solvation with high shielding ability effectively modifies the binding configuration of magnesium polysulfides, which lowers the energy barrier for their stepwise conversion, thus improving reaction reversibility. Furthermore, under solvation modulation, magnesium sulfide deposition exhibits a three-dimensional nucleation morphology, providing more active sites for redox. Consequently, the developed cells achieve the equilibrium potential ~1.1 V (vs. Mg/Mg2+) and extended cycle life (0.1 C over 120 cycles,1 C (60 min) = 1675 mA g-1), with pouch cells showing high discharge capacity (940.80 mAh g−1 in the 1st cycle; > 600 mAh g-1 after 18 cycles).

Introduction
Lithium–sulfur batteries (LSBs), known for their high theoretical specific capacity and environmental friendliness, face practical problems such as polysulfide shuttling and lithium dendrite formation1-3. In contrast, magnesium–sulfur batteries (MSBs) have emerged as a prospective alternative, which mitigates the dendrite growth issues and offers a large theoretical volumetric capacity (3833 mAh cm−3)4-9. Despite these advantages, the practical development of MSB is hindered by low sulfur conversion efficiency and severe capacity decay10,11. The strong coulombic interactions between Mg2+ and polysulfides that significantly retard redox kinetics. Additionally, the poor electronic/ionic conductivity of discharge products, magnesium sulfide (MgS), leads to pore blockage and premature capacity termination12. To enhance reaction kinetics and optimize battery performance, these critical challenges must be effectively addressed.

The high energy density of MSB originates from multi–electron redox reactions involving complex phase transitions from solid sulfur to MgS13,14. During the transition processes, polysulfides (PS) solvated in the electrolyte serve as fundamental intermediates and directly govern the performance of the batteries15. Prior work has established that strong Mg2+–PS coordination dominates MgPS behavior16,17, but the specific solvation structure of MgPS and its profound influence on the fundamental conversion chemistry itself have remained elusive. Early attempts based on conventional magnesium electrolytes (such as the Grignard reagent/aluminum chloride system) were made, and the instability issue in the presence of polysulfides was pointed out18. Breakthroughs in the design of electrolyte systems for MSB, such as boron-based and phenolate-based systems with better compatibility, have also been achieved19,20. Besides, recent strategies demonstrate that introducing additives21-23 or catalyst24 to the electrolyte can modify the interphase to suppress polysulfide dissolution and diffusion. Nevertheless, the fundamental role of solvent–directed MgPS solvation in steering conversion kinetic and its critical impact on battery performance remains largely unexplored.

In this work, we reveal that the solvation structure of MgPS is a previously overlooked regulator of the conversion chemistry. To demonstrate this, a series of common ether solvents were selected to evaluate their ability of solvation modulation for MgPSs: 1,2-dimethoxyethane (DME), diethylene glycol dimethyl ether (G2), triethylene glycol dimethyl ether (G3) and tetraethylene glycol dimethyl ether (G4). The lone pair electrons on oxygen atoms of ether solvents enable strong coordination capability, which can stabilize the MgPS complexes. Although these solvents show analogous molecular structures, they demonstrate different electrochemical performance when applied in MSB. To systematically explore the role of solvent, we investigate the solvation–modulated property of polysulfide speciation and explore the relationship between the solvation environment of MgPS and battery performance. We found that solvent shields the interaction between Mg2+ and PS and governs MgPS solvation. Is/p, calculated by the Mayer bond order ratio of Mg-solvent (Is) to Mg-PS (Ip), was defined as a parameter to evaluate the shielding ability: solvation environment with high Is/p value exhibits higher shielding ability and exerts stronger effect on the Mg-S interaction, facilitating efficient growth of MgS with the uniform three-dimensional (3D) morphology (Fig.1). Based on the simulation results, solvents affect the intrachain bonding of polysulfides, which concurrently changes the energy barriers during sulfur conversion. Electrochemical characterization and spectroscopy analysis prove that this solvation effect on MgPS manifest distinctly in voltage profiles, polysulfide conversion, and electrode thermodynamics. Meanwhile, solution–mediated nucleation and growth of MgS also show strong correlation with solvation effect. Among the prepared electrolytes, the G2–based electrolyte, which facilitates a favorable solvation environment with high shielding ability for MgPS, promotes the polysulfide speciation and conversion. High sulfur utilization with 3D morphology deposition of MgS was observed in G2–based electrolyte. Moreover, the G2–based electrolyte demonstrated stable cycling for 120 cycles at 0.1 C in coin cells and exhibits high capacity in pouch cells (940.80 mAh g−1 in the 1st cycle; > 600 mAh g-1 after 18 cycles), underscoring its suitability for MSBs. These results not only shed light on attainable reaction mechanisms of MgPS speciation and MgS electrodeposition, but also give guidance for solvent selection criteria in designing practical electrolytes for MSB.

Results
Solvation–mediated Mg2+–PS coordination chemistry
Polysulfide species serve as the solvated intermediates in the electrochemical conversion of sulfur electrodes. Their formation and evolution directly govern the energy efficiency, cycling stability, and reaction kinetics of the battery25. To screen the interactions among Mg2+, solvents, and polysulfides and gain deeper insights into the solvation behavior of MgPS, we employed molecular dynamics (MD) simulations to investigate the structures of polysulfide species in four ether solvents :1,2-dimethoxyethane (DME), diethylene glycol dimethyl ether (G2), triethylene glycol dimethyl ether (G3) and tetraethylene glycol dimethyl ether (G4). Supplementary Fig. 1 exhibits the radial distribution function (RDF) analyses and coordination numbers in four electrolytes (The atomic coordinates of the optimized structure can be found in the Supplementary Data 1). It can be found that, Mg2+ not only coordinates with O from the solvents and TFSI- in the solvation shell, but also coordinates with S from polysulfides due to the strong Lewis interaction. Here, the representative reaction intermediate MgS4, which bridges the gap in the solid–liquid–solid transformation process, was selected as the focus for a detailed illustration. The solvation structures of MgS4 derived from electrolytes with different solvents are shown in Fig.2a. The calculated values of Mayer bond orders and Mulliken charge distribution are listed in Supplementary Table 1–4. (The solvation structures and corresponding atomic numbers are shown in the Supplementary Fig. 2). According to Supplementary Table 1–4., it can be seen that for four solvation environments, the terminal sulfur atoms show a more negative Mulliken charge value, indicating a great charge enrichment in two ends of S chain. At the same time, the Mayer order of Mg-S bond exhibits a strong–weak–strong sequence along the S4 chain and the Mayer bond order of middle S–S bond is weaker than other S-S bonds, suggesting that Mg2+ mainly interacts with sulfur atoms at both ends of S4. Fig.2b and Supplementary Table. 5 demonstrate the average Mayer bond order of Mg-O(Is) and Mg-S(Ip) in four coordination environments from Fig.2a. It can be inferred that, due to the strong interaction between Mg2+ and PS, Ip is stronger than the Is for all solvents in Fig.2b. Moreover, the values of Is and Ip changes significantly with the solvation environments, indicating solvation shields the strong electrostatic interaction between Mg2+ and PS. To assess the role of the solvent on tailoring the solvation structure, the Is/p ratio (the value of Is divided by the value of Ip) is introduced as a parameter and represents the shielding ability of solvation. Among four solvents, G2-based solvation environment shows the highest Is/p and thus has the strongest effect on MgPSs, indicating effective modulation of Mg-PS interaction. Meanwhile, the Mayer bond order of middle S–S bond in G2–based solvation environment is the weakest (Fig.2a) and is prone to cleavage, which benefits the MgPS solvation dynamics as well as sulfur conversion kinetics. Moreover, because the high shielding ability of G2, the Mulliken charge of Mg in G2–based solvation environment is the highest (Fig.2b), suggesting that the valence state of Mg is less influenced. The reaction pathway in the implicit solvation environment by considering the MgS8, MgS4, and MgS2 is built in Fig.2c. Attributed to the solvation effect on the coordination between Mg2+ and PS, the energy barrier from solvated polysulfides to terminal solid MgS shows different values. It can be shown that G2 with high shielding ability also facilitates a lower Gibbs free energy change (ΔG), enabling a reduced conversion barrier and accelerated redox kinetics for MgPS. The simulations explicitly deciphered the cooperative effects of solvent–Mg2+–PS and demonstrated the critical role of solvation mediation in Mg2+–PS coordination, providing a molecular–scale basis for MgPS solvation dynamics.

During the charging and discharging process, active sulfur species in both solid and solvated phases control the reaction thermodynamics and provide the capacity26. To figure out how the solvation–mediated intermediate influence the energy density of the battery, constant current charge–discharge and cyclic voltammetry tests were conducted using various electrolyte solvents and iodine additive. The galvanostatic charge–discharge profiles were displayed in Supplementary Fig. 3a–d and the profile of the third cycle at 0.05 C was shown in Fig.2d. During the discharge reaction, magnesium metal is oxidized, producing Mg2+ ions that migrate toward the positive electrode through the separator and electrolyte. Simultaneously, electrons are transferred to the active sulfur materials via an external circuit to keep the positive electrode at a neutral charge state. It can be concluded that four types of electrolytes exhibited a certain capacity and featured a long plateau with a significant portion of the capacity, originating from the liquid–solid conversion step. Supplementary Fig. 3a shows that in the 1st cycle, DME–based cell shows the high discharge capacity around 1100 mAh g-1 at 0.05 C and exhibits two main voltage plateaus (~1.4 V and ~1.0 V). However, because of the irreversible MgS conversion, the high polarization leads to premature termination during charge (coulombic efficiency ~69%), leading to the gradual capacity decay in the following cycles and the cell failure. Based on the discussion in simulation results, it can be inferred that G2 provides the favorable solvation environment for polysulfides, which facilitating the reaction kinetics. Therefore, the charge–discharge profiles of G2-based cell have small polarization(~0.62 V) and reasonably high reversibility during charge and discharge. Meanwhile, the two redox peaks observed at approximately 1.9 V and 2.0 V in Supplementary Fig. 3b attribute to the redox reaction of iodine additive. For the G3–based cell, in the first discharge cycle, compared to G2, the solvation environment modulated by G3 has weaker promoting effect on polysulfide conversion, resulting in lower sulfur utilization and reduced capacity release. Notably, unexpectedly prolonged charging profiles near 1.8 V can be observed in the charge curves of G3-based cell in Supplementary Fig. 3c. During this process, the conversion of long-chain polysulfides to short-chain species exhibits slow kinetics, leading to the polysulfide shuttle effect and continuous loss of active sulfur. For G4–based cell, the high viscosity and poor wettability with the electrodes resulted in erratic curve in the first cycle. Then in the following cycles, sulfur undergoes incomplete conversion, which results in low voltage plateaus and low discharge capacity. Furthermore, because of the high dissociation energy barrier of MgS, G4-based cell suffers from a large polarization in charging process, ultimately leading to insufficient charge capacity (Supplementary Fig. 3d). These results confirm that the solvation environment critically determines polysulfide conversion efficiency, with the optimal coordination strength of G2 enabling both low polarization and stable cycling, and this is the core requirements for practical MSBs.

The key results derived from the galvanostatic charge–discharge profiles can be correlated with the CV analysis of four ether–based cells at a scan rate of 0.2 mV s-1 (Supplementary Fig. 3e–h). Two main reduction peaks at ~ 1.23 V and ~ 0.88 V were observed both in cells with DME and G2 baseline solvent, which can be attributed to the reaction from long–chain polysulfides to short–chain polysulfides and short–chain polysulfides to MgS, respectively. During charging, the two overlapped oxidation peaks belonged to the MgS oxidation in the G2–based cell start at 1.55 V. In contrast, the cell with the DME solvent shows two separate oxidation peaks at 1.60 V and 1.96 V, which demonstrates a higher energy barrier during the transformation from MgS to elemental sulfur. The small peak at ~1.47 V is ascribed to the iodide reduction in the DME– and G2–based electrolyte27. In the case of cells with the G3 and G4 solvent, although the first reduction peak appears around 1.23 V, the peak voltage originated from MgS formation in G3 and G4 is lower than DME and G2, revealing different polysulfides speciation in the electrolyte. Notably, apart from the obvious charge/discharge voltage variations and the pronounced reduction peak shifts, when comparing the value of peak current and capacities of discharge plateaus (Fig.2e), it is recognizable that the significant numerical fluctuations exhibit a certain regularity consistent with the replacement of solvent. This variation implies that the electrochemical motive forces for the reversible conversion between elemental sulfur and MgS are drastically changed by solvation of MgPS.

Supplementary Table 6 presents the inherent properties of various solvents, such as viscosity and dielectric constant, which have been proved to have impacts on the sulfur transformation and its reaction mechanism. However, based on the above analysis, these properties cannot independently govern the electrochemical performance. Combined with the molecular simulation, we can comprehensively evaluate the role of solvent and make judgments from a more profound perspective: solvent tunes the coordination of Mg2+–PS and can provide favorable MgPS solvation environment. The feeble binding force of Mg2+ toward polysulfide species, coupled with the diminished inter-bond S-S coordination, promotes the acceleration of the reduction reaction via electron acquisition, which contributes to high capacity and reversibility of MSB during the charge/discharge process.

Solvation–Mediated MgPS Phase Transition and Nucleation
Redox–active MgPSs play an important role in phase transition at the electrode–electrolyte interface. The solubility of MgPS directly relates to the reaction pathways and kinetic behavior of sulfur, causing following impact on shuttle effect, battery efficiency, and cycling stability for enhancing overall battery performance28,29. Polysulfides dissolved in different solvents with/without iodine additive were analyzed using UV−vis spectroscopy (Fig.3a and Supplementary Fig. 4). It can be learned that soluble S8 show signals at 260 and 280 nm30. Owing to the difference in Donor number (Supplementary Table 6), the intensity of S8 peak in DME and G2 is higher than G3 and G4. The UV peak at 315 cm-1 attributed to S62–/S42– and the absorbance distributed in 355 and 409 nm belong to S62- and S42-, respectively31. Notably, overlapped with the sulfur peak, the UV peak at 234, 294 and 366 nm originates from iodide ions32,33.The stability of these main intermediates determines the reaction rates of the disproportionation/dissociation/recombination of polysulfides and thereby affects the phase transition of the MSB.

As illustrated in Fig.3b, the configuration of Operando Raman spectra was constructed to monitor the MgPS speciation in DME– and G2–based electrolyte during discharge process (Fig.3c and d). The Raman spectra at open–circuit voltage was obtained and shows peaks at 155, 218, and 470 cm–1 in DME– and G2–based cell, which can be assigned to the signals of elemental sulfur34-36. For DME–based cell, as the voltage decreases, the MgSx (x=4–8) band was observed from 279–325 cm–1, which implies that bulk sulfur is initially reduced, triggering the opening of S8 rings and the formation of liquid–phase chain polysulfides. The band of S62– species appears at a high potential of 1.43 V and gradually diminishes with Sx2–(x=4–8) as the voltage decreases, while a weak S8 peak can still be detected. In contrast, in the G2 electrolyte, liquid–phase polysulfides emerge at 1.39 V, and their intensity increases gradually during discharge, with almost no detectable S8 peak. This is attributed to the faster conversion of solid–phase S8 to long–chain polysulfides and the subsequent shortening of long–chain polysulfides in the G2 environment, generating the absence of the short plateau region observed around 1.4 V in DME. During the conversion from S8 to MgS, the step involving the transformation of S22– to MgS and MgS2 contributes to the most capacity37. It can be seen that the band of S22– and MgS are observed at 492 and 273 cm–1, respectively. In DME, the S22– species appear at a higher potential around 1.12 V and convers rapidly to MgS. However, in the G2 electrolyte, S22⁻ species can still be detected at low potentials (0.85 V) and gradually converts to MgS, resulting a longer plateau with more capacity contribution compared to DME. In addition, the solid species S22– spans in different voltage range in DME and G2, indicating S nucleation and deposition may relate to solvation.

To further explore the thermodynamic and kinetic properties of MSB during discharge process, galvanostatic intermittent titration technique (GITT) at a current density of 0.05 C was adopted in four ether–based coin cell (Fig.4a). Each step of GITT was set to be 15 min following 2 hours relaxation step in order to ensure the full recovery to thermodynamic equilibrium state(see Supplementary Fig. 5 and Fig. 6: Time-dependent potential plots). The corresponding polarizations of the electrochemical reaction in four ether–based cells (Fig.4b and Supplementary Table 7) were obtained by comparing the equilibrium potential (grey) with the transient potential in Fig.4a. Obviously, four curves show staging features and the equilibrium potential at long plateau follow a trend of DME (1.125 V) > G2 (1.116 V) > G3 (1.098 V) > G4 (1.088 V). Besides, the polarization status can be divided into three typical stages: S8–MgSx, MgSx–MgS2 and MgS2–MgS38. Firstly, active sulfur materials receive electrons through the external electrical circuit, which triggers the successive reduction from elemental sulfur to soluble polysulfides MgSx (stage 1). Then the soluble polysulfides cleave and convert to solid MgS2 (stage 2). Solid–state transition from MgS2 to MgS occurs at the end of the discharge process (stage 3). The Stage 1 corresponds to the solid-to-liquid conversion of sulfur to polysulfides in MSB, specifically the dissolution of solid sulfur in the electrolyte environment. A short plateau is observed during this stage, corresponding to the emergence of polysulfides. In the case of G3 and G4, high viscosity leads to wettability issues, which reduces the rate of electrochemical reactions and results in low active material utilization. Consequently, the first plateau exhibits a lower voltage with a distinct sloping profile. In contrast, as seen in the G2 solvent environment, better wettability and higher ionic conductivity allow the active sites on the positive electrode to function effectively and sulfur can readily transform to polysulfides without rapid diffusion away from the reaction interface, thereby resulting in a well-defined plateau region. Meanwhile, DME-based cell suffers from passivation by irreversible residues formed in previous cycles, which hinders the accessibility of active sulfur. This directly leads to the shortage of the plateau and results in incomplete sulfur conversion. In stage 2, long potential plateau and negligible hysteresis were observed in all curves, indicating that the reaction at this stage is relatively stable and proceeds continuously with lower energy barrier for conversion. Since soluble polysulfides can easily diffuse to the electron available site and acts as redox mediators to transfer electron, the reaction kinetics is faster in stage 1 and 2 than stage 3. For stage 3, the dissolved polysulfides are gradually consumed and high polarization of the solid–state reaction leads to premature termination of the discharge. Moreover, comparing the GITT curves and overpotentials of four ether–based cells, it can be concluded that the stability of the reactions and the ability to overcome the conversion barrier in each stage are relevant to the solvation of the electrolyte. Among the cells, G2–based cell has the smallest variation in polarization and show a more effectively accelerate redox kinetics of the positive electrode. This not only proves the effect of solvation on polysulfides during sulfur reduction, but also illustrates that the charge transfer process and ion diffusion rely on the solvation of the electrolyte39.

Based on the GITT results, it can be found that the unbearable polarization with the sluggish kinetics in stage 3 appears with the MgS nucleation and deposition. During the solid–solid conversion, the morphology and structure of the final product MgS may influence the achievable discharge capacity and the following oxidation process, which directly relates to the utilization and reversibility of active sulfur materials40. The quasi–equilibrium potential of MgPS to solid MgS was ascertained by utilizing the potentiostatic intermittent titration technique (PITT). Subsequently, the current−time transients were normalized based on the peak current (im) and the corresponding time (tm: specific time when the current reaches maximum). Then, the mechanism of MgS electrodeposition was compared with four classical electrodeposition models including Scharifker−Hills (SH) models41 (3D instantaneous (3DI) nucleation and 3D progressive nucleation (3DP) nucleation) and Bewick−Fleischman−Thirsk (BFT) models42 (2D instantaneous (2DI) nucleation and 2D progressive nucleation (2DP) nucleation) were used to fit the current–time responses. It can be seen that the PITT curves of DME and G2 (Supplementary Fig. 7a and Fig.4c) show obvious current responses, which is ascribed to the processes of nucleation and growth in liquids/solids. By fitting and analyzing the deposition curves of MgS at the responsive current (corresponding voltage: DME at 0.94 V and 0.80 V; G2 at 0.94 and 0.84 V), the nucleation and growth of MgS in DME– and G2–based electrolytes follow the 3D growth mode, performing a transition from 3DI to 3DP as the discharge proceeds (Supplementary Fig. 8 and Fig.4d). The 3D precipitation avoids clogging of the positive electrode and exposes more active sites, thereby improving sulfur utilization and stabilizing the electrode structure43. As for G3 and G4, there is no current signals in PITT curves (Supplementary Fig. 7b and c), suggesting that the nucleation mode is not suitable for G3– and G4–based electrolyte.

To further compare the diffusion state of polysulfides in DME and G2, we simulate the polysulfides diffusion and calculate the standard deviation by densities (DME: 0.074; G2: 0.065) (Fig.5a–b). In DME, the polysulfides exhibit low dispersity and tend to aggregate into clusters on the positive electrode surface, which hinders the conversion and deposition. In contrast, in G2, polysulfides have higher dispersity, enabling better contact with the active sites of the electrode and facilitating more uniform deposition. Nanosized nuclei MgS particles were observed on the carbon paper (CP) via scanning electron microscopy (SEM) (Fig.5c–f, Supplementary Fig. 9) and confirmed by the EDS (Supplementary Fig. 10). It can be observed that compared with the blank CP, the MgS particles generated in four different solvents all remain spherical shape at a size of 50 ~ 100 nm. In DME and G2, the small MgS granules deposit densely on CP and continue to precipitate on top of existing nuclei. Gradually, the particles connect together and the deposition become concentrated, which conforms to 3D nucleation mode. In contrast, in G3 and G4, a small amount of MgS particles are independently deposited on CP. Besides, reducing current density or raising sulfur concentration enhances the compactness of MgS particles (Supplementary Fig. 9b, c). The deposition of MgS on the positive electrode surface after cycling was further confirmed by XPS and TEM characterization (Supplementary Fig. 11-12).

Mg–S Battery Performance Evaluation
Based on the preceding discussion and the comprehensive consideration of solvation effect, compared with DME, G3 and G4, the G2–based solvent electrolytes with iodine additive can better enable the operation of MSB. By using the three–electrode cell with Mg metal as the reference electrode, the contribution of G2–based electrolyte to the negative electrode and the positive electrode was clarified (Supplementary Fig. 13). It can be revealed that the three–electrode Mg–S cell shows similar voltage profile with two–electrode cell (Fig.2d). The overpotential between Mg stripping and deposition at the negative electrode is only 0.4 V. Meanwhile, the positive electrode exhibits a polarization of ~0.7 V between the discharge and charge voltages. What’s more, G2–based Mg symmetric cells exhibit cycling performance of over 900 hours with low polarization about 0.2 V (Supplementary Fig. 14), identifying the G2–based electrolyte exhibits chemical compatibility with electrophilic sulfur and possesses the capability for reversible magnesium deposition and dissolution. Consequently, G2 solvent is chosen to enhance the performance of MSB. By employing the G2-based electrolyte, the assembled Mg–S full cell exhibits improved performance. CV curves at different scan rates operated in G2–based electrolyte present similar shape and well–maintained redox peaks (A, B, C, D) (Fig.6a), and there is a linear relationship between the peak response currents and square root of scan rates, proving that the highly reversible Mg storage process is driven by diffusion dynamics (Fig.6b). Rate capability and long–term cycling performance were illustrated in Fig.6c and Fig.6d, respectively. It can be found that the G2–based MSB displays a high average specific capacity of 946.61 mAh g−1 at relatively low current density of 0.05 C, which gradually decreases to 350.20 mAh g−1 at 1.0 C, respectively. When the current density was restored to 0.5 C and subsequently to 0.2 C, the MSB presented high restoration capability. Moreover, the G2–based cell demonstrated high cyclic stability over 120 cycles at 0.1 C. The MSB pouch cell with sulfur loading of ~1 mg cm–2 was also assembled and the open– circuit voltage maintained around ~2 V (Fig.6e). The voltage profiles of the pouch cell are displayed in Fig.6f, showing the performance with high discharge capacity of the 1st cycle (940.80 mAh g−1) and high retention after 18 cycles. The corresponding specific energy are 31.44 Wh kg–1 in the 3rd cycle and 22.87 Wh kg–1 in the 10th cycle (Supplementary Table 8). These results suggest that G2–based electrolyte effectively facilitate kinetics and optimize the performance, demonstrating the prospects for practical MSB.

According to the above observations and analysis, we demonstrate that strategic solvent selection enables precise control over the solvation structures of MgPS, directly governing polysulfide speciation and ultimately dictating the morphology and reversibility of MgS deposition (Fig.7). The sulfur reduction pathway initiates with the direct electrochemical conversion of solid S8 to S82- on the electrode–electrolyte interface, followed by its stepwise dissociation into short–chain species. In the solvation environment of MgPS, solvent shields the high electrostatic interaction between PS and Mg2+, which attenuates the bonding strength of Mg2+–PS and activates the cleavage steps of the polysulfides. Facilitated by the supply of external electrons, MgPS subsequently migrate to the electrode–electrolyte interface and achieve phase conversion. Benefitting from the optimized solvation environment, MgPS contact well with the active sites of the electrode by diffusion force and the formed MgS perform uniform deposition, promoting a 3D morphology nucleation. The 3D nucleation provides efficient charge transfer and improves the utilization rate of sulfur. The continuous growth of MgS is inevitably impeded because of the electron–insulating property, so the consequent deposition mainly depends on the conversion of MgPS. During this process, under the solvation effect, the MgPS with a strong affinity to the solid particles prefer to deposit on the pre–existing particles along the 3D directions, constructing an ideal interphase and enhancing the reversibility of MgS conversion.

Methods
General Materials
Mg(TFSI)2 (99.5%, Duoduo Chemical Reagent Co., Ltd.) was vacuum-dried at 200 °C for 24 hours before using. Magnesium sulfide (MgS, 99.98%, Adamas) and iodine (99.995%, Adamas) were sourced from Adamas–beta. Sulfur/carbon composites (S/C, 99.5%) were obtained from Xiamen TOB New Energy Technology Co., Ltd. Polyvinylidene difluoride (PVDF) and Super-P were obtained from Shenzhen Kejing Star Technology Co., Ltd. Solvents including 1,2–dimethoxyethane (DME, 99.5%), diethylene glycol dimethyl ether (G2, 99.5%), triethylene glycol dimethyl ether (G3, 99.5%), tetraethylene glycol dimethyl ether (G4, 99.5%,), dimethyl sulfoxide (DMSO, 99.5%) and N–Methylpyrrolidone (NMP, 99.9%) were all purchased from Adamas–beta and dried by molecular sieves (4Å, Adamas–beta) for over 24 hours before using. All chemicals were used as received.

Synthesis of the electrolyte and magnesium polysulfides (MgPS) solution
The 0.5 M Mg(TFSI)2 electrolytes were prepared by dissolving dried Mg(TFSI)2 in DME, G2, G3 and G4, respectively. After 24 hours of standing, iodine was added to achieve a final concentration of 0.5 M Mg(TFSI)2 –0.1 M iodine electrolytes. Then, they were kept at 25℃ for three days to ensure the components were well mixed.

Magnesium polysulfide (MgSx) was synthesized by reacting MgS and elemental sulfur in a 1:3 molar ratio in DMSO under continuous stirring at 70 °C for 6 days to ensure complete dissolution and homogeneous reaction. Subsequently, the solution was vacuum-dried under 70 °C for 48 hours to obtain polysulfide powders. The powders were then dissolved in solvents of DME, G2, G3 and G4 with/without iodine additive to prepare the MgSx solution. All preparation steps described above were performed in an argon-filled glove box, with both water and oxygen concentrations maintained below 0.1 ppm.

Preparation of S electrodes
The positive electrodes were prepared using the homogeneous slurry mixed from S/C, Super P, and PVDF in a 7:2:1 weight ratio in NMP. The slurry was cast onto Cu foils (16 μm, SHENZHEN KEJING STAR TECHNOLOGY CO., LTD.) by doctor blade, followed by vacuum heating of the electrode at 50 °C for 12 hours. The dried electrodes were cut into discs with a diameter of 12 mm for cell assembly. The active sulfur loading was around 0.6–1.0 mg cm−2.

Electrochemical measurements
Cell assembly were all carried out in an argon-filled glove box with water and oxygen contents < 0.1 ppm. Electrochemical testing was conducted at 25 ℃. The ionic conductivity of the electrolytes was measured using a conductivity meter (Mettler Toledo). To make coin cells (CR2025–type) for electrochemical tests, the glass fibre separator (GF/D, thickness of 675 μm, Whatman) was cut into 19-mm-diameter discs and vacuum-dried overnight at 120 °C. A piece of S cathode (diameter of 12 mm) and Mg foil (1×1 cm-2, SHENZHEN KEJING STAR TECHNOLOGY CO., LTD.) were separated by the GF/D separator with 90 μl of electrolyte. The galvanostatic charge‒discharge tests were conducted at 0.05 C (0.08 A g-1) or 0.1 C (0.17 A g-1) using the NEWARE battery test system. The cut-off potential ranges employed were 0.4–2.2 V for DME, 0.4–2.0 V for G2, 0.4–1.9 V for G3, and 0.4–2.1 V for G4, respectively. Corresponding cyclic voltammetry (CV) measurements were carried out on an IVIUMnSTEAT analyzer at a sweep rate of 0.2 mV s-1 with the same respective potential ranges. The three–electrode electrochemical tests were conducted in hermetically sealed Teflon Swagelok cell. For the galvanostatic intermittent titration technique (GITT) tests, the cells were activated for 1 cycle and discharged at 0.05 C for 15 min followed by relaxation at open circuit for 2 hours in each step. For the potentiostatic intermittent titration technique (PITT) test, the cell was potentionstatically discharged with current responses until the current was below 0.01 mA under –20 mV potential steps. The cells for MgS nucleation were assembled using carbon paper (CP, HCP020N) as the working electrode, magnesium foil as the counter electrode and Celgard 2325 as separator. A volume of 20 μL of MgSx catholyte was applied onto the CP, while 20 μL of the corresponding MgSx free electrolyte was added to the negative electrode compartment.

The rate performance and cycling performance of the Mg–S coin cells were assembled using positive electrodes with 0.62 mg cm-2 and 0.91 mg cm-2, respectively. The Coulombic efficiency is calculated by the ratio of charge capacity to discharge capacity. The pouch cells (one layer) were fabricated incorporating the negative electrode (Mg foil, 3 × 4 cm2, thickness of 50 μm), positive electrode with S loading of ~1 mg cm−2 (3 × 4 cm2), 200 μL electrolyte and GF/A separator (3 × 4 cm2, thickness of 260 μm, Whatman). Mg foils were freshly polished before being applied. Ni tabs(SHENZHEN KEJING STAR TECHNOLOGY CO., LTD.) were used for assembly. The cycling was performed at 0.1 C without external pressure applied during cycling. The specific energy of pouch cell is calculated based on the information listed in Supplementary Table 8. The calculation of the specific energy (𝑊) follows : 
𝑊 = (𝐶 × 𝑈) / 𝑀
Here, 𝐶 stands for the cell capacity. 𝑈 represents the average discharge voltage and 𝑀 represents the total weight. The mass of the packaging and tabs was not taken into the calculations. For each electrochemical condition, a minimum of five cells were evaluated and the performance curves presented in the main figures were derived from a representative cell that best reflects the median or typical behavior observed across all replicates.

Materials Characterization
Ultraviolet–visible (UV–vis) spectra of MgSx were detected on a Shimadzu UV2600 UV–vis spectrometer. Operando Raman spectroscopy monitor under a 532 nm laser was detected with a perforated stainless steel positive case and a quartz window. The cell was prepared by the stainless steel collector (300 mesh, nominal pore size 40 μm, diameter of 12 mm discs) coated with the positive electrode materials (sulfur loading: 1.0 mg cm–2), Celgard 2325 separator, negative electrode (Mg, diameter of 12 mm discs), and 90 μL electrolyte. Each spectrum was acquired for 20 s with the frequency of 10%. The cells were tested at a current rate of 0.1 C (167.5 mA g-1). The Raman signals were acquired with a Raman microscope (Renishaw, British).XPS analysis was performed after five cycles. All spectra were measured after treating the sample for 10 min by Ar sputtering to remove impurities formed from the XPS instrument chamber as well as the oxide film formed during the sample transport. X-ray photoelectron spectroscopy (XPS) investigations were carried out with ULVAC-PHI, GENESIS 900, and the binding energies were calibrated against the C 1s reference peak. Small electrode fragments were taken out after the potentiostatic discharging tests. The samples were washed with the corresponding solvent to remove additional Mg(TFSI)2. Samples for transmission electron microscope (TEM, Talos F200X G2), tests were prepared after further sonication of the positive electrode in corresponding solvents and loaded onto Na Cu grid. All samples were vacuum dried overnight before transferring to the microscope chamber. The microstructures of MgS on CP were revealed using scanning electron microscopy (SEM, Apreo 2S) accompanied an energy dispersive X–ray spectrometer (EDS).

Theoretical Calculation
Density functional theory (DFT) calculations for geometry optimization were conducted by Gaussian 16 software package44. Geometry optimizations and energy calculations were performed using B3LYP/6–311G(d,p)45, and the convergence tolerance quality was the default. The solvation effect was considered with the universal solvation model based on density (SMD)46,47.

All the classic molecular dynamic simulations conducted in this work were performed using the Large–scale Atomic/Molecular Massively Parallel Simulator (LAMMPS, http://lammps.sandia.gov). OPLS–AA48 parameters with 1.14*CMIA partial atomic charges were generated by LigParGen49-51 for the organic solvent molecules. The OPLSAA force field developed by Jorgensen was used for Mg2+52. To get the electrolyte structure, isothermalisobaric (NPT) ensemble runs were first performed at 328 K for 5 ns and then 298 K/328 K for 5 ns to ensure the equilibrium salt dissociation53. Then, 5 ns long canonical (NVT) ensemble runs were conducted and the last 0.5 ns were used to obtain the structure of electrolyte.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The solvation structure of magnesium polysulfides is considered to dominate the conversion chemistry and regulate the performances of Mg−S batteries. In this work, the discharging behaviors, kinetic characteristics and electrochemical deposition models were detailed analyzed in this work by introducing a series of solvents with different coordination ability. Cells with a G2 based electrolyte demonstrate the considerable improvements in cyclic stability and specific capacity. However, the novelty and the insight of this work are still lacking. I cannot support this work to be published in Nat. Comm. My comments are as follow: 1. The crucial role of the polysulfides solvation structure in determining the charging/discharging behaviors and cell performance has been reported by some works, e.g. Adv. Mater. 2022, 34, 2205284; Angew. Chem. Int. Ed. 2025, 64 e202424547; Angew. Chem. Int. Ed. 2023, 62, e202309968. Particularly, the selected solvents in this work have been identified to function in tailoring the electrochemical deposition morphologies and models (Adv. Energy Mater. 2019, 9, 1802207). So, what is the key difference between this work and the previously reported works? 2. It is accepted that the coordination ability from G1 to G2 gradually increase. In this case, the abilities of the solvents in shielding the interaction between Mg2+ and PS should be orderly enhanced. However, this work did not detailly discuss why the G2 based electrolyte delivered the extraordinary cell performance from the perspective of solvation structure. Besides, what is identification of the \"high shielding ability\" of solvents mentioned in this work and how does it dominate the solvation structures of MgPS and determine the cell performance?", "reviewer_2": "Recommendation: Reject. The manuscript contains several critical flaws. Most importantly, the solvent nomenclature is incorrect: the four solvents are listed as “DME: 1,2-dimethoxyethane; G2: dimethyl ether; G3: diethylene glycol dimethyl ether; G4: tetraethylene glycol dimethyl ether.” Labeling G2 as dimethyl ether is a fundamental error—dimethyl ether is a gas at room temperature and not the intended glyme (diglyme). This mislabeling propagates through the text and figures and renders the “G2 is best” conclusion ambiguous: are the simulations and experiments referring to diglyme or something else? At minimum, all figures, captions, analyses, and simulations must be rechecked and corrected with the proper glyme assignments; until then, the solvent–structure–property relationships are not credible. I would be happy to reconsider and provide a revised review once the authors correct and clarify the solvent identities throughout.", "reviewer_3": "Solvating Magnesium Polysulfides Enables Low-Barrier Speciation for Magnesium Sulfur Batteries General Comments: This manuscript presents an insightful analysis of the Mg-S battery discharge mechanism, particularly focusing on the role of ether-based electrolytes through GITT and PITT. The structure of the discharge into three stages is a clear framework for discussion. A key strength is the direct comparison between four different ether-based electrolytes, linking differences in electrochemical performance to solvation effects. While the central hypothesis, methodology, and results are excellent, technical concerns about the consistency of the data presentation remain relevant to the study's execution, particularly in how the key kinetic limitations are quantified. Overall, the work is well conducted and holds potential for publication after minor revisions. Here are some of the comments that the authors should address in order to strengthen the manuscript further. 1. The Introduction section could be strengthened by including a more detailed literature review on Mg–S batteries, with particular emphasis on previous electrolyte studies. This would provide a clearer context and highlight the novelty of the present work. 2. The Introduction does not clearly specify the types of solvents used in this study. The terminology for the solvents (e.g., G2) appears without prior explanation. Please introduce and briefly describe each solvent early in the text for better clarity. 3. There appears to be a noticeable discrepancy between the charge and discharge capacities across the different solvents. Please discuss the possible reasons for these variations and provide an explanation for each solvent system. 4. The authors must provide a quantitative comparison of the overpotential for all four electrolytes in each of the three stages. A table or bar chart showing the MgS nucleation overpotential across DME, G2, G3, and G4 should be added to prove the claim that solvation lowers the energy barrier numerically. 5. The claim that a 2-hour relaxation step is sufficient to ensure the full recovery to the thermodynamic equilibrium state is an assumption that needs more substantial justification. The authors should present a time-dependent potential plot during relaxation to prove the potential truly reaches a stable, equilibrium value, especially at later stages of discharge. 6. The observation that Stage 1 has a short plateau with capacity contribution and low sulfur utilization during this stage should be linked back to the electrolyte differences. The authors should elaborate the mechanism for this initial reduction, whether is it due to poor initial wetting, slow Mg2+, migration, or a kinetic bottleneck in the first step? 7. The authors must quantitatively determine the relative concentrations of the different MgPS species as a function of discharge capacity for both DME and G2 electrolytes. 8. Given that S22- is the precursor to MgS, what is the mechanistic explanation for S22- persistence down to 0.5 V in G2? Does the solvation structure of G2 stabilize S22- or does it inhibit the final MgS nucleation? 9. In Figure S3(b), two redox peaks are observed at approximately 1.9 V and 2.0 V. Please elaborate on the origin and electrochemical significance of these peaks. 10. If Mg²⁺ exhibits weaker interactions with S₄²⁻, a higher degree of polysulfide dissolution would be expected. However, the charge–discharge profiles do not indicate pronounced polysulfide dissolution in the G2 system. Please clarify this apparent inconsistency with an appropriate discussion (see Figure S3)."}, "author_response": "Dear Editor: We appreciate for your kind consideration of our manuscript entitled “Solvating Magnesium Polysulfides Enables Low–Barrier Speciation for Magnesium Sulfur Batteries” (No. NCOMMS-25- 61539A). We also thank the three reviewers for their time, efforts, and valuable comments on this manuscript. Having carefully gone through the evaluation and comments of the reviewers, we provide a point-by-point response to the comments. Listed below are our responses/answers to both the comments and suggestions. For ease of reference, the reviewers’s comments and suggestions are reproduced in blue, our responses are in black, and the resulting changes to the manuscript and supporting information are in black bold. All changes have been highlighted in yellow background in the revised manuscript and supporting information. Hope that the revision would satisfactorily address the comments and meet the requirements of Nature Communications. Thanks again for your kind consideration. Reviewer #1: The solvation structure of magnesium polysulfides is considered to dominate the conversion chemistry and regulate the performances of Mg-S batteries. In this work, the discharging behaviors, kinetic characteristics and electrochemical deposition models were detailed analyzed in this work by introducing a series of solvents with different coordination ability. Cells with a G2 based electrolyte demonstrate the considerable improvements in cyclic stability and specific capacity. However, the novelty and the insight of this work are still lacking. I cannot support this work to be published in Nat. Comm. 1. The crucial role of the polysulfides solvation structure in determining the charging/discharging behaviors and cell performance has been reported by some works, e.g. Adv. Mater. 2022, 34, 2205284; Angew. Chem. Int. Ed. 2025, 64 e202424547; Angew. Chem. Int. Ed. 2023, 62, e202309968. Particularly, the selected solvents in this work have been identified to function in tailoring the electrochemical deposition morphologies and models (Adv. Energy Mater. 2019, 9, 1802207). So, what is the key difference between this work and the previously reported works? Response: Thank you for the thoughtful comment regarding the degree of innovation. We have carefully reviewed the publications you referenced, and we fully acknowledge that these works elucidate the crucial role of the polysulfides solvation structure in sulfur-based batteries, as noted: 1) Adv. Mater. 2022, 34, 2205284. This study employs a concept of encapsulating-polysulfide electrolytes (EPSE), which lies in suppressing the side reactions between polysulfides and the lithium metal anode by weakening the solvation power of the outer solvent. 2)Angew. Chem. Int. Ed. 2025, 64 e202424547. This study designs the localized high-concentration electrolyte (LHCE) that modulates the electrical double layer (EDL) chemistry, drastically reducing sodium polysulfides (NaPS) dissolution and enabling the dendrite-free sodium plating/stripping. 3)Angew. Chem. Int. Ed. 2023, 62, e202309968. This study explores that the medium solvating power of the electrolyte stabilizes electrode kinetics of LIBs, underscoring the pivotal role of polysulfides solvation in governing reaction kinetics. Other paper beyond this: 1)J. Am. Chem. Soc. 2025, 147, 15435−15447. This study discovers that the solvation structure of Li polysulfides changes non-linearly with weakly solvating solvent (WSS) content and the fundamental change in the composition of the inner solvation shell before and after the critical transition point is the root cause of the two-stage deterioration in redox kinetics. 2)J. Am. Chem. Soc. 2025, 147, 26414−26424. This work designs the locally confined polysulfide- reactive electrolyte (PRE) by hierarchically controlling solvation from suppressing free solvents and tuning diluent–solvent interactions to creating a localized reaction zone, facilitating a uniform cathode- electrolyte interphase and enabling shuttle-free operation via a quasi-solid-state sulfur conversion mechanism. These works make valuable contributions to the lithium-sulfur(Li-S) and sodium-sulfur(Na-S) batteries, and we would like to respectfully note that these works demonstrate a significant common concept and design logic in the solvation of polysulfides, especially in how to suppress the polysulfide shuttle effect by regulating the solvation structure. However, distinct from other sulfur-based battery systems, the multivalent nature of magnesium ions (Mg2+) and their high charge density lead to stronger interaction between Mg2+ and sulfur in magnesium polysulfides(MgPSs), resulting in more complex solvation behavior and sluggish conversion reactions in Mg-S batteries. As illustrated in the Fig.R1, in Li-S batteries, the solvation structure typically involves two Li+ coordinating with a long-chain polysulfide. In contrast, in Mg-S batteries, one Mg2+ coordinates with polysulfide anion. This significant difference may cause different interaction strength and regulation criterion among solvents, cations and polysulfides. Besides, compared with the well-characterized Li-S system, the sulfide conversion mechanisms in Mg-S batteries are not as thoroughly elucidated. It is generally accepted that MgPSs exhibit lower solubility than LiPSs, which results in different conversion mechanism between Li-S and Mg-S systems1,2. Therefore, our work considerably varies to the previously reported works. Figure R1. The solvation structure of LiPS and MgPS. Particularly, as you mentioned, by evaluating the low-donicity glyme-based solvents (DME, G2, G3 and G4) and high-donicity solvents, Yichun Lu et.al (Adv. Energy Mater. 2019, 9, 1802207.) demonstrated that the solvent properties including donicity, dielectric constant, and viscosity collectively dictate the Li S deposition and overall electrochemical performance of Li-S batteries. 2 We totally agree that both Lu’s work and our work have experimentally demonstrated that solvents influence the formation of final discharge products such as MgS or Li S. However, their work 2 remains at the level of establishing the correlation between solvent properties (such as donicity) and macroscopic properties or morphology. A more specific mechanism remained less explored. Since MgS formation originates from its precursors, polysulfides(PS), and PS are governed by the solvation, in our study, we found that solvent shields the strong Mg-PS interaction and used the shielding ability as a descriptor to evaluate the solvation of polysulfides and the role of solvent, thereby investigating its impact on MgS nucleation and battery performance. In other words, we proposed a new criterion for solvent evaluation. More importantly, given that MgS exhibits considerably high formation energy due to the pronounced multi-valent character and high strength of the Mg-S bond, there is a huge difference in solvation modulation between Mg-S and Li-S systems. In conclusion, we propose the shielding ability of solvation as a novel descriptor to evaluate the important part of solvent in Mg-S batteries and establish the strong correlation between solvation and MgPSs conversion. We prove solvents can effectively control MgPS speciation and MgS deposition. We believe this work makes a meaningful contribution to the field. 2.It is accepted that the coordination ability from G1 to G2 gradually increase. In this case, the abilities of the solvents in shielding the interaction between Mg2+ and PS should be orderly enhanced. However, this work did not detailly discuss why the G2 based electrolyte delivered the extraordinary cell performance from the perspective of solvation structure. Besides, what is identification of the \"high shielding ability\" of solvents mentioned in this work and how does it dominate the solvation structures of MgPS and determine the cell performance? Response: We would like to express our gratitude to your professional comment. As illustrated in Fig. R2, it is true that the intrinsic coordination tendency of the G2 solvent toward a Mg2+ is generally stronger than that of DME—this conclusion is supported by computational modeling under idealized conditions, where all three oxygen atoms of G2 are able to coordinate with the cation, resulting in a notably higher binding energy compared to DME. However, such a scenario represents an ideal coordination environment without interference. In the actual electrolyte system for Mg-S batteries, the situation is far more complex. Due to factors such as steric hindrance, solvent concentration, and competitive coordination from other species, G2 may not achieve full three-oxygen coordination. Figure R2. The solvation coordination of the Mg-DME and Mg-G2. In the actual electrolyte for Mg-S battery, polysulfides(PS), present as the competitor, drive the reconfiguration of the solvation structure via the strong Mg-S interaction, triggering a distinct coordination environment. In this dynamic coordination environment, the coordination of Mg2+ has changed and the role of the solvent in this environment should be reconsidered. To clarify this point, our work focuses on a more critical and functional property in the MgPS chemistry: the shielding ability. Shielding ability is not solely determined by the intrinsic coordination strength. It is a comprehensive descriptor that reflects the effectiveness in forming a stable primary solvation sheath around Mg2+ and competing against PS. In the revised manuscript, we introduce the parameter, I (the s/p Mayer bond order ratio of Mg-solvent (I ) to Mg-PS (I )), to evaluating the solvation environment. As s p illustrated in Fig.1, solvation environment with high I has high shielding ability, facilitating the s/p optimal reaction in terms of kinetics and thermodynamics for polysulfides and promoting the efficient MgS deposition. Moreover, given that MgPSs serve as the important intermediate in the reversible conversion of sulfur, the solvation behavior of the MgPSs directly regulate the energy release mechanism and fundamentally link to the battery performance. Figure 1. Conceptual diagram illustrating the role of solvation modulation in Mg-S batteries. The text in the revised manuscript has been updated as follows(Page 8): Fig.2b and Supplementary Table. 5 demonstrate the average Mayer bond order of Mg-O(I ) s and Mg-S(I ) in four coordination environments from Fig.2a. It can be inferred that, due to the p strong interaction between Mg2+ and PS, I is stronger than the I for all solvents in Fig.2b. p s Moreover, the values of I and I changes significantly with the solvation environments, s p indicating solvation shields the strong electrostatic interaction between Mg2+ and PS. To assess the role of the solvent on tailoring the solvation structure, the I ratio (the value of I divided by s/p s the value of I ) is introduced as a parameter and represents the shielding ability of solvation. p Among four solvents, G2-based solvation environment shows the highest I and thus has the s/p most significant effect on MgPSs, indicating effective modulation of Mg-PS interaction. Figure 2b. Coordination bonds of Mg–O(I ), Mg-S(I ) and the Mulliken charge of Mg within the s p solvation structures of MgPS. Reviewer #2 Reject. The manuscript contains several critical flaws. Most importantly, the solvent nomenclature is incorrect: the four solvents are listed as DME: 1,2-dimethoxyethane; G2: dimethyl ether; G3: diethylene glycol dimethyl ether; G4: tetraethylene glycol dimethyl ether; Labeling G2 as dimethyl ether is a fundamental error; dimethyl ether is a gas at room temperature and not the intended glyme (diglyme). This mislabeling propagates through the text and figures and renders the G2 is best; conclusion ambiguous: are the simulations and experiments referring to diglyme or something else? At minimum, all figures, captions, analyses, and simulations must be rechecked and corrected with the proper glyme assignments; until then, the solvent/structure/property relationships are not credible. I would be happy to reconsider and provide a revised review once the authors correct and clarify the solvent identities throughout. Response: Thank you for your thorough review and such valuable feedback. We sincerely apologize for the oversight in incorrectly naming the solvents. We have now made comprehensive revisions and clarifications throughout the manuscript. Specifically, the four ether solvents used in our work are 1,2-dimethoxyethane (DME), diethylene glycol dimethyl ether (G2), triethylene glycol dimethyl ether (G3) and tetraethylene glycol dimethyl ether (G4). Additionally, in all figures, tables, and computational data, the solvents are labeled consistently with the main text. In our experimental and computational results, the solvent identified as the best-performing is diethylene glycol dimethyl ether (G2). We kindly request your review and reconsideration. Thank you once again for your time and effort. Best regards. Reviewer #3: This manuscript presents an insightful analysis of the Mg-S battery discharge mechanism, particularly focusing on the role of ether-based electrolytes through GITT and PITT. The structure of the discharge into three stages is a clear framework for discussion. A key strength is the direct comparison between four different ether-based electrolytes, linking differences in electrochemical performance to solvation effects. While the central hypothesis, methodology, and results are excellent, technical concerns about the consistency of the data presentation remain relevant to the study's execution, particularly in how the key kinetic limitations are quantified. Overall, the work is well conducted and holds potential for publication after minor revisions. Here are some of the comments that the authors should address in order to strengthen the manuscript further. 1.The Introduction section could be strengthened by including a more detailed literature review on Mg/S batteries, with particular emphasis on previous electrolyte studies. This would provide a clearer context and highlight the novelty of the present work. Response: We sincerely thank you for this valuable suggestion. We agree that this suggestion will help better contextualize our work and clarify its novelty. In the revised version of the manuscript, we have significantly expanded the Introduction section to include a more detailed and structured discussion of prior research on Mg-S battery electrolytes. Revised in manuscript (Page 3) “The high energy density of MSB originates from multi–electron redox reactions involving complex phase transitions from solid sulfur to MgS13,14. During the transition processes, polysulfides (PS) solvated in the electrolyte serve as fundamental intermediates and directly govern the performance of the batteries15. Prior work has established that strong Mg2+–PS coordination dominates MgPS behavior16,17, but the specific solvation structure of MgPS and its profound influence on the fundamental conversion chemistry itself have remained elusive. Early attempts based on conventional magnesium electrolytes (such as the Grignard reagent/aluminum chloride system) were made, and the instability issue in the presence of polysulfides was pointed out18. Breakthroughs in the design of electrolyte systems for Mg-S batteries, such as boron-based and phenolate-based systems with better compatibility, have been achieved19,20. Recent strategies demonstrate that introducing additives21-23 or catalyst24 to the electrolyte can modify the interphase to suppress polysulfide dissolution and diffusion. Nevertheless, the fundamental role of solvent–directed MgPS solvation in steering conversion kinetic and its critical impact on battery performance remains largely unexplored.” We believe the revised introduction now provides a clearer foundation and more effectively emphasizes the significance and innovation of our study. Thank you again for this constructive suggestion, which has undoubtedly strengthened the manuscript. 2.The Introduction does not clearly specify the types of solvents used in this study. The terminology for the solvents (e.g., G2) appears without prior explanation. Please introduce and briefly describe each solvent early in the text for better clarity. Response: Thank you for this suggestion. We agree that defining the solvents at the beginning of the manuscript is crucial for readability and clarity. We apologize for this oversight in the original submission. In response, we have revised the Introduction to describe each solvent selected for this study. Revised in manuscript: “To demonstrate this, a series of common ether solvents were selected to evaluate their ability of solvation modulation for MgPSs: 1,2-dimethoxyethane (DME), diethylene glycol dimethyl ether (G2), triethylene glycol dimethyl ether (G3) and tetraethylene glycol dimethyl ether(G4). The lone pair electrons on oxygen atoms of ether solvents enable strong coordination capability, which can stabilize the MgPS complexes.” We believe these revisions have significantly improved the clarity of the manuscript and thank the reviewer again for this constructive comment. 3.There appears to be a noticeable discrepancy between the charge and discharge capacities across the different solvents. Please discuss the possible reasons for these variations and provide an explanation for each solvent system. Response: Thank you for your professional comments. We have given a potential mechanism about the discrepancy between the charge and discharge capacities in the revised manuscript and made an explanation for each solvent. The modified text now reads(Page 9-10): “Supplementary Fig. 3a shows that in the 1st cycle, DME–based cell shows the high discharge capacity around 1100 mAh g-1 at 0.05 C (1 C = 1675 mA g−1) and exhibits two main voltage plateaus (~1.4 V and ~1.0 V). However, because of the irreversible MgS conversion, the high polarization leads to premature termination during charge (coulombic efficiency ~69%), leading to the gradual capacity decay in the following cycles and the cell failure. Based on the discussion in simulation results, it can be inferred that G2 provides the favorable solvation environment for polysulfides, which facilitating the reaction kinetics. Therefore, the charge–discharge profiles of G2-based cell have small polarization(~0.62 V) and reasonably high reversibility during charge and discharge. Meanwhile, the two redox peaks observed at approximately 1.9 V and 2.0 V in Supplementary Fig. 3b attribute to the redox reaction of iodine additive. For the G3–based cell, in the first discharge cycle, compared to G2, the solvation environment modulated by G3 has weaker promoting effect on polysulfide conversion, resulting in lower sulfur utilization and reduced capacity release. Notably, unexpectedly prolonged charging profiles near 1.8 V can be observed in the charge curves of G3-based cell in Supplementary Fig. 3c. During this process, the conversion of long-chain polysulfides to short-chain species exhibits slow kinetics, leading to the polysulfide shuttle effect and continuous loss of active sulfur. For G4–based cell, the high viscosity and poor wettability with the electrodes resulted in erratic curve in the first cycle. Then in the following cycles, sulfur undergoes incomplete conversion, which results in low plateaus and low discharge capacity. Furthermore, because of the high dissociation energy barrier of MgS, G4-based cell suffers from a large polarization in charging process, ultimately leading to insufficient charge capacity (Supplementary Fig. 3d).” 4.The authors must provide a quantitative comparison of the overpotential for all four electrolytes in each of the three stages. A table or bar chart showing the MgS nucleation overpotential across DME, G2, G3, and G4 should be added to prove the claim that solvation lowers the energy barrier numerically. Response: Thanks for your valuable suggestion. Following your recommendation, we have made a table (Supplementary Table 7) to more accurately prove the comparison of the overpotential for all four electrolytes in each of the three stages. Stage 3 can represent the MgS nucleation process. Supplementary Table 7. The value of overpotentials of four solvents according to GITT. DME G2 G3 G4 290.2 412.5 457.9 403 Overpotential 333.4 96.3 282.4 342.4 in Stage 1 221.6 238.9 305 323.4 (mV) 127.2 292.8 307.9 299.3 167.5 191.8 244.5 120.4 138.2 155.6 163 115.1 129.4 157.7 109.2 Overpotential 115.7 119.4 158.6 108.7 in Stage 2 123.3 111.9 160.2 110.8 (mV) 110 160.6 112.8 110.7 113.9 138.1 115.8 162.1 128.6 179.3 131.2 163 154.7 Overpotential 329.1 178.1 166.3 221.2 in Stage 3 538.5 332.2 222.7 401 (mV) 635.7 550.6 462.8 669.3 646.8 674.9 5.The claim that a 2-hour relaxation step is sufficient to ensure the full recovery to the thermodynamic equilibrium state is an assumption that needs more substantial justification. The authors should present a time-dependent potential plot during relaxation to prove the potential truly reaches a stable, equilibrium value, especially at later stages of discharge. Response: Thanks for your valuable suggestion. Following your recommendation, we have now provided a plot of the time-dependent potential and shows the close-up view of the later stages of discharge (Supplementary Fig.5). It can be concluded that the potential change in every step nearly reach to zero after two hours relaxation, indicating the equilibrium of voltage and a full recovery. Supplementary Figure 5. The time-dependent potential plot and the close-up view of the later stages of discharge of a, b DME, c, d G2, e, f G3 and g, h G4 the analyzed from Fig.4a 6.The observation that Stage 1 has a short plateau with capacity contribution and low sulfur utilization during this stage should be linked back to the electrolyte differences. The authors should elaborate the mechanism for this initial reduction, whether is it due to poor initial wetting, slow Mg2+, migration, or a kinetic bottleneck in the first step? Response: Thank you for this insightful comment. We have discussed the mechanism for Stage 1 in detail and stated the differences for four electrolytes in the revised manuscript: “The Stage 1 corresponds to the solid-to-liquid conversion of sulfur to long-chain polysulfides in Mg-S batteries, specifically the dissolution of solid sulfur in the electrolyte environment. A short plateau is observed during this stage, corresponding to the emergence of polysulfides. In the case of G3 and G4, high viscosity leads to wettability issues, which reduces the rate of electrochemical reactions and results in low active material utilization. Consequently, the first plateau exhibits a lower voltage with a distinct sloping profile. In contrast, as seen in the G2 solvent environment, better wettability and higher ionic conductivity allow the active sites on the sulfur cathode to function effectively and sulfur can readily transform to polysulfides without rapid diffusion away from the reaction interface, thereby resulting in a well-defined plateau region. Meanwhile, DME-based cell suffers from passivation by irreversible residues formed in previous cycles, which hinders the accessibility of active sulfur. This directly leads to the shortage of the plateau and results in incomplete sulfur conversion.” 7.The authors must quantitatively determine the relative concentrations of the different MgPS species as a function of discharge capacity for both DME and G2 electrolytes. Response: Thanks for this professional comments. According to your instructions, we have segmented the actual discharge curve based on the theoretical capacity contribution of each polysulfide(TableR.1), with the depth of discharge (DOD) as the X axis and the voltage change as the Y axis. The orange circles mark the corresponding polysulfide species at specific depths of discharge, and we have used these marks to represent the highest relative concentration of each polysulfide for plotting(Fig.R3). It is important to note that these results reflect the ideal sulfur conversion; in fact, the disproportionation of polysulfides during battery discharge inevitably affect the actual concentration profiles. Table R.1 Theoretical Sulfur Reduction Path and Corresponding Specific Capacity Cumulative theoretical capacity Species (mAh g-1) S8 / MgS 209 8 MgS 279 6 MgS 419 4 MgS 838 2 MgS 1675 Figure R3. The capacity-dependent concentrations of the major species considered: S , MgS , MgS , 8 8 6 MgS , MgS and MgS in DME and G2-based batteries. 4 2 8.Given that S22- is the precursor to MgS, what is the mechanistic explanation for S22- persistence down to 0.5 V in G2? Does the solvation structure of G2 stabilize S22- or does it inhibit the final MgS nucleation? Response: We sincerely appreciate your insightful comments. During the in-situ Raman test, we observed the presence of S 2- peaks in G2 even at 0.5 V. We attribute this to the residual S 2- species, which 2 2 cannot be fully converted to MgS. Besides, compared with theoretical capacity (1675 mAh g-1) for full conversion to MgS, the delivered capacity (~670 mAh g-1) shown in Fig.3f is relatively lower, indicating the residual species still exist. Furthermore, regarding the theory you mentioned that G2 may stabilize S 2- or inhibit MgS nucleation, our coin cell data (Fig.2d) shows that the discharge 2 capacity(~1091 mAh g-1) is higher than the theoretical capacity for sulfur conversion solely to S 2- 2 (838mAh g-1). Therefore, the effect of G2 in this regard might not be significant, and its primary role appears to be more prominent in mediating the solvation of polysulfides. 9.In Figure S3(b), two redox peaks are observed at approximately 1.9 V and 2.0 V. Please elaborate on the origin and electrochemical significance of these peaks. Response: Thank you for raising this important question. Given that we use Iodine(I⁻/I₃⁻ redox couple) as the additive in four electrolytes to facilitate the conversion of sulfur, the two redox peaks at approximately 1.9 V and 2.0 V in FigS.3b may attribute to the redox reaction of iodine. Besides, we have also made clarification in the revised manuscript. 10.If Mg exhibits weaker interactions with S, a higher degree of polysulfide dissolution would be expected. However, the charge/discharge profiles do not indicate pronounced polysulfide dissolution in the G2 system. Please clarify this apparent inconsistency with an appropriate discussion (see Figure S3). Response: Thank you for your critical and professional comments. For Mg-S batteries, the solubility of magnesium polysulfides in ether-based solvents is inherently low, especially when compared to lithium polysulfides3. Under this condition, although solvent like G2 modifies the interactions of Mg-S bond, UV-vis spectroscopy confirms that it does not significantly alter the polysulfide solubility (Fig.3a). Consequently, the charge-discharge profiles do not exhibit signs of pronounced polysulfide dissolution. Figure 3.a UV−vis absorption spectra of MgSx in various solvents. References 1 Luo, T. et al. Polysulfides in Magnesium‐Sulfur Batteries. Advanced Materials 36, doi:10.1002/adma.202306239 (2023). 2 Bieker, G. et al. Cation-Dependent Electrochemistry of Polysulfides in Lithium and Magnesium Electrolyte Solutions. The Journal of Physical Chemistry C 122, 21770- 21783, doi:10.1021/acs.jpcc.8b06560 (2018). 3 Bieker, G. et al. Influence of cations in lithium and magnesium polysulphide solutions: dependence of the solvent chemistry. Physical Chemistry Chemical Physics 19, 11152-11162, doi:10.1039/c7cp01238a (2017)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Authors have addressed my concerns, and it can be published as it.", "reviewer_2": "The author's responses are generally satisfactory, and I have no further questions at this time.", "reviewer_3": "The authors have addressed all the comments, and there is significant improvement in the manuscript. I recommend the manuscript for publication."}, "author_response": ""}]

Decision letter