Asymmetric sulfonamide design enabling high-voltage sodium-ion pouch cells in wide temperature

Domain: biology
Published: 2026-03-24
Editorial outcome: major_revision

Abstract
To advance toward commercial viability, sodium-ion batteries are required to operate under high cut-off voltages and low temperatures. This necessitates electrolyte designs that provide sufficient oxidative stability for high-voltage cycling while maintaining high ionic conductivity at low temperatures. Herein we design a non-flammable sulfonamide solvent molecule, N-ethyl-N-methyl-trifluoromethanesulfonamide, by introducing asymmetric alkyl substituents that create a geometric kink to hinder efficient crystal packing during cooling, leading to a low melting point (−86 °C). The resulting sulfonamide-based electrolyte exhibits weak ion–dipole interaction and favorable solvation structures enriched in contact ion pairs and aggregates, which promotes the formation of highly stable and conductive interphases with both the positive and negative electrodes. Benefitting from these features, the sulfonamide-based electrolyte enables 1-Ah-level hard carbon||NaNi1/3Fe1/3Mn1/3O2 pouch cells to retain 69.8% and 42.3% of room-temperature capacity even at −60 °C and −70 °C, while achieving capacity retentions of 90.0% and 81.6% after 1500 and 1000 cycles at high upper cut-off voltages of 4.15 V and 4.2 V, respectively. The sulfonamide-based electrolyte also improves the high-temperature cycling stability and delays the onset and trigger of thermal runaway at the pouch-cell level. This work offers fundamental insights into solvent molecule and electrolyte design for advancing high-energy and wide-temperature sodium-ion batteries.

Introduction
Rechargeable sodium-ion batteries (SIBs) have attracted significant attention as promising candidates for energy storage applications due to the abundant Na resources and their potential low cost1–5. However, the practical deployment of SIBs is still hindered by their relatively low energy density6–9, which compromises their competitiveness compared to commercial lithium-ion batteries (LIBs, e.g., graphite||LiFePO4, ~180 Wh kg−1)10,11. To boost their energy density, a promising approach is to elevate the charging voltage (e.g., ≥4.2 V vs. Na/Na+, 4.2 VNa) of the O3-type layered transition metal (TM) oxide positive electrodes (NaxTMO2, 0＜x≤1; TM: Ni, Co, Mn, Fe, etc.) to extract more Na-ions12–14. However, this strategy often leads to interphasial instability and poor cycling performance due to severe electrolyte oxidation at high potentials15. Moreover, although the desolvation of Na+ in low temperatures is theoretically easier than that of Li+—due to its ~30% larger ionic radii16,17 and thus weaker electrostatic interactions with solvent molecules—the low temperature performance of SIBs remains unsatisfactory, especially under low temperature conditions such as −60 ℃18,19. These challenges under high voltages and temperature conditions highlight the critical need for electrolyte designs that can simultaneously withstand high voltages and enable efficient ion transport at low temperatures.

Unlike metal negative electrodes20–22, where Na0 (NaN-metal) electrodeposition typically occurs in a surface-confined and planar manner with minimal ion diffusion, hard carbon (HC) negative electrodes rely on intercalation chemistry23,24 that requires Na⁺ cations to diffuse across a several-tens-μm-thick electrode through tortuous internal pores. This longer and more complex transport pathway makes efficient Na⁺ intercalation/deintercalation particularly challenging under practical conditions (assuming a typical HC areal loading of 5 mg cm−2 and a compressed density of 1 g cm−3, which corresponds to a ~50 μm electrode thickness) especially at low temperatures. In this scenario, electrolyte design should particularly focus on facilitating Na+ diffusion and weakening ion‒dipole interactions, while also ensuring high-voltage stability and sufficient ionic conductivity. Recent work in this field has relied on advances in introducing effective additives25,26 into conventional carbonate-based electrolytes, as well as developing efficient ether-based27–29 and acetonitrile-based30 electrolytes. While these strategies have led to great progress, it is still challenging to simultaneously meet all the above mentioned requirements—particularly when scaling up to ampere-hour-level pouch cells13. Further progress requires molecular design to simultaneously address the demands of high-voltage stability, low temperature performance, and practical scalability.

Ion‒dipole interactions31,32 play an important role in desolvation, which dictates the kinetics. From a fundamental perspective, the Na+ cation can be treated as a charged point charge. As illustrated in Fig. 1a, consider a Na+ cation located at a distance r from the center of a polar molecule with dipole moment μ, forming an angle θ relative to the dipole axis. The maximum attractive interaction (i.e., maximum negative energy) occurs when the dipole is aligned directly away from the ion (θ=0°). In realistic electrolyte environments, thermal fluctuations lead to continuous reorientation of dipolar solvent molecules with a rotation angle of β (around θ=0°). To account for this rotational averaging, the ion–dipole interaction energy ω is expressed in an integral form over all possible orientations:

𝛽 𝑒𝜇𝑐𝑜𝑠𝜃
𝜔 =−∫ 𝑑𝜃 (1)
0 4𝜋𝜀0𝜀𝑟2

Where ε0 and εr are dielectric permittivities of the dipole and vacuum33. As temperature decreases, the rotational degrees of dipolar solvent molecules are progressively restricted, leading to reduced molecular reorientation (β↓). This effect becomes particularly pronounced as the temperature approaches the solvent’s melting point (Tm), where molecular (i.e., dipolar) rotations effectively freeze (Fig. 1b). The dielectric constant of the solvent (ε) drops sharply upon solidification, reflecting the diminished ability of the medium to reorient dipoles in response. According to the equation (1), it leads to much higher ion‒dipole interaction at low temperature than at room temperature (25 °C, RT) (ωLT>>ωRT). Therefore, the molecular design of solvents with Tm far below the intended operating temperature is critical to ensure sufficient dipolar mobility and maintain a high dielectric constant for effective ion transport.

To design solvent molecules with intrinsically low Tm, one must consider not only the strength of intermolecular attractive forces but also the molecular geometry that governs crystal packing. While boiling points are largely dictated by attractive interactions, Tm are primarily influenced by how well molecules can pack into an ordered solid lattice. According to this principle, molecules with rigid, symmetrical structures tend to pack efficiently and thus exhibit high Tm. In contrast, introducing asymmetry or steric irregularities disrupts lattice formation and lowers Tm. Based on this rationale, a non-flammable sulfonamide molecule, N-ethyl-N-methyl-trifluoromethanesulfonamide (EMTMSA) was rationally designed by incorporating asymmetric alkyl substituents (methyl and ethyl groups) on the sulfonamide nitrogen (Fig. 1c), which introduce a geometric kink and hinder efficient crystalline packing. This molecular asymmetry disrupts solid-state packing, resulting in poor crystallinity and a low melting point of −86 °C (Fig. 1d). In addition, its high flash and boiling points, along with moderate viscosity and dielectric constant, render it a well-balanced and practically viable electrolyte solvent (Supplementary Table 1). Moreover, it also successfully inherits the anodic stability of its structural analogue, N,N-dimethyltrifluoromethanesulfonamide (DMTMSA, Fig. 1c), which was previously developed by our group for high-voltage chemistries34,35. Specifically, as expected, the formulated EMTMSA-based electrolyte shows sufficient molecular rotational mobility at low temperatures revealed by the longer spin-spin relaxation time in variable-temperature nuclear magnetic resonance (VT-NMR). The electrolyte also exhibits large portions of contact-ion pairs (CIPs) and ion‒solvent‒anion aggregates (AGGs) with preferential solvation characteristics, promoting the formation of solid‒electrolyte interphase (SEI) with low resistance and stability, effectively supporting low temperature and long-term cycling stability at high voltages. Benefiting from these, our EMTMSA-based electrolyte successfully enables an Ah-level HC||NaNi1/3Fe1/3Mn1/3O2 (NFM) pouch cell to maintain 69.8% and 42.3% of its room-temperature capacity even at −60 ℃ and −70 ℃, respectively, while simultaneously delivering high temperature stability and delayed thermal runaway. The pouch cells also exhibit long-term cycling stability with high capacity retentions (with respect to (w.r.t.) the third cycle) of 90.0% (cut-off voltage ~4.15 VNa) and 81.6% (cut-off voltage ~4.2 VNa) after 1500 and 1000 cycles, respectively. This work provides valuable insights for designing electrolyte molecules or recipes for versatile and practical SIBs.

Results
Molecular and electrolyte design. While polar molecules are beneficial for dissociating sodium salts to support high ionic conductivity, they often exhibit higher Tm than their nonpolar counterparts. This is primarily due to stronger intermolecular Coulombic interactions—such as dipole–dipole interactions or hydrogen bonding—which enhance molecular ordering and raise the crystallization temperature. To elucidate the nature of intermolecular interactions, molecular dynamics (MD) simulations were performed for three sulfonamides with different alkyl substitutions: DMTMSA, EMTMSA, and DETMSA (N,N-diethyl-trifluoromethanesulfonamide) at different temperatures (Supplementary Fig. 1 and Supplementary Data 1). In Fig. 1e, the Coulomb and van der Waals forces of DMTMSA are set as the reference (zero point), indicated by the dashed line at the center. The relative differences in interaction forces for EMTMSA and DETMSSA are plotted accordingly. Substituting the two methyl groups in DMTMSA with longer or bulkier alkyl chains—either one ethyl (EMTMSA) or two ethyls (DETMSA)—results in enhanced van der Waals interactions. This increase is attributed to the larger molecular size and greater intermolecular contact area, which promote stronger dispersion forces. Conversely, the Coulombic interactions become weaker with increasing alkyl chain length, likely due to slightly reduced molecular polarity and dielectric response as more nonpolar alkyl groups are introduced. Importantly, the combined intermolecular force (Coulomb + van der Waals) is lower for both DETMSA and EMTMSA compared to DMTMSA, suggesting a weaker overall interaction. This is consistent with both the experimentally measured Tm and MD simulated diffusion behavior. Differential scanning calorimetry (DSC) shows that EMTMSA and DETMSA exhibit no sharp endothermic peaks above −85 °C, while DMTMSA displays a distinct melting transition at −60 °C (Fig. 1d). MD-simulated diffusion coefficients exhibit a clear transition in slope around their Tm with molecular mobility increasing more rapidly above the phase transition (Fig. 1f).

To formulate a practical electrolyte suitable for pouch cells, propylene carbonate (PC, Tm=−49 °C) and ethyl methyl carbonate (EMC, Tm=−53 °C)36 were introduced as co-solvents with EMTMSA. The resulting sulfonamide-based electrolyte consists of 1 M sodium bis(fluorosulfonyl)imide (NaFSI) dissolved in an EMTMSA/PC/EMC mixture with a volume ratio of 6:1:3. DMTMSA- and DETMSA-based electrolytes are prepared using the same recipe, with EMTMSA simply replaced by DMTMSA or EMTMSA, respectively. In this recipe, 10 vol% PC primarily facilitates the dissociation of NaFSI due to its high dielectric constant, while 30 vol% EMC serves to reduce the overall viscosity of the electrolyte and enhance ion mobility. All solvents were subjected to molecular-sieve drying to eliminate residual water and consequently suppress hydrogen fluoride (HF) generation (Supplementary Fig. 2), which is vital for maintaining stable electrochemical performance. Correspondingly, a carbonate electrolyte consisting of 1 M NaFSI in PC/EMC (3:7 vol) was selected as the reference electrolyte. The DMTMSA- and EMTMSA-based electrolytes maintain high oxidative stability up to ≈4.8 VNa, whereas the DETMSA-based electrolyte exhibits a greatly reduced anodic stability (Supplementary Fig. 3a). This decrease correlates with the substituent’s electronic structure: longer alkyl chains donate more electron density, weakening the FSO2– group’s electron-withdrawing effect and elevating the local electron density, consistent with the density functional theory (DFT) results showing higher highest occupied molecular orbital (HOMO) levels for longer chains (Supplementary Fig. 3b and Supplementary Data 2). The ionic conductivities of all three sulfonamide-based electrolytes decrease with decreasing temperature, but the EMTMSA-based electrolyte consistently maintains the highest conductivity. Notably, the DMTMSA-based electrolyte exhibits a sudden drop in conductivity at −60 °C, indicating a possible crystallization around this temperature (Fig. 1g). Furthermore, we performed ignition tests by exposing both electrolytes to an open flame (Fig. 1h, Supplementary Movie 1 and 2). Compared to the carbonate reference electrolyte, the EMTMSA-based electrolyte demonstrates largely improved flame retardancy, effectively suppressing ignition and sustaining non-flammability, which is critical for improving the battery safety. Moreover, the EMTMSA-based electrolyte exhibits much better wettability than the reference carbonate electrolyte (Supplementary Fig. 4). Therefore, considering the ionic conductivities at low temperatures and electrochemical stability windows of the sulfonamide-based electrolytes, the designed EMTMSA-based electrolyte with an asymmetric molecular structure was selected as the optimal formulation.

The coordination interactions and solvation structures in the electrolytes. The characteristics of the Na+ solvation structures play an important role in influencing the decomposition of electrolyte components on both the positive electrode and negative electrode surfaces. The S‒N‒S symmetric stretching vibrational mode for the neat NaFSI salt is around 770–780 cm−1 (Supplementary Fig. 5), which redshifts when dissolved in solvents, driven by the Coulombic interactions between ions and solvent molecules. Raman spectra of both the EMTMSA- and carbonate-based electrolytes were deconvoluted to identify the various ion-solvent clusters including solvent-separated ion pairs (SSIP, ~720 cm−1), contact ion pairs (CIP, ~735 cm−1), and ion-pair aggregates (AGG, ~745 cm−1)37. While SSIP (56.4%) is the dominant cluster type in the carbonate-based electrolyte (Fig. 2a), AGG (10.2%) and CIP (84.2%) are the main species in the EMTMSA-based electrolyte (Fig. 2b), indicating that more FSI− anions participate in the first solvation sheath. It would promote increased anion decomposition near the charged electrode surfaces, which is beneficial for enhancing the electrochemical stability of the electrolyte. To elucidate the exact coordination sites between ions and solvents, NMR technique was conducted on the electrolytes before and after incorporating NaFSI. An apparent upfield shift of 9.22 ppm for PC was observed from the 17O NMR in the carbonate-based electrolyte on dissolving 1 M NaFSI (Fig. 2c), indicating the preferential solvation of Na+ by PC molecules, attributed to the much higher dielectric constant of PC than that of EMC. When using EMTMSA/PC/EMC as the solvent system, a similar preferential solvation phenomenon was observed (Fig. 2d, Supplementary Fig. 6). An upfield shift of 1.01 ppm for the O atoms in the EMTMSA molecule was also detected upon dissolving 1 M NaFSI (Fig. 2d), suggesting that EMTMSA participates in the Na⁺ solvation sheath. The 1.01 ppm shift is appreciable considering EMTMSA constitutes the largest volume fraction (60%) among the mixed solvents. A downfield shift in 19F NMR can also be noted from the EMTMSA molecule after salt incorporation (Supplementary Fig. 7). It should be noted that NMR spectroscopy reflects an ensemble-averaged chemical environment, which supports the conclusion that EMTMSA contributes to the Na+ solvation structure. Based on the above arguments, schematic figures of the representative Na+ solvation structures in both electrolytes were presented in Fig. 2e and f.

Besides the characterizations of the solvation structures at 25 °C, VT-NMR was also employed to reveal the chemical environment and molecule mobility of the electrolytes at low temperatures38. When the temperature decreases from RT to −50 °C, upfield shifts were noted for all 1H NMR spectra (Fig. 2g, Supplementary Fig. 8). This could be attributed to the reduced thermal motion of electrons at lower temperatures, leading to increased electron density around each nucleus and enhanced shielding effects. More importantly, noticeable peak broadening was observed for all signals in the 1H NMR spectra of the PC/EMC solvent mixture (Fig. 2g), suggesting slower molecular dynamics and increased viscosity at lower temperatures. Higher viscosity leads to a shorter spin-spin relaxation time (T2), which is inversely proportional to the NMR line width. In stark contrast, the peaks in the EMTMSA/PC/EMC solvent mixture remain sharp and almost unchanged even at −50 °C (Fig. 2h), indicating preserved molecular mobility and lower viscosity. Notably, the 1H NMR signals of pure EMTMSA exhibit minimal changes during cooling (Supplementary Fig. 9), suggesting that EMTMSA itself can maintain stable molecular dynamics upon cooling. This highlights the positive effect of EMTMSA, which helps maintain a fluidic environment and prevents molecule rotational motions from slowing, validating our rationale for designing the asymmetric EMTMSA molecule with an intrinsically low Tm.

Electrochemical performance of SIB pouch cells at room and low/high temperatures. To evaluate the electrolyte performance under practical cell conditions, Ah-level pouch cells were assembled by multilayer-stacking NFM positive electrodes (areal loading 40 mg cm−2, double-side) and HC negative electrodes (areal loading 20 mg cm−2, double-side, detailed cell parameters are provided in Supplementary Table 2). It is worth noting that the mass transport characteristics and failure mechanisms in pouch cells with high-loading electrodes differ substantially from those in coin cells. The more stringent conditions in pouch cells demand adequate electrolyte wettability to ensure uniform infiltration across the entire electrode stack. Furthermore, gas evolution within pouch cells can induce rapid capacity decay due to cell swelling and interfacial disconnection that are negligible in coin cell configurations. The gas generated during the formation cycle was completely removed by a vacuum degassing process during the final sealing step after formation (Supplementary Fig. 10). A small gas pocket remains after final sealing, serving as a visible indicator for qualitative observation of gas evolution during long-term cycling, after which the pouch cells are placed in a mechanical fixture for cycling tests (Supplementary Fig. 10). The long-term cycling performance of pouch cells using the carbonate-based and EMTMSA-based electrolytes was first evaluated with an upper charging voltage of 4.1 V at a charge-discharge rate of 1 C (4.15 VNa, Fig. 3a). Upon desodiation to 4.15 VNa, a short voltage plateau was observed (Supplementary Fig. 11), which corresponds to the detrimental P3 to OP2 phase transformation13,39. Such a transformation evokes appreciable instability within the bulk lattice and the interphasial region, eventually deteriorating cycling stability. To avoid such degradation and achieve a long-term cycling stability (e.g., >1000 cycles), it is generally necessary to restrict the upper cut-off voltage to ≤4.0 VNa. However, even under an elevated upper cut-off voltage of 4.15 VNa, our EMTMSA-based electrolyte can enable cycling stability with 90.0% capacity retention (w.r.t. the third cycle) after 1500 cycles (Fig. 3a). It outperforming the carbonate-based electrolyte, which showed only 72.0% capacity retention (w.r.t. the third cycle) under the same conditions (Fig. 3a). More importantly, when evaluated at a higher cut-off voltage of 4.2 VNa (Fig. 3b), the pouch cell with our EMTMSA-based electrolyte can still retain 81.6% of its initial capacity (w.r.t. the third cycle) after 1000 cycles at charging-discharging rate of 0.5 C (close to the capacity retention 81.9% achieved by the pouch cell with the DMTMSA-based electrolyte, Supplementary Fig. 12), outperforming the carbonate-based and DETMSA-based electrolytes, which retain 65.6% and 64.5% of its original capacity for 1000 cycles under the same conditions, respectively.

The elevation in the charging voltage from 4.0 to 4.2 VNa enables a ~25% increase in the specific capacity of the NFM positive electrode (Supplementary Fig. 13), attributed to the extended voltage plateau near 4.15 VNa during the charging process (Fig. 3c, d, indicated by dashed boxes). These results demonstrate that our EMTMSA-based electrolyte can effectively passivate the aggressive NFM surfaces by forming a highly conductive and stable cathode‒electrolyte interphase (CEI). In stark contrast, the carbonate-based electrolyte gives rise to an unstable and resistive CEI, as evidenced by its much lower discharge capacity of ~0.71 Ah at 0.5 C, compared to that of the cell with the EMTMSA-based electrolyte (~0.91 Ah, Fig. 3b). Compared to the carbonate-based electrolyte, the pouch cell with the EMTMSA-based electrolyte also delivers improved average Coulombic efficiency (CE) at 4.2 VNa (~99.94%, Supplementary Fig. 14) and better rate performance (Supplementary Fig. 15). The formation of an unstable CEI in the carbonate-based electrolyte under high-voltage cycling is further evidenced by the pronounced gas evolution, as indicated by the noticeable swelling of the gas pocket (Fig. 3e). In contrast, negligible gas generation was observed in the cell employing the EMTMSA-based electrolyte (Fig. 3e), highlighting its enhanced interphasial stability. To analyze the impedance evolution during cycling, galvanostatic intermittent titration technique (GITT) measurements were performed on the pouch cells after 200 cycles in different electrolytes. The overpotentials observed in the cell with the EMTMSA-based electrolyte were significantly lower than those in the cell with the carbonate-based electrolyte (Fig. 3f), further verifying the different interphasial stability in both electrolytes. Moreover, the evolution of desodiation and sodiation kinetics was investigated using in situ electrochemical impedance spectroscopy (EIS), and the distribution of relaxation times (DRT) technique was employed to decouple electrochemical processes over different timescales during the charge–discharge process40,41. The four distinct regions in DRT data (Fig. 3g) are attributed to Re (electrolyte resistance), RSEI/CEI (SEI and CEI resistance), Rct (charge transfer resistance), and Wdiff (solid-state diffusion). During the charge–discharge process, the cell with the carbonate-based electrolyte exhibits significant fluctuations in Rct, with high values especially near the high-voltage region (~4.2 V). In contrast, the Rct is almost negligible in the EMTMSA-based electrolyte, which further elucidates the low impedance of pouch cells with the EMTMSA-based electrolyte after high-voltage cycling. Additionally, the EMTMSA-based electrolyte also shows a smaller RSEI/CEI compared to the carbonate-based electrolyte, suggesting the formation of a more robust CEI/SEI and more efficient interphasial charge transfer. These results can also be verified by the impedance analysis on the pouch cells before and after long-term cycling (Supplementary Fig. 16 and Supplementary Table 3).

Applications such as grid-scale energy storage and electric tools in high-latitude countries demand that batteries operate reliably across a wide temperature range (−60 °C to 60 °C)42. Under low temperatures, it is crucial for SIBs to retain appreciable discharge capacity. This challenge is further amplified in pouch cells with high-loading electrodes, where low temperatures not only limit intercalation kinetics but also severely hinder ion transport, making it difficult to sustain sufficient ionic mobility throughout the electrode stack. Ah-level pouch cells were also used to evaluate the practical electrochemical performance at low temperatures (−20 °C, −40 °C, −60 °C, and −70 °C) at an upper charging voltage of 4.2 VNa with the electrolytes. While the pouch cell using the carbonate-based electrolyte retains 78.1% of its RT capacity at −20 °C, its performance deteriorates sharply with further temperature reduction by retaining only 14.8% at −40 °C and dropping to nearly zero at −60 °C (Fig. 4a). Similarly, the capacity of the pouch cell using the DMTMSA-based electrolyte declines to nearly zero at −60 °C (Supplementary Fig. 17). In sharp contrast, the pouch cell with the EMTMSA-based electrolyte exhibits much better low temperature performance, retaining 93.8%, 80.6%, 69.8%, and 42.3% of its RT capacity at −20 °C, −40 °C, −60 °C, and −70 °C, respectively (Fig. 4b). The pouch cell with the EMTMSA-based electrolyte maintained high capacity retention, even under simultaneous low temperature charging and discharging at 0.01 C, retaining 81.1%, 59.7%, and 41.9% of its RT capacity at −20 °C, −40 °C, and −60 °C, respectively (Supplementary Fig. 18).

This stark difference in low temperature electrochemical performance can be attributed to the distinct physical appearances of the two electrolytes after overnight storage at −60 °C: the carbonate-based electrolyte had solidified, while the EMTMSA-based electrolyte remained in a liquid state (Fig. 4c). Benefiting from its intrinsically low Tm (−86 °C), EMTMSA constitutes 60 vol% of the EMTMSA-based electrolyte and serves as a fluidic medium that accommodates both PC and EMC molecules as well as solvated Na+ ions. This enables the electrolyte to maintain a liquid state even at −60 °C, ensuring continuous ion transport under low temperature conditions. Furthermore, we also evaluated the cycling performance of the pouch cells using different electrolytes, where both charging and discharging were performed at −20 °C. This scenario presents greater challenges to cycling stability, as the increased overpotential at low temperatures can readily drive the negative electrode potential well below 0 VNa. Such conditions promote the irreversible Na0 plating on the hard carbon surface, leading to the loss of Na inventory and thus rapid capacity fading. Compared to the carbonate-based electrolyte, the pouch cell with the EMTMSA-based electrolyte delivers improved cycling stability and CE at −20 °C (Fig. 4d; Supplementary Fig. 19). After 500 cycles at −20 °C, the cell using the EMTMSA-based electrolyte retained 59.2% of its initial capacity in contrast to only 37.0% retention observed with the carbonate reference electrolyte. In comparison with recent studies on low temperature electrolytes for SIBs with layered oxide positive electrodes, our results with the EMTMSA-based electrolyte demonstrate notable high-voltage (4.2 VNa) and capacity retention under low temperature (−60 °C and −70 °C) conditions (Fig. 4e and Supplementary Table 4). Notably, these results were achieved in practical pouch cells. Moreover, the high temperature stability at 45 °C and 60 °C was evaluated in pouch cells at an upper charging voltage of 4.2 VNa. Pouch cells with the EMTMSA-based electrolyte exhibit better cycling stability compared with those employing the carbonate-based electrolyte, achieving 87.7% capacity retention (w.r.t. the third cycle) after 200 cycles at 45 °C (vs. 75.7%) and 82.1% retention at 60 °C (vs. 71.4%, Supplementary Fig. 20). In addition to improved high temperature cycling stability, the EMTMSA-based electrolyte also delivers advantages under thermal-abuse conditions. Pouch cells charged to 100% state of charge (4.2 V) were evaluated by accelerated rate calorimetry (ARC) to track key thermal-runaway temperatures (Fig. 4f). The EMTMSA-based electrolyte increases both the self-generated heat initiation temperature (T1, from 163 °C to 206 °C) and the thermal-runaway trigger temperature (T2, from 233 °C to 278 °C), while showing a slight reduction in the maximum thermal-runaway temperature (T3, from 471 °C to 468 °C). These ARC data verify that the flame-retardant properties of the EMTMSA molecule are effectively propagated to the pouch-cell level, highlighting the value of molecular-level flame-retardant electrolyte design.

Characterizations of SEIs formed on HC negative electrodes. The pronounced difference in low temperature cycling performance between the two electrolytes could be primarily attributed to their distinct interactions with the HC negative electrode, which guides us to conduct postmortem analysis on the cycled HC negative electrodes. After 200 cycles at −20 °C, the HC negative electrodes were extracted from the pouch cells in an argon-filled glovebox. A large amount of white precipitates was observed on the HC electrode cycled in the carbonate-based electrolyte, indicative of severe Na0 precipitation (Fig. 5a). In contrast, the HC negative electrode cycled in the EMTMSA-based electrolyte retained a uniform black appearance, suggesting a more stable Na0-free interphase (Fig. 5a). The presence of Na0 precipitation was further confirmed by scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS) analysis. Mossy-like Na0 dendrites43–45—characteristic of sodium metal deposition from carbonate-based electrolytes—were clearly observed on the HC electrode cycled in the carbonate-based electrolyte (Fig. 5b). In contrast, the HC electrode maintained a smooth surface without Na agglomeration after cycling in the EMTMSA-based electrolyte (Supplementary Fig. 21). The suppressed Na0 precipitation observed in the EMTMSA-based electrolyte can be attributed to its lower cell impedance at low temperatures (Supplementary Fig. 22), as well as the formation of a more ionic conductive and stable SEI on the HC negative electrode. While the SEI formed in the carbonate-based electrolyte is approximately 8–10 nm (Fig. 5c), observed in the high-resolution transmission electron microscopy (HRTEM), it is only 2–4 nm (Fig. 5d) in the EMTMSA-based electrolyte. These observations indicate that the two electrolytes should undergo distinct decomposition pathways, despite both having PC as the primary solvent coordinating with Na+. In addition, the polarity differences between the two electrolytes also lead to different SEI solubility. The extent of SEI dissolution was further evaluated using Na||Cu cells by monitoring capacity changes after controlled resting intervals according to a previous report46–48. An increase in capacity after each pause reflects continued electrolyte decomposition (Fig. 5e), primarily due to the dissolution and reformation of SEI during open-circuit rest. The carbonate-based electrolyte exhibited a larger capacity increase after each pause compared to the EMTMSA-based electrolyte (Fig. 5f), indicating more severe SEI dissolution and inferior ability to suppress parasitic reactions. The higher polarity of the carbonate-based electrolyte may promote continuous dissolution and re-deposition of SEI components, leading to uncontrolled SEI growth and increased interfacial resistance. In contrast, the lower polarity of the EMTMSA-based electrolyte helps preserve SEI integrity and suppress excessive thickening49.

SEI compositions play an important role in ion transport and dissolution behavior, which was further investigated by surface chemistry analysis. X-ray photoelectron spectroscopy (XPS) results show that the SEI formed on HC in the ETMTMSA-based electrolyte contains a higher proportion of NaF species and a lower proportion of organic species, even including NaCx, which represents dead sodium precipitation, compared to that formed in the carbonate-based electrolyte (Fig. 5g, Supplementary Fig. 23). As a representative inorganic component, NaF contributes to enhancing the chemical robustness of the SEI. This inorganic-rich SEI is less prone to dissolution in organic electrolytes, in contrast to organic-rich SEI components, which tend to dissolve more readily due to the like-dissolves-like principle. To get more accurate insights into the depth distribution of SEI components, Na||Cu cells were repeatedly cycled within a voltage window of 0.005 to 1.5 V in different electrolytes. This procedure facilitates the formation of a relatively flat and uniform SEI layer on the Cu substrate, which is more suitable for depth profiling compared to the inherently rough and irregular SEI surface typically formed on HC particles. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) was conducted to obtain the depth profiling of different fragments in the SEI. TOF-SIMS was employed to investigate the depth profiles of characteristic SEI fragments. As shown in Fig. 5h, i and Supplementary Fig. 24, the SEIs formed in both electrolytes consist of inorganic (NaF−) and organic (CHO2−) components, but their content and spatial distributions differ significantly. In the carbonate-based electrolyte, the SEI exhibits a strong CHO2− signal and a gradual increase in the Cu− signal throughout the sputtering process (1000 s), accompanied by a concurrent decrease in NaF− and CHO2− intensities (Fig. 5h). This indicates the formation of a relatively thick SEI. In contrast, the Cu− signal in the ETMTMSA-derived SEI rapidly saturates within 300 s of sputtering, suggesting a much thinner interphase Fig. 5i. These distinctions are more intuitively visualized in the corresponding 3D reconstructed TOF-SIMS maps (Fig. 5h and i), where the more compact and uniform distribution of NaF− and CHO2− in the ETMTMSA system further corroborates the formation of a thinner and more homogeneous SEI layer, which are consistent with the HRTEM and XPS results.

Characterizations of CEIs formed on NFM positive electrodes. Operating at high cut-off voltages, such as 4.2 V vs. Na/Na+ (equivalent to 4.5 V vs. Li/Li+), poses a significant challenge to the anodic stability of electrolytes. At these elevated potentials, the TM oxide positive electrode is subjected to both high chemical and mechanical stress. If positive electrode–electrolyte interfacial side reactions are not effectively suppressed, continuous chemical corrosion can occur. Deep charging the layered oxide positive electrode beyond 4.0 VNa often induces excessive Na extraction from the lattice, triggering a O3 to OP2 phase transformation13 accompanying by the formation of highly oxidative, high-valence Ni species at the positive electrode surface. Under such conditions, intensified parasitic reactions at the CEI promote TM dissolution and oxygen loss, destabilizing the layered framework. The resulting structural degradation, marked by irreversible layer sliding and cation mixing, drives the formation of an electrochemically inactive, cation-disordered rock-salt phase50,51, which not only hampers Na⁺ mobility but also introduces a pronounced stress mismatch with the parent layered framework. Although single-crystalline NFM particles can tolerate higher mechanical stress34,35 than their polycrystalline counterparts due to the absence of fragile grain boundaries, their effective surface area for Na+ transport is inherently limited. In contrast, polycrystalline particles, despite being mechanically weaker, can expose additional intergranular surfaces upon transgranular cracking, thus increasing Na+ transport pathways. From this perspective, the impedance imposed by the resistive rock-salt layer is more detrimental to single-crystalline NFM, where the restricted surface area magnifies the blocking effect of this passivating phase. After long-term deep cycling, the positive electrode in the carbonate-based electrolyte develops a thick rock-salt reconstruction layer that reaches several tens of micrometers beneath the CEI (Fig. 6a). In contrast, the positive electrode cycled in our EMTMSA-based electrolyte exhibits only a few-micrometer-thick rock-salt region that is discontinuously distributed in isolated nanoscale patches, as outlined by the finely dashed white contours (Fig. 6b). Fast Fourier transform (FFT) and inverse FFT patterns taken from the marked regions confirm the coexistence of layered and rock-salt structures, allowing direct visualization of the reconstructed rock-salt phase at the surface. Moreover, the CEI formed in the carbonate-based electrolyte is relatively thick (5–10 nm, Fig. 6a), whereas the interphase generated in the EMTMSA-based electrolyte is much thinner at only 2–3 nm (Fig. 6b). This thinner and more compact CEI formed in our EMTMSA-based electrolyte effectively protects the positive electrode surface from further degradation while facilitating Na+ transport, which is consistent with the improved cycling stability observed over 1000 cycles. TOF-SIMS depth profiling of the NFM particles after cycling in the carbonate-based electrolyte reveals that organic components, represented by the CHO2− fragment, are present throughout the entire sputtering process (Fig. 6c). Moreover, the CHO2− signal intensity is higher than that observed in the CEI formed in the EMTMSA-based electrolyte, as evidenced by both the 2D and 3D reconstructed distribution maps (Fig. 6c and d). In contrast, the decomposition products of the salt NaFSI, such as NaF⁻ and SO⁻ fragments, are primarily confined to the surface region for the carbonate-derived CEI (Fig. 6c). Notably, the intensity of the SO− fragment decreases rapidly during sputtering, suggesting very limited depth penetration. These results indicate that the majority of the interphase formed in the carbonate-based electrolyte originates from the solvent decomposition rather than the salt. However, for the CEI formed in the EMTMSA-based electrolyte, the NaF⁻ and SO⁻ fragments have stronger intensities and a broader spatial distribution (Fig. 6d) compared to those in the carbonate-based electrolyte. This suggests a more extensive incorporation of salt- or EMTMSA- derived species into the CEI, contributing to its inorganic-rich composition. The NaF-rich CEI formed in the EMTMSA-based electrolyte can be further verified by the XPS results (Fig. 6e, f). This reflects the advantage of our EMTMSA molecule with a sulfonimide-salt-like molecular structure that can deduce similar decomposition products as the sulfonimide salt52 to effectively passivate the high-voltage aggressive positive electrode surface (Supplementary Fig. 25 and Supplementary Data 2). To further characterize the parasitic reaction between positive electrode and electrolyte, the dissolution of TMs (Ni, Fe, and Mn) after long-term cycling was quantitatively measured by inductively coupled plasma optical emission spectrometry (ICP–OES). Compared with the carbonate-based electrolyte, the EMTMSA-based electrolyte led to an order-of-magnitude reduction in the dissolution of Ni, Fe, and Mn (Fig. 6g). This further suggests that the CEI formed in the EMTMSA-based electrolyte can effectively mitigate the parasitic reaction and thus suppress the TM dissolution from the NFM positive electrodes.

Methods
Materials and electrolyte preparation
Sodium metal (Na) circular disks (12 mm in diameter, 0.45 mm in thickness, 99.7% purity) were obtained from Guangdong Canrd Technology Co. Ltd. Copper (Cu) foil (9 μm in thickness) and aluminum (Al) foil (20 μm in thickness) were purchased from Guangdong Canrd Technology Co. Ltd. and employed after light wiping with anhydrous ethanol, without further processing. NaFSI (99.90%) salt was purchased from Solvionic Co. Ltd. Organic solvents including PC (battery-grade) and EMC (battery-grade) were provided by Guangdong Canrd Technology Co. Ltd. The detailed synthesis method of DMTMSA53 is as follows: First, 28 mL of 2 M dimethylamine solution in tetrahydrofuran (56.0 mmol), 7.8 mL of triethylamine (56.0 mmol), and 50 mL of dichloromethane (DCM) were added to a 250 mL round-bottomed flask equipped with a magnetic stirring bar. Under a nitrogen atmosphere and at −78 °C, with continuous stirring, trifluoromethanesulfonyl chloride (8.0 g, 47.0 mmol) dissolved in 10 mL DCM was added dropwise to the reaction mixture using a syringe pump. The resulting mixture was stirred at RT overnight. Next, 100 mL of 1 M hydrochloric acid solution was added to separate the organic phase, which was then washed with brine solution (2 × 40 mL). The organic layer was dried over anhydrous sodium sulfate, filtered, and concentrated by rotary evaporation. Finally, the pure colorless liquid product DMTMSA was obtained by distillation twice (115–120 °C, 1 atm). For the synthesis of EMTMSA and DETMSA, dimethylamine was replaced with methylethylamine and diethylamine54, respectively, as the starting amine, and tetrahydrofuran was not required. All solvents were dried over 4 Å molecular-sieves for 3 days prior to use. The carbonate electrolyte 1 M NaFSI in PC/EMC (3/7, volume ratio, v/v) is used as a reference electrolyte. The sulfonamide-based electrolyte is 1 M NaFSI in EMTMSA/PC/EMC (6/1/3, v/v). Other sulfonamide-based electrolytes were prepared by replacing EMTMSA with DMTMSA or DETMSA, while keeping the composition of 1 M NaFSI in sulfonamide/PC/EMC (6/1/3, v/v) unchanged. Electrolytes were precisely prepared in glass sample vials using a pipette fitted with a polypropylene tip and homogenized by shaking. After use, the vials were sealed with parafilm and stored. All preparation and storage procedures were performed in an argon-filled glove box (both H2O and O2 levels < 0.1 ppm, Shanghai Mikrouna) at 25 °C.

Pouch cell preparation
Dry Ah-level HC||NFM pouch cells were purchased from Beijing Bosonlight Technology Co. Ltd. To fabricate the pouch cells, a slurry was first prepared by mixing 95.3 wt% NFM (Jiangsu Xiangying Amperex Technology Co. Ltd.) powder, 2.4 wt% polyvinylidene difluoride (Guangdong Canrd Technology Co. Ltd.), 1.3 wt% Super P (Guangdong Canrd Technology Co. Ltd.) conductive agent, and 1 wt% carbon nanotubes (Guangdong Canrd Technology Co. Ltd.) with N-methylpyrrolidone (battery-grade, Guangdong Canrd Technology Co. Ltd.). For negative electrode, a slurry was prepared by mixing 94.5 wt% HC powder (Kuarary Co. Ltd.), 2.5 wt% carboxymethyl cellulose (Guangdong Canrd Technology Co. Ltd.), 1 wt% styrene-butadiene rubber (Guangdong Canrd Technology Co. Ltd.), 2 wt% Super P conductive agent with deionized water. The slurry was homogenized using a mixer machine (AR100, THINKY) at 25 °C in an argon atmosphere. The slurry for positive electrode and negative electrode were coated by a doctor-blade method on Al foils, which were dried under vacuum at 120 °C and 60 °C, respectively. Subsequently, the prepared NFM positive electrodes and HC negative electrodes were punched into rectangular sheets with dimensions of 60×80 mm2 and 63×84 mm2, respectively, using a semi-automatic electrode punching machine (MSK-180-S, Shenzhen Kejing Technology Co.). The positive electrode and negative electrode were then carefully stacked layer by layer with Celgard 2320 separator (polypropylene/polyethylene/polypropylene trilayer structure, 20 μm thick, 39% porosity) followed by Al tab welding in an argon-filled glove box at 25 °C. Finally, the dry pouch cells were injected with the electrolyte (6 g Ah−1) and vacuum-sealed. Cell parameters are summarized in Supplementary Table 2. Following electrolyte filling and hot sealing in the glove box, the pouch cells were pre-cycled at 40 °C at a low rate of 0.05 C for the formation step. After formation, the cells underwent a vacuum degassing followed by final sealing (Supplementary Fig. 10). In this process, the edge of the gas pocket is gently punctured to release the accumulated gas under vacuum, and the pouch is then hot-sealed under vacuum to ensure complete removal of residual gas. This process effectively eliminates the influence of formation-induced gas generation on the subsequent cycling tests. A small gas pocket remains after final sealing (Supplementary Fig. 10), the pouch cells are placed in a mechanical fixture (5–6 kg cm−2) for cycling tests.

Electrochemical performance
All cycling tests were carried out using a Neware battery testing system (CT-4008Tn-5V6A-S1), and all the pouch cells were tested inside a battery forced convection thermostat chamber (BTC-706, Shanghai Boling instrument) to ensure a constant temperature under different temperatures, with temperature fluctuations kept within ±0.5 °C of the set value. The cells were initially charged and discharged at 0.1 C for two formation cycles, followed by long-term cycling tests at higher rates. The 1 C rate corresponds to 1 A, equivalent to a current density of approximately 2.60 mA cm−2 based on the total geometric area of the positive electrode (8 layers × 60 mm × 80 mm = 384 cm2). Moreover, the capacity of third cycle was used as the initial capacity to calculate the capacity retention. All electrochemical performance tests were conducted without employing constant voltage techniques. Rate capability of the pouch cells with different electrolytes was evaluated at rates of 0.1 C, 0.2 C, 0.5 C, and 1 C for five cycles for each rate between 1.5 V and 4.2 VNa. To evaluate the discharge capacities at different low temperatures, the pouch cells with different electrolytes were first charged to 4.2 VNa at 0.1 C at RT followed by discharging at −20 °C, −40 °C, −60 °C at 0.1 C. At −70 °C, the cell was discharged at 0.01 C to test its intrinsic discharging performance at such a low temperature. GISTT measurements were performed after cycling by repeatedly applying a rate of 0.5 C for 10 minutes followed by a 50-minute rest period. During the constant current pulses, data points were recorded every 10 s, while during the rest periods, data points were recorded every 30 s. EIS during the cell operation, as well as before and after cycling, were performed on a Solartron Energy Lab electrochemical workstation with an amplitude of 5 mV in the frequency range from 10−2 to 105 Hz. Prior to measurement, the cell was allowed to stabilize at open-circuit voltage for 12 h. The EIS data fitting was performed using Solartron Energy Lab electrochemical workstation. In situ EIS measurements were conducted using standard pouch cells. The test program was pre-set, and EIS spectra were automatically acquired every 0.1 V during the charge/discharge process. Electrochemical stability window of electrolyte was determined via linear scan voltammetry (LSV) using Na||Al cells, scanned from open-circuit voltages to 6 V at a scan rate of 10 mV s−1. Ionic conductivity (σ) of electrolyte was measured using steel||steel symmetric cells at various temperatures. Prior to each measurement, the cells were equilibrated at the designated temperature for at least 1 hour to reach temperature balance. Ionic conductivity was then calculated using the equation (2).

σ= L / (S*R) (2)

Here, L is the distance between the two steel electrodes, S is the surface area of the steel, and R is the measured resistance of the cell. For the SEI dissolution test, Na||Cu cells were assembled with 50 µL electrolyte. The cells were cycled within a voltage range of 0.005–1.5 V at a constant current density of 2.5 μA cm−2 (based on the area of Na electrode) to form SEI layer on the Cu surface. After every five cycles, the cells were paused at 1.5 V for a period of time to allow SEI dissolution before resuming cycling. Dissolution tests were repeated three times for each electrolyte using parallel Na||Cu cells to ensure reproducibility. All electrochemical test data were obtained from three parallel cells to ensure reproducibility.

Characterizations
Melting points of the solvents were determined by DSC using a NETZSCH DSC 3500 instrument with a cooling rate of 10 K min−1. The flash point was measured using a closed-cup flash point tester (SYD-3536-1). The dielectric constant was determined using a Wayne Kerr 6530B impedance analyzer. Viscosity was evaluated using an AMETEK Brookfield viscometer at 25 °C. The boiling point of EMSTMSA was obtained experimentally during synthesis. The water content of the electrolyte was measured by Karl Fisher titration (Mettler–Toledo). The HF content of the electrolyte was measured by NMR (400 MRHz, JEOL). Prior to use, NMR tubes were dried under vacuum at 80 °C. To evaluate the wettability of the electrolytes on the Celgard 2320 separator, contact angle measurements were conducted using a contact angle meter (SDC-200S, SINDIN). Raman spectroscopy (InVia Qontor, Renishaw) was employed to investigate the Raman spectra with an excitation wavelength of 785 nm. VT-NMR was employed to investigate the Na+ solvation structure and interactions in the electrolyte. The targeted electrolytes were carefully transferred into NMR tubes in an argon-filled glovebox. Chloroform-d (CDCl3) or D2O was hot-sealed in a capillary tube as internal references, which was then coaxially immersed into the NMR tube for locking field in the NMR tests. A cooling system comprising liquid nitrogen and a temperature controller was used to regulate the sample temperature during VT-NMR experiments. Cycled pouch cells were disassembled in an argon-filled glovebox with O2/H2O contents <0.1 ppm. The extracted positive electrodes and negative electrodes were then cut into small pieces and rinsed using dimethyl carbonate (DMC) and diethylene glycol dimethyl ether (DEGDME) to remove residual salts, respectively. Following the rinsing process, positive electrodes, negative electrodes, and separators were transferred to the glovebox’s antechamber and subjected to vacuum for 12 h to ensure thorough drying. The dried samples were subsequently sealed and stored in an argon-filled glove box for subsequent characterizations. Dissolution of TMs was quantified by ICP-OES (Agilent 5110). Surface chemistry of the cycled electrodes was analyzed by XPS (Thermo Fisher Escalab Xi+) and TOF-SIMS (ION-TOF M6, sputtering beam 250×250 µm2, sputtering rate 0.1 nm s−1). XPS samples were prepared in glove box and transferred using a vacuum transfer chamber. TOF-SIMS samples were sealed with aluminum-plastic film inside the glove box and transferred to a dedicated glove box attached to the TOF–SIMS instrument, without any exposure to air throughout the entire process. The morphology of the electrodes was examined using SEM (SU8230, 10 kV) equipped with EDS. HRTEM (JEM-F200) was employed to observe the morphology of the CEI and SEI. SEM and HRTEM samples were also prepared in an argon-filled glove box, sealed with aluminum-plastic film, and stored/transported under an argon atmosphere. They were exposed to air for only a short time (less than 5 s) immediately before insertion into the instrument and evacuation. Thermal runaway study of HC||NFM pouch cells are conducted using an ARC (BTC130, HEL, England). The pouch cells are charged to 4.2 V at 0.1 C before test. During heat-wait-search mode, the heating step is set as 5 °C per step and the detection limit is 0.03 °C min−1. The thermal runaway point is set as 1 °C min−1.

Computation methods
To explore the annealing and diffusion behaviors of DMTMSA, EMTMSA, and DETMSA molecules, MD simulations were conducted using the GROMACS software suite55. The simulations employed the General AMBER Force Field (GAFF)56 along with AEM1-BCC atomic charges. All topological files for the molecules and ions were automatically generated using the AuToFF web server57. Simulation boxes containing 400 molecules were initially generated using the Packmol program58. After energy minimization, each system underwent a rapid annealing process in which the temperature was decreased from 400 K to 240 K over a period of 2 ns, with a time step of 1 ps to facilitate equilibration. Subsequently, the temperature was further reduced from 240 K to 160 K over a 160 ns simulation using the velocity-rescale thermostat59 with a relaxation time constant of 1 ps. The pressure was maintained at 1.01325×105 Pa using the C-rescale barostat with an isothermal compressibility constant of 4.5×10−5 bar−1. Periodic boundary conditions were enforced in all directions. Electrostatic interactions along with van der Waals forces were calculated using the particle-mesh Ewald (PME) method, with a cutoff distance set to 15 Å.

A linear fit of the mean square displacement (MSD) of the molecular center of mass was used to calculate the diffusion coefficient, following the equations (3) and (4):

MSD(𝜏)=<(𝐫(𝑡+𝜏)−𝐫(𝑡))2> (3)

D = MSD(𝑡) / 6𝜏 (4)

where 𝜏 represents the lag time between two positions. The coefficient before the MSD takes the value of 1/6 for a three-dimensional system. The lowest unoccupied molecular orbital (LUMO) and HOMO energy levels of the FSI−, NaFSI, DMTMSA, EMTMSA, DETMSA, and Na+–EMTMSA were calculated using the DFT in Gaussian 09 W with B3LYP60–62/6-311+g (d, p)63,64 optimization.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In the present work, the authors rationally designed a novel sulfonamide molecule bearing asymmetric alkyl groups, which exhibits an impressively low melting temperature down to −86 °C. For such a polar molecule, it is remarkable that the authors successfully balanced polarity, melting temperature, and anodic stability simultaneously. The discussion on the solvation structure and the formulation of the electrolyte is well organized and convincing. From a practical perspective, this work is also timely, as sodium-ion batteries are approaching the stage of industrialization, which requires to further improve their energy density and performance under extreme conditions. Notably, the authors carried out comprehensive electrochemical evaluations using Ah-scale pouch cells, which convincingly demonstrate the superior overall performance of the designed electrolyte. The results highlight excellent cycling stability at high voltages (4.2 V) and superior discharging performance even under extremely low temperatures. Therefore, I recommend to accept it after addressing the following comments.\n1. In Fig. 1d, the authors measured the freezing point of the DMTMSA molecule by DSC. It is also recommended to further supplement its practical electrochemical performance of the DMTMSA-based electrolyte prepared with this molecule at low temperatures.\n2. The authors demonstrate that the as-prepared EMTMSA-based electrolyte is flame retardant (Fig. 1h). To more comprehensively highlight its flame-retardant characteristics, it is recommended to provide the video of the electrolyte ignition test in as supplementary materials.\n3. Pouch cells using the EMTMSA-based electrolyte still maintain an impressive capacity retention even at extremely low temperatures, and exhibit excellent long-term cycling stability at −20 °C, fully demonstrating the outstanding low-temperature performance of this electrolyte. However, from a practical application perspective, how about its high-temperature electrochemical performance?\n4. To better meet the requirements of practical applications, it is also recommended to provide the rate performance of cells with this electrolyte.\n5. The impedance evolution of pouch cells upon cycling is also important to evaluate electrolyte. Please conduct EIS analysis on the pouch cells before and after cycling from a kinetic perspective.\n6. The authors performed TEM analyses on the formed CEIs after cycling in different electrolytes. It is also required to give a more detailed analysis of surface phase transition of the cathode as it is a main degradation mechanism for layered oxide cathodes.", "reviewer_2": "This work offers some fundamental insights into solvent molecule and electrolyte design for advancing high-energy and low-temperature sodium-ion batteries. It is indicated that the EMTMSA-based electrolyte can enable 1-Ah-level NaNi1/3Fe1/3Mn1/3O2||hard carbon pouch cells to retain 69.8% and 42.3% of room-temperature capacity even at −60 °C and −70 °C, while achieving outstanding capacity retentions of 90.0% and 81.6% after 1500 and 1000 cycles at high upper cut-off voltages of 4.15 V and 4.2 V, respectively. This paper presents some interesting results. Therefore, it is recommended to be accepted. However, before acceptance, a number of questions need to be carefully addressed and relevant revisions are mandatory:\n1.In the experimental section, all solvents were dried over 4 Å molecular sieves for 3 days prior to use. Although it is pointed out that the water could be removed, from a comprehensive perspective, it is suggested that the water and impurity contents in the electrolytes should be measured and provided in order to evaluate the performance of the electrolyte.\n2.The cycled pouch cells were disassembled in the glovebox, and the cathodes, anodes, and separator were extracted for subsequent characterizations, those information on how to prepare test samples and ensure their intrinsic characteristics should be presented.\n3.Both the DMTMSA- and EMTMSA-based electrolytes exhibit similarly high electrochemical stability up to 4.2 VNa. The author explained that the gradual elongation of the alkyl substituents leads to a decrease in the oxidative stability of the electrolyte, likely due to the changed electron-donating effect that compromises anodic resistance. How to verify this conjecture ?\n4.EMTMSA-based electrolyte demonstrates largely improved flame retardancy, effectively suppressing ignition and sustaining non-flammability. Does this electrolyte have a negative impact on battery performance, such as capacity and coulombic efficiency? If there are negative impacts, how to strike a balance.\n5.The safety of batteries is related to the self-generated heat initiation temperature T1, the thermal runaway trigger temperature T2, and the maximum thermal runaway temperature T3. Whether the EMTMSA-based electrolyte can improve battery safety by changing T1, T2, and T3? Please provide relevant evidence.\n6.In the battery cycle test depicted in Figure 3a and 3b, the airbag of the pouch cell is not removed. For the control group cell, gas is produced during the formation stage. If the gas is not completely removed, it will also have a negative impact on the battery cycle test, making it impossible to determine the effect of functional electrolyte components.\n7.The use of EMTMSA-based electrolyte cell demonstrates excellent low-temperature performance, but the high-temperature performance of the cell is not mentioned in the article. How about the cycling performance at 45 ℃ and 60 ℃?\n8.For electrolyte solvents, physicochemical parameters such as dielectric constant, melting and boiling points, flash point, and viscosity are crucial. Please provide the relevant parameters of EMTMSA to demonstrate its potential as a promising electrolyte solvent for sodium-ion batteries.\n9.The author investigated the unique effects of several components, including DMTMSA, DETMSA, and EMTMSA, in different battery systems. Can these components also exhibit similar functional characteristics in the same battery material system?", "reviewer_3": "This manuscript introduces a novel non-flammable, low-melting-point sulfonamide molecule, EMTMSA, as a solvent for sodium-ion battery electrolytes. The system demonstrates fast sodium-ion transport and stable electrode–electrolyte interphase formation. The authors conducted extensive material characterizations and evaluated electrochemical performance in Ah-level pouch cells, which showed promising high-voltage and low-temperature performance with potential for practical applications. However, several conclusions and discussions require clarification or further justification. I therefore recommend a major revision before this manuscript can be considered for publication. Specific comments are as follows:\n1. What is the relative percentage of SSIP, CIP, and AGG in both EMTMSA- and carbonate-based electrolytes? The manuscript states that AGG and CIP dominate in the EMTMSA-based electrolyte, yet the solvation structure in Fig. 2f does not show any FSI− anions.\n2. For low-temperature operation of SIBs, charging is usually much more difficult than discharging. Since Figs. 4a and 4b only display LT discharge capacity, can the cells also be charged under the same conditions? Long-term viability at extreme temperatures should also be demonstrated.\n3. The authors mentioned that the suppressed Na precipitation observed in the EMTMSA-based electrolyte can be attributed to its lower cell impedance at low temperatures. However, the evolution in cell impendence at LT is not provided.\n4. How to interpret the “negative” capacity losses associated with EMTMSA electrolyte in Fig. 5f.\n5. SEI formed on hard carbon and Cu substrates can differ in chemical composition and thickness. It is recommended to include SEI characterization of hard carbon anodes after cycling.\n6. What sputtering rate was used in TOF-SIMS? Some experimental details are missing in the Supporting Information and should be clarified.\n7. Given the structural similarity between NaFSI and EMTMSA, how can one distinguish whether the inorganic SEI/CEI components originate from salt or EMTMSA decomposition? Does this imply that EMTMSA also contributes to interfacial stability via sacrificial decomposition? A DFT analysis of HOMO–LUMO levels may help identify decomposition preference.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Point-to-point response\nReviewer #1 (Remarks to the Author):\nIn the present work, the authors rationally designed a novel sulfonamide molecule bearing\nasymmetric alkyl groups, which exhibits an impressively low melting temperature down to\n−86 °C. For such a polar molecule, it is remarkable that the authors successfully balanced\npolarity, melting temperature, and anodic stability simultaneously. The discussion on the\nsolvation structure and the formulation of the electrolyte is well organized and convincing.\nFrom a practical perspective, this work is also timely, as sodium-ion batteries are approaching\nthe stage of industrialization, which requires to further improve their energy density and\nperformance under extreme conditions. Notably, the authors carried out comprehensive\nelectrochemical evaluations using Ah-scale pouch cells, which convincingly demonstrate the\nsuperior overall performance of the designed electrolyte. The results highlight excellent cycling\nstability at high voltages (4.2 V) and superior discharging performance even under extremely\nlow temperatures. Therefore, I recommend to accept it after addressing the following comments.\nResponse: We appreciate Reviewer #1 for the positive comments on our work.\nComment 1: In Fig. 1d, the authors measured the freezing point of the DMTMSA molecule by\nDSC. It is also recommended to further supplement its practical electrochemical performance\nof the DMTMSA-based electrolyte prepared with this molecule at low temperatures.\nResponse: We sincerely thank the reviewer for this valuable suggestion. To validate the\npractical low-temperature performance of the DMTMSA-based electrolyte, we evaluated the\ndischarge performance of the Ah-level NFM||HC pouch cell with the DMTMSA-based\nelectrolyte at room temperature to −60 °C (Fig. R1). At −20 °C and −40 °C, the discharge\ncapacity of the pouch cell utilizing the DMTMSA-based electrolyte reached 90.1% and 79.8%\nof their room temperature capacity, respectively. This performance is slightly lower compared\nto the EMTMSA-based electrolyte’s pouch cell (which retained 93.8% and 80.6% of capacity\nat the same temperatures). At a lower temperature of −60 °C, the DMTMSA-based pouch cell\ndelivered only 0.2% of its room-temperature discharge capacity, whereas the EMTMSA-based\nelectrolyte retained 69.8%. One possible reason is that the melting point of the DMTMSA\nmolecule is only −60 °C, much higher than that of EMTMSA (−86 °C), limiting its fluidity and\nion transport at ultra-low temperatures. Fig. R1 has been updated in the Supplementary\nInformation, and the related discussion in the main text has been revised accordingly. All\nmodifications are highlighted in yellow.\n1\n4.5\n4.0\n3.5\n3.0\n2.5\n2.0\n1.5\n1.0\n0.0 0.2 0.4 0.6 0.8 1.0\nFig. R1 Voltage profiles of the pouch cell with the DMTMSA-based electrolyte during discharging\nat 25 °C, −20 °C, −40 °C, and −60 °C.\nChanges:\nMain text:\n“Similarly, the capacity of the pouch cell using the DMTMSA-based electrolyte declines to nearly\nzero at −60 °C (Supplementary Fig. 17).”\nSupplementary Information:\nUpdated Supplementary Fig. 17 Voltage profiles of the pouch cell with the DMTMSA-based\nelectrolyte during discharging at 25 °C, −20 °C, −40 °C, and −60 °C.\nComment 2: The authors demonstrate that the as-prepared EMTMSA-based electrolyte is\nflame retardant (Fig. 1h). To more comprehensively highlight its flame-retardant characteristics,\nit is recommended to provide the video of the electrolyte ignition test in as supplementary\nmaterials.\nResponse: We appreciate the reviewer’s comments. To further visualize the flame-retardant\nproperty of the EMTMSA-based electrolyte, we have uploaded the videos of the ignition tests\nfor both electrolytes as supplementary videos. These videos clearly contrast the rapid\n2\n)V(\negatloV\nDMTMSA RT\n−20 °C\n−40 °C\n−60 °C\n0.2% 79.8% 90.1%\nCapacity (Ah)\n4.5\n4.0\n3.5\n3.0\n2.5\n2.0\n1.5\n1.0\n0.0 0.2 0.4 0.6 0.8 1.0\n)V(\negatloV\nDMTMSA RT\n−20 °C\n−40 °C\n−60 °C\n0.2% 79.8% 90.1%\nCapacity (Ah)\ncombustion of the carbonate-based electrolyte with the flame retardancy of our EMTMSA-\nbased electrolyte. These videos have been included in the Supplementary information as\nSupplementary Videos S1 and S2.\nChanges:\nMain text:\n“Furthermore, we performed ignition tests by exposing both electrolytes to an open flame (Fig. 1h,\nSupplementary Video S1 and S2).”\nComment 3: Pouch cells using the EMTMSA-based electrolyte still maintain an impressive\ncapacity retention even at extremely low temperatures, and exhibit excellent long-term cycling\nstability at −20 °C, fully demonstrating the outstanding low-temperature performance of this\nelectrolyte. However, from a practical application perspective, how about its high-temperature\nelectrochemical performance?\nResponse: We appreciate the reviewer’s comments, which highlights the importance of the\nhigh-temperature stability. We evaluated the electrochemical performance of pouch cells using\nboth the carbonate-based and EMTMSA-based electrolytes at high temperatures (45 °C and\n60 °C). Pouch cells with the EMTMSA-based electrolyte exhibit better cycling stability at\nelevated temperatures compared to those with the carbonate-based electrolyte. After 200 cycles\nat 45 °C, the capacity retention improves from 75.7% (carbonate-based electrolyte) to 87.7%\n(EMTMSA-based electrolyte), and at 60 °C, from 71.4% to 82.1% (Fig. R2). This result further\nindicates the advantage of our electrolyte for wide-temperature operation and practical\napplications.\nFig. R2 Cycling performance of pouch cells with the carbonate-based and EMTMSA-based\nelectrolytes at high temperatures with an upper cut-off voltage of 4.2 V at 0.5 C. (a) 45 °C. (b)\nNa\n60 °C.\nChanges:\nMain text:\n3\na\nb\n)hA(\nyticapaC\n)hA(\nyticapaC\n1\n1\n0\n0\n1\n1\n0\n0\n.5\n.0\n.5\n.0\n.5\n.0\n.5\n.0\n0\n0\n42E\n42E\n0 m\n0 m\n/C =\n0 m\n0 m\n/C =\ngg6\ngg6\n−2 cm N\n−2 cm H−1\ng A h\n−2 cm N\n−2 cm H−1\ng A h\nFC\nFC\nC u t-o ff vo lta g e =\nM (d o u b le -sid e )\n(d o u b le -sid e )\n5 0\nC u t-o ff vo lta g e =\nM (d o u b le -sid e )\n(d o u b le -sid e )\n5 0\n4 .2 V\n4 .2 V\n, C h a rg e /d isch a rg e a t 0 .5 C\nNa\n1 0 0\nC y c le\n, C h a rg e /d isch a rg e a t 0 .5 C\nNa\n1 0 0\nC y c le\n, 4 5 °C\n1 5 0\n, 6 0 °C\n1 5 0\nCE\nCE\naM\naM\n8 7 .7 %\n7 5 .7 %\nrb o n a te\nT M S A\n2\n8 2 .1 %\n7 1 .4 %\nrb o n a te\nT M S A\n2\n0\n0\n1\n9\n9\n8\n80\n1\n9\n9\n8\n80\n0\n5\n0\n5\n0\n0\n5\n0\n5\n0\n0\n0\n)%\n( EC\n)%\n( EC\n“Moreover, the high-temperature stability at 45 °C and 60 °C was evaluated in pouch cells at an\nupper charging voltage of 4.2 V . Pouch cells with the EMTMSA-based electrolyte exhibit better\nNa\ncycling stability compared with those employing the carbonate-based electrolyte, achieving 87.7%\ncapacity retention after 200 cycles at 45 °C (vs. 75.7%) and 82.1% retention at 60 °C (vs. 71.4%,\nSupplementary Fig. 20).”\nSupplementary Information:\nUpdated Supplementary Fig. 20 Cycling performance of pouch cells with the carbonate-based and\nEMTMSA-based electrolytes at high temperatures with an upper cut-off voltage of 4.2 V at 0.5 C.\nNa\n(a) 45 °C. (b) 60 °C.\nComment 4: To better meet the requirements of practical applications, it is also recommended\nto provide the rate performance of cells with this electrolyte.\nResponse: We appreciate the reviewer’s comment. Rate performance of the NFM||HC pouch\ncells with the carbonate-based and EMTMSA-based electrolytes was evaluated (Fig. R3, 1 C =\nAh at 0.1 C, 0.2 C, 0.5 C, and 1 C, respectively, whereas the carbonate-based cell delivered\n0.951, 0.828, 0.713, and 0.632 Ah at the corresponding rates. It is noted that the cell with our\nEMTMSA-based electrolyte showed better rate capability than the carbonate-based electrolyte.\n4\na\nb\n)hA(\nyticapaC\n)hA(\nyticapaC\n1\n1\n0\n0\n1\n1\n0\n0\n.5\n.0\n.5\n.0\n.5\n.0\n.5\n.0\n0\n0\n42E\n42E\n0 m\n0 m\n/C =\n0 m\n0 m\n/C =\ngg6\ngg6\n−2 cm N\n−2 cm H−1\ng A h\n−2 cm N\n−2 cm H−1\ng A h\nFC\nFC\nC u t-o ff vo lta g e =\nM (d o u b le -sid e )\n(d o u b le -sid e )\n5 0\nC u t-o ff vo lta g e =\nM (d o u b le -sid e )\n(d o u b le -sid e )\n5 0\n4 .2 V\n4 .2 V\n, C h a rg e /d isch a rg e a t 0 .5 C\nNa\n1 0 0\nC y c le\n, C h a rg e /d isch a rg e a t 0 .5 C\nNa\n1 0 0\nC y c le\n, 4 5 °C\n1 5 0\n, 6 0 °C\n1 5 0\nCE\nCE\naM\naM\n8 7 .7 %\n7 5 .7 %\nrb o n a te\nT M S A\n2\n8 2 .1 %\n7 1 .4 %\nrb o n a te\nT M S A\n2\n0\n0\n1\n9\n9\n8\n80\n1\n9\n9\n8\n80\n0\n5\n0\n5\n0\n0\n5\n0\n5\n0\n0\n0\n)%\n( EC\n)%\n( EC\nFig. R3 Rate performance of the pouch cells using the carbonate-based and EMTMSA-based\nelectrolytes at different rates and upper cut-off voltage of 4.2 V . (a) Comparison of discharge\nNa\ncapacity of the pouch cells at different rates. Corresponding voltage profiles for the (b) carbonate-\nbased and (c) EMTMSA-based electrolytes.\nChanges:\nMain text:\n“Compared to the carbonate-based electrolyte, the pouch cell with the EMTMSA-based electrolyte\nalso delivers improved average Coulombic efficiency (CE) at 4.2 V (~99.94%, Supplementary\nNa\nFig. 13) and better rate performance (Supplementary Fig. 15).”\nSupplementary Information:\n5\nb\n)V(\negatloV\n4\n4\n3\n3\n2\n2\n1\n.5\n.0\n.5\n.0\n.5\n.0\n.50\nC\n.0\na rb o n a\n0\nte\n.2\na\n)hA(\nyticapaC\n0C\n1\n1\n0\n0\n0\n.4a\np\n.2\n.0\n.8\n.6\n.4\na c\n0\n0 .6\nity (A\n0 .1\nh )\nC\n0\n5\n.8\nC u t-o\n0 .2 C\n1 0\n0 .1 C 0 .2 C\n0 .5 C\n1 .0\nff vo lta g e\n0 .5 C\nC y c le\nc\n)V(\negatloV\n= 4 .2 V\nNa\n1\n1 5\n4 .5\nE M\n4 .0\n3 .5\n3 .0\n2 .5\n2 .0\n1 .50\n.0\nC\nT M\n2 0\nS A\n0 .2\nC\nE\n0 .1 C\narbonate\nM T M S A\n2\n0 .4\nC a p a\n5\nc\n0 .6\nity (A h )\n0 .8\n0 .1 0 .2\n0 .5\n1 .0\nC C\nC\nUpdated Supplementary Fig. 15 Rate performance of the pouch cells using the carbonate-based\nand EMTMSA-based electrolytes at different rates and upper cut-off voltage of 4.2 V . (a)\nNa\nComparison of discharge capacity of the pouch cells at different rates. Corresponding voltage\nprofiles for the (b) carbonate-based and (c) EMTMSA-based electrolytes.\nExperimental section:\n“Rate capability of the pouch cells with different electrolytes was evaluated at rates of 0.1 C, 0.2 C,\n0.5 C, and 1 C for five cycles for each rate between 1.5 V and 4.2 V .”\nNa\nComment 5: The impedance evolution of pouch cells upon cycling is also important to evaluate\nelectrolyte. Please conduct EIS analysis on the pouch cells before and after cycling from a\nkinetic perspective.\nResponse: We agree with the reviewer’s comment on the evolution of impedance, which can\ngive us more information on the kinetics. As suggested, we performed EIS analysis on the\npouch cells using both electrolytes before and after 1000 cycles at an upper cut-off voltage of\n4.2 V . The Nyquist plots, along with the equivalent circuit model and fitted parameters, are\nNa\npresented in Fig. R4. The impedance from the electrolyte (R), corresponding to the high-\ne\nfrequency intercept with the x-axis, dramatically increased from 279.9 mΩ to 334.8 mΩ in the\ncarbonate-based electrolyte, but only marginally grew from 183.1 mΩ to 212.3 mΩ in the\nEMTMSA-based electrolyte. After long-term cycling, the interphase impedance (R )\nSEI/CEI\nincreased in both electrolytes. However, the carbonate-based electrolyte showed a pronounced\nrise from 85.4 to 155.6 mΩ (nearly a two-fold increase), whereas the EMTMSA-based\nelectrolyte exhibited only a slight increase from 12.9 to 16.6 mΩ. These results indicate that\nthe EMTMSA-based electrolyte establishes a mechanically and chemically stable interphase\non both electrodes, enabling reliable operation at a high cutoff voltage of 4.2 V .\nNa\n6\nb\n)V(\negatloV\n4\n4\n3\n3\n2\n2\n1\n.5\n.0\n.5\n.0\n.5\n.0\n.50\nC\n.0\na rb o n a\n0\nte\n.2\na\n)hA(\nyticapaC\n0C\n1\n1\n0\n0\n0\n.4a\np\n.2\n.0\n.8\n.6\n.4\na c\n0\n0 .6\nity (A\n0 .1\nh )\nC\n0\n5\n.8\nC u t-o\n0 .2 C\n1 0\n0 .1 C 0 .2 C\n0 .5 C\n1 .0\nff vo lta g e\n0 .5 C\nC y c le\nc\n)V(\negatloV\n= 4 .2 V\nNa\n1\n1 5\n4 .5\nE M\n4 .0\n3 .5\n3 .0\n2 .5\n2 .0\n1 .50\n.0\nC\nT M\n2 0\nS A\n0 .2\nC\nE\n0 .1 C\narbonate\nM T M S A\n2\n0 .4\nC a p a\n5\nc\n0 .6\nity (A h )\n0 .8\n0 .1 0 .2\n0 .5\n1 .0\nC C\nC\nFig. R4 The Nyquist plots, along with the equivalent circuit model and parameters (insets), fitted\nfrom EIS analysis on the NFM||HC pouch cells using the (a) carbonate-based and (b) EMTMSA-\nbased electrolytes before and after long-term cycling at 4.2 V .\nNa\nChanges:\nMain text:\n“These results can also be verified by the impedance analysis on the pouch cells before and after\nlong-term cycling (Supplementary Fig. 16) .”\nSupplementary Information:\nSupplementary Fig. 16 The Nyquist plots, along with the equivalent circuit model and parameters\n(insets), fitted from EIS analysis on the NFM||HC pouch cells using the (a) carbonate-based and (b)\nEMTMSA-based electrolytes before and after long-term cycling at 4.2 V .\nNa\nExperimental section:\n“Electrochemical impedance spectroscopy (EIS) during the cell operation, as well as before and\nafter cycling, were performed on a Solartron Energy Lab electrochemical workstation with an\namplitude of 5 mV in the frequency range from 10−2 to 105 Hz.”\nComment 6: The authors performed TEM analyses on the formed CEIs after cycling in\ndifferent electrolytes. It is also required to give a more detailed analysis of surface phase\ntransition of the cathode as it is a main degradation mechanism for layered oxide cathodes.\nResponse: We agree with the reviewer’s comment and include a more detailed discussion on\nthe surface degradation. Deep charging the layered oxide cathode beyond 4.0 V often induces\nNa\n7\na\na\n)Ω(\n''Z−\n)Ω(\n''Z−\n0\n0\n0\n0\n0\n0\n0\n0\n0\n0\n0\n0\n.5\n.4\n.3\n.2\n.1\n.0\n.5\n.4\n.3\n.2\n.1\n.0\n0\n0\n.0\n.0\nBRAR\nBRAR\ne fo re :\n= 2 7 9 .9 m\nefte\nr:\n= 3 3 4 .8 m\ne\n0 .2\ne fo re :\n= 2 7 9 .9 m\nefte\nr:\n= 3 3 4 .8 m\ne\n0 .2\nΩ\nΩ\nΩ\nΩ\nR ,\nR ,\nR ,\nR ,\nS E I/C E\nS E I/C E\n0 .4\nZ\nS E I/C E\nS E I/C E\n0 .4\nZ\nC a\nC a\n= 8 5\nI\n= 1 5\nI\n' ( Ω\nC a\nC a\n= 8 5\nI\n= 1 5\nI\n' ( Ω\n' ( Ω\nrb o n a te , b e fo re c y c lin\nrb o n a te , a fte r c y c lin g\nR .4 m Ω , = 3 7 7 .9 m Ω\nct\nR 5 .6 m Ω , = 4 5 1 .2 m Ω\nct\n0 .6 0 .8\n)\nrb o n a te , b e fo re c y c lin\nrb o n a te , a fte r c y c lin g\nR .4 m Ω , = 3 7 7 .9 m Ω\nct\nR 5 .6 m Ω , = 4 5 1 .2 m Ω\nct\n0 .6 0 .8\n)\ng\n1\ng\n1\n.0\n.0\nb\nb\n)Ω(\n''Z−\n)Ω(\n''Z−\n0\n0\n0\n0\n0\n0\n0\n0\n0\n0\n0\n0\n.5\n.4\n.3\n.2\n.1\n.0\n.5\n.4\n.3\n.2\n.1\n.0\n0\n0\n.0\n.0\nBRAR\nBRAR\ne fo re :\n= 1 8 3 .1\nefte\nr:\n= 2 1 2 .3\ne\n0 .2\ne fo re :\n= 1 8 3 .1\nefte\nr:\n= 2 1 2 .3\ne\n0 .2\nm\nm\nm\nm\nΩ\nΩ\nΩ\nΩ\nR ,\nR ,\nR ,\nR ,\nS\nS\nS\nE I/C E\n.4\nZ\nE I/C E\nE I/C E\n.4\nZ\nE M\nE M\n= 1 2\nI\n= 1 6\nI\n' ( Ω\nE M\nE M\n= 1 2\nI\n= 1 6\nI\n' ( Ω\n' ( Ω\nT M S\nT M S\n.9 m Ω\n.6 m Ω\n0 .6\n)\nT M S\nT M S\n.9 m Ω\n.6 m Ω\n0 .6\n)\nA , b e fo re c y c lin\nA , a fte r c y c lin g\nR , = 9 9 .3 m Ω\nct\nR , = 1 8 6 .3 m Ω\nct\n0 .8\nA , b e fo re c y c lin\nA , a fte r c y c lin g\nR , = 9 9 .3 m Ω\nct\nR , = 1 8 6 .3 m Ω\nct\n0 .8\ng\n1\ng\n1\n.0\n.0\n“excessive” Na extraction from the lattice, triggering a O3 to OP2 phase transformation (Adv.\nMater. 37, 2415611 (2025)) accompanying by the formation of highly oxidative, high-valence\nNi species at the cathode surface. Under such conditions, intensified parasitic reactions at the\ncathode–electrolyte interface promote transition-metal dissolution and oxygen loss,\ndestabilizing the layered framework. The resulting structural degradation, marked by\nirreversible layer sliding and cation mixing, drives the formation of an electrochemically\ninactive, cation-disordered rock-salt phase (J. Mater. Chem. A 12, 12443–12451 (2024); Mater.\nToday 89, 35–43 (2025)), which not only hampers Na⁺ mobility but also introduces a\npronounced stress mismatch with the parent layered framework. Although single-crystalline\nNFM particles can tolerate higher mechanical stress than their polycrystalline counterparts due\nto the absence of fragile grain boundaries, their effective surface area for Na+ transport is\ninherently limited. In contrast, polycrystalline particles, despite being mechanically weaker, can\nexpose additional intergranular surfaces upon transgranular cracking, thus increasing Na+\ntransport pathways. From this perspective, the impedance imposed by the resistive rock-salt\nlayer is more detrimental to single-crystalline NFM, where the restricted surface area magnifies\nthe blocking effect of this passivating phase. After long-term deep cycling, the cathode in the\ncarbonate-based electrolyte develops a thick rock-salt reconstruction layer that reaches several\ntens of micrometers beneath the CEI (Fig. R5a). In contrast, the cathode cycled in our\nEMTMSA-based electrolyte exhibits only a few-micrometer-thick rock-salt region that is\ndiscontinuously distributed in isolated nanoscale patches, as outlined by the finely dashed white\ncontours (Fig. R5b). Fast Fourier transform (FFT) and inverse FFT patterns taken from the\nmarked regions (Fig. R5) confirm the coexistence of layered and rock-salt structures, allowing\ndirect visualization of the reconstructed rock-salt phase at the surface. Moreover, the CEI\nformed in the carbonate-based electrolyte is relatively thick (5–10 nm, Fig. R5a), whereas the\ninterphase generated in the EMTMSA-based electrolyte is much thinner at only 2–3 nm (Fig.\nR5b). This thinner and more compact CEI formed in our EMTMSA-based electrolyte\neffectively protects the cathode surface from further degradation while facilitating Na+ transport,\nwhich is consistent with the markedly improved cycling stability observed over 1000 cycles.\na\nCarbonate Ⅰ Rock-salt\n5–10 nm\nFFT\nⅠ\nⅠ\nd=0.14 nm\nInverse FFT\nb Fig. 11R00 n5nmm High-resolution TEM images and corresponding FFT/inverse FFT patterns of the selected\nareas of the NFM particles cycled in the carbonate-based (a) and EMTMSA-based (b) electrolytes.\nScale bars, 10 nm. Area I and II indicate rock-salt and layered structures, respectively. The straight\ndashed line marks the CEI layer (5–10 nm in a, and 2–3 nm in b). In (b), the white finely dashed\ncontours highlight the thin nanoscale rock-salt regions (I), whereas in (a) the entire area beneath the\nCEI is composed of rock-salt.\nChanges:\n8\n11 00 n n mm\na\nEMTMSA Ⅱ Layered\nⅠ\nⅡ\nⅠ\nFFT\n2–3 nm Ⅰ Ⅱ\nⅠ\nd(101)=0.25 nm\nInverse FFT\n11 00 n n mm\nCarbonate Ⅰ Rock-salt\n5–10 nm\nFFT\nⅠ\nⅠ\nd=0.14 nm\nInverse FFT\nb\nEMTMSA Ⅱ Layered\nⅠ\nⅡ\nⅠ\nFFT\n2–3 nm Ⅰ Ⅱ\nⅠ\nd(101)=0.25 nm\nInverse FFT\n1100 nnmm\nMain text:\n“Deep charging the layered oxide cathode beyond 4.0 V often induces “excessive” Na extraction\nNa\nfrom the lattice, … This thinner and more compact CEI formed in our EMTMSA-based electrolyte\neffectively protects the cathode surface from further degradation while facilitating Na+ transport,\nwhich is consistent with the markedly improved cycling stability observed over 1000 cycles.”\nCarbonate\nUpdated Fig. 6 Characterizations of the bulk/surface phase transition and CEIs formed on NFM\ncathodes with different electrolytes. High-resolution TEM images and corresponding FFT/inverse\nFFT patterns of the selected areas of the NFM particles cycled in the carbonate-based (a) and\nEMTMSA-based (b) electrolytes. Scale bars, 10 nm. Area I and II indicate rock-salt and layered\nstructures, respectively. The straight dashed line marks the CEI layer (5–10 nm in a, and 2–3 nm in\nb). In (b), the white finely dashed contours highlight the thin nanoscale rock-salt regions (I), whereas\nin (a) the entire area beneath the CEI is composed of rock-salt. TOF-SIMS depth profiles and 2D/3D\nreconstructed images of the interested species for CEIs formed in the carbonate-based (c) and\nEMTMSA-based (d) electrolytes. Scale bars, 25 μm. Blue and red colors correspond to the\nminimum and maximum intensities respectively. F1s XPS spectra of CEIs formed in the carbonate-\nbased (e) and EMTMSA-based (f) electrolytes. (g) TM dissolution measured by ICP-OES after 1000\ncycles in different electrolytes.\nReference:\n“49. Zhao, X. et al. Insights into the capacity fading and failure mechanism of an O3-\nNaNi Fe Mn O layered oxide cathode material for sodium-ion batteries. J. Mater. Chem. A 12,\n1/3 1/3 1/3 2\n12443–12451 (2024).”\n“50. Liu, M. et al. Layered-to-rocksalt atomic reconfiguration on O3-type cathodes surface for high-\nenergy and durable sodium-ion batteries. Mater. Today 89, 35–43 (2025).”\n9\nytisnetni\ndezilamroN\nNaF2−\nC 58N H i O −2− SO−\nNaF2−\n0 100 200 300 400 500\nSputtering time (s)\n80 60\n40\n20 00 25 50 μmNaF-2\nMC: 199; TC: 2.194e+06\n75\n2∙101.8 80 1.51.3 60 1.0\n40 0.80.5\n20 0.30.0 00 25 50 μmCHO-2\nMC: 504; TC: 4.698e+06\n75\n2∙10 4 3\n2\n1 0 CHO2−\n80 60\n40\n20 00 25 50 μmPO-3MC:\n232; TC: 1.648e+06\n75\n2∙102.0 1.6 1.2\n0.8\n0.4 0.0 SO−\nEMTMSA NaF2− C\n58N\nH\ni\nO −2−\nSO− ytisnetni dezilamroN\n0 100 200 300 400 500\nSputtering time (s)\n80 60 40 20 00 25 50 μmNaF-2 MC: 269; TC: 3.223e+06 75\n2∙102.4\n2.0 1.6 1.2 0.8 0.4 0.0 NaF2− 80 60 40 20 00 25 50 μmCHO-2 MC: 361; TC: 3.313e+06 75\n2∙10\n3.02.52.01.51.00.50.0\na c\nb d\n∙102\n80 2 2 . . 5 0 60 1.5 40 1.0 20 0.5 CHO2− μm00SMOC-: 285 2 ; 5 TC S : 3.0 O 3 5 8 0 e+ − 06 75 0.0\nF 1s Carbonate SOxFy\nOne order of\nmagnitude lower\nNaF\n692 690 688 686 684 682\nBinding energy (eV)\n)mpp(\nnoitulossid\nMT\ne f g\nF 1s EMTMSA 1 N Fe i SOxFy Mn\nNaF\n0. 1\n0.01\n692 690 688 686 684 682 Carbonate EMTMSA\nBinding energy (eV)\n10 nm 10 nm\nCarbonate ⅠRock-salt\n5–10 nm\nFFT\nⅠ\nⅠ\nd=0.14 nm\nInverse FFT\n10 nm 10 nm\nEMTMSA Ⅱ Layered\nⅠ Ⅱ Ⅰ FFT Ⅰ\n2–3 nm Ⅱ\nⅠ\nd(101)=0.25 nm\nInverse FFT\nReviewer #2 (Remarks to the Author):\nThis work offers some fundamental insights into solvent molecule and electrolyte design for\nadvancing high-energy and low-temperature sodium-ion batteries. It is indicated that the\nEMTMSA-based electrolyte can enable 1-Ah-level NaNi Fe Mn O ||hard carbon pouch\n1/3 1/3 1/3 2\ncells to retain 69.8% and 42.3% of room-temperature capacity even at −60 °C and −70 °C,\nwhile achieving outstanding capacity retentions of 90.0% and 81.6% after 1500 and 1000 cycles\nat high upper cut-off voltages of 4.15 V and 4.2 V, respectively. This paper presents some\ninteresting results. Therefore, it is recommended to be accepted. However, before acceptance,\na number of questions need to be carefully addressed and relevant revisions are mandatory:\nResponse: We appreciate the reviewer’s recognition and positive comments on our work.\nComment 1: In the experimental section, all solvents were dried over 4 Å molecular sieves for\n3 days prior to use. Although it is pointed out that the water could be removed, from a\ncomprehensive perspective, it is suggested that the water and impurity contents in the\nelectrolytes should be measured and provided in order to evaluate the performance of the\nelectrolyte.\nResponse: We appreciate the reviewer’s comment on the detailed water content, which is\nimportant for electrolyte quality. The most common impurities in electrolyte are water and\nhydrogen fluoride (HF). Accordingly, the water content was assessed by Karl Fischer titration\n(Mettler–Toledo), and the HF content was analyzed by NMR for the EMTMSA-based\nelectrolyte (Fig. R6). The EMTMSA-based electrolyte contained 67 ppm of water prior to\ndrying, which was markedly reduced to 26 ppm after treating the solvent with 4 Å molecular\nsieves for 3 days (Fig. R6a). 19F NMR (Fig. R6b) and 1H NMR (Fig. R6c) data of the\nEMTMSA-based electrolyte after molecular-sieve drying show no detectable HF impurities.\nThe low impurity level in our electrolyte is essential for achieving its high electrochemical\nperformance.\n10\n).u.a(\nytisnetnI\n1 0 0 5 0 0\n19F C h e m\n-5\nic a\n0l\ns h ift (p\n-185\n-1 0 0\np m )\nN o H\n-190\n-1\nF\n5 0\n-195\n-2 0 0\na b\n80\n67 ppm\n)m60 Dried over\np p 4 Å molecular sieves\n(\ntn\nfor 3 days\ne tn 40\no\nc\nre 26 ppm\ta W20\n0\nc\n).u\n.a\n(\ny\ntis\nn\ne\ntn\nI\n8 7 6 5 4 3 2 1 0\n1H Chemical shift (ppm)\nFig. R6 Characterizations of impurity in the EMTMSA-based electrolyte. (a) Water content of the\nEMTMSA-based electrolyte prepared with solvents not subjected to 4 Å molecular-sieve\ndehydration (left bar) and with solvents dehydrated over 4 Å molecular-sieves for 3 days (right bar).\n(b) 19F NMR and (c) 1H NMR of the dehydrated EMTMSA-based electrolyte. The molecular\nstructures in (b) and (c), annotated with circles and arrows, indicate the peak assignments in the 19F\nand 1H NMR spectra. All observed peaks are fully assigned, and no signals appear in the HF\nchemical-shift region, confirming the absence of detectable HF.\nChanges:\nMain text:\n“All solvents were subjected to molecular-sieve drying to eliminate residual water and consequently\nsuppress hydrogen fluoride (HF) generation (Supplementary Fig. 2), which is vital for maintaining\nstable electrochemical performance.”\nSupplementary Information:\nUpdated Supplementary Fig. 2 Characterizations of impurity in the EMTMSA-based electrolyte.\n(a) Water content of the EMTMSA-based electrolyte prepared with solvents not subjected to 4 Å\nmolecular-sieve dehydration (left bar) and with solvents dehydrated over 4 Å molecular-sieves for\n3 days (right bar). (b) 19F NMR and (c) 1H NMR of the dehydrated EMTMSA-based electrolyte.\nThe molecular structures in (b) and (c), annotated with circles and arrows, indicate the peak\nassignments in the 19F and 1H NMR spectra. All observed peaks are fully assigned, and no signals\nappear in the HF chemical-shift region, confirming the absence of detectable HF.\nExperimental section:\n“The water content of the electrolyte was measured by Karl Fisher titration (Mettler–Toledo). The\nhydrogen fluoride (HF) content of the electrolyte was measured by nuclear magnetic resonance\n(NMR, 400 MHz, JEOL). Prior to use, NMR tubes were dried under vacuum at 80 °C.”\nComment 2: The cycled pouch cells were disassembled in the glovebox, and the cathodes,\nanodes, and separator were extracted for subsequent characterizations, those information on\nhow to prepare test samples and ensure their intrinsic characteristics should be presented.\nResponse: We appreciate your comments on the preparation of the sensitive samples in glove\nbox. Cycled pouch cells were disassembled in an argon-filled glovebox with O /H O contents\n2 2\n11\na\n)mpp(\ntnetnoc\nretaW\nc\n8\n6\n4\n2\n).u.a(\nytisnetnI\n0\n0\n0\n0\n0\n8\n6 7 p\n7\np m\nD rie d o ve r\n4 Å m o le cu la\nfo r 3 d a ys\n2 6 p p m\n6\nr sie ve s\n5\nb\n).u.a(\nytisnetnI\n1H\n1 0\nC\n0\nh e m ic\n5 0\n4a\nl s h ift (p p m\n0\n19F\n3\n)\nC h e m\n-5\nic a\n0l\ns h ift (p\n2\n-185\n-1 0 0\np m )\nN o H\n-190\n-1\n5 0\n-195\n-2 0\n0\n0\n<0.1 ppm. The extracted cathodes and anodes were then cut into small pieces and rinsed using\ndimethyl carbonate (DMC) and diethylene glycol dimethyl ether (DEGDME) to remove\nresidual salts, respectively. Following the rinsing process, cathodes, anodes, and separators\nwere transferred to the glovebox’s antechamber and subjected to vacuum for 12 h to ensure\nthorough drying. The dried samples were subsequently sealed and stored in the glovebox for\nsubsequent characterizations. We have supplemented the Experimental section of the\nmanuscript with detailed steps for sample preparation:\nChanges:\nExperimental section:\n“Cycled pouch cells were disassembled in an argon-filled glovebox with O /H O contents <0.1 ppm.\n2 2\nThe extracted cathodes and anodes were then cut into small pieces and rinsed using dimethyl\ncarbonate (DMC) and diethylene glycol dimethyl ether (DEGDME) to remove residual salts,\nrespectively. Following the rinsing process, cathodes, anodes, and separators were transferred to the\nglovebox’s antechamber and subjected to vacuum for 12 h to ensure thorough drying. The dried\nsamples were subsequently sealed and stored in the glovebox for subsequent characterizations.”\nComment 3: Both the DMTMSA- and EMTMSA-based electrolytes exhibit similarly high\nelectrochemical stability up to 4.2 V . The author explained that the gradual elongation of the\nNa\nalkyl substituents leads to a decrease in the oxidative stability of the electrolyte, likely due to\nthe changed electron-donating effect that compromises anodic resistance. How to verify this\nconjecture?\nResponse: We appreciate the reviewer’s comment on the molecular design. As the alkyl chain\nlength increases, its greater electron-donating capacity attenuates the electron-withdrawing\neffect of the FSO – group on the terminal substituent, resulting in a higher local electron density\n2\ncompared with a –CH terminus. This mechanistic trend is supported consistently by both\n3\nelectrochemical measurements and theoretical calculations.\nOn the one hand, linear sweep voltammetry (LSV) was performed using Na||Al cells to evaluate\nthe anodic decomposition onset potentials of the three electrolytes. As shown in Fig. R7a\n(previous Supplementary Fig. 2), the DETMSA-based electrolyte exhibits the lowest onset\npotential (~4.1 V), whereas the DMTMSA- and EMTMSA-based electrolytes show higher\nvalues (~4.8 V). This trend experimentally supports the analysis that elongating the alkyl\nsubstituent diminishes oxidative stability.\nOn the other hand, density functional theory (DFT) calculations were carried out to compare\nthe frontier molecular orbitals of DMTMSA, EMTMSA, and DETMSA. The highest occupied\nmolecular orbital (HOMO) energy levels show a clear monotonic increase with the gradual\nextension of the alkyl group (−8.0319 eV for DMTMSA, −7.8983 eV for EMTMSA, and\n−7.7963 eV for DETMSA; Fig. R7b), directly indicating that the enhanced electron-donating\neffect renders the molecule thermodynamically more prone to oxidation.\nthree sulfonamide derivatives DMTMSA, EMTMSA, and DETMSA. (a) Electrochemical stability\nwindow of different electrolytes via LSV. (b) LUMO and HOMO energy levels of the DMTMSA,\nEMTMSA, and DETMSA calculated by DFT.\nChanges:\nMain text:\n“The DMTMSA- and EMTMSA-based electrolytes maintain high oxidative stability up to ≈4.8 V ,\nNa\nwhereas the DETMSA-based electrolyte exhibits a greatly reduced anodic stability (Supplementary\nFig. 3a). This decrease correlates with the substituent’s electronic structure: longer alkyl chains\ndonate more electron density, weakening the FSO – group’s electron-withdrawing effect and\n2\nelevating the local electron density, consistent with the density functional theory (DFT) results\nshowing higher highest occupied molecular orbital (HOMO) levels for longer chains\n(Supplementary Fig. 3b).”\nSupplementary Information:\nUpdated Supplementary Fig. 3 The relationship between oxidative stability and the length of the\nalkyl substituent of the three sulfonamide derivatives DMTMSA, EMTMSA, and DETMSA. (a)\nElectrochemical stability window of different electrolytes via LSV. (b) LUMO and HOMO energy\nlevels of the DMTMSA, EMTMSA, and DETMSA calculated by DFT.\nExperimental section:\n“The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO)\nenergy levels of the NaFSI, FSI−, DMTMSA, EMTMSA, DETMSA, and Na+–EMTMSA were\ncalculated using the density functional theory (DFT) in Gaussian 09 W with B3LYP8–10/6-311+g (d,\np)11,12 optimization).”\nComment 4: EMTMSA-based electrolyte demonstrates largely improved flame retardancy,\n13\na\na\n)2−mc\nAm(\nytisned\ntnerruC\n)2−mc\nAm(\nytisned\ntnerruC\n0\n0\n0\n-0\n0\n0\n0\n-0\n.1\n.0\n.0\n.0\n.1\n.0\n.0\n.0\n0\n5\n0\n5\n0\n5\n0\nE M\nD E\n2 .5\nD M\nE M\nD E\n2 .5\nT M\nT M\nT M\nT M\nT M\nT M\nS A -b a se d\nS A -b a se d\nS A -b a se d\n3 .0\nP\nS A -b a se d\nS A -b a se d\nS A -b a se d\n3 .0\nP\no\no\nte\nte\nn\nn\n3 .5\ntia l (V\n3 .5\ntia l (V\nv\nv\n~ 4 .1 V\n4 .0\ns . N a /N\n~ 4 .1 V\n4 .0\ns . N a /N\na\na\n+\n+\n)\n)\n4\n4\n.5\n.5\n~\n~\n4\n4\n.8 V\n5 .0\n.8 V\n5 .0\nb\nb\n)Ve(\nlevel\nygrenE\n)Ve(\nlevel\nygrenE\n2\n0\n-2\n-4\n-6\n-8\n-1 0\n2\n0\n-2\n-4\n-6\n-8\n-1 0\nL\nH\nL\nH\nU M O\n− 0 .3 0 1 3\n− 8 .0 3 1 9\nO M O\nD M T M S\nU M O\n− 0 .3 0 1 3\n− 8 .0 3 1 9\nO M O\nD M T M S\nA\nA\n− 0 .3 3 9 1\n− 7 .8 9 8 3\nE M T M S\n− 0 .3 3 9 1\n− 7 .8 9 8 3\nE M T M S\nA\nA\n− 0\n− 7\nD E\n− 0\n− 7\nD E\n.3 3\n.7 9\nT M\n.3 3\n.7 9\nT M\n7 5\n6 3\nS\n7 5\n6 3\nS\nA\nA\neffectively suppressing ignition and sustaining non-flammability. Does this electrolyte have a\nnegative impact on battery performance, such as capacity and coulombic efficiency? If there\nare negative impacts, how to strike a balance.\nResponse: We appreciate the reviewer for raising this important question regarding practical\napplication. Our results clearly indicate that the EMTMSA-based electrolyte, while providing\nexcellent flame retardancy, shows no observable negative impact on battery performance in\nterms of capacity and coulombic efficiency (CE). Specifically, the initial discharge capacity of\nNa||NFM cells (Fig. R8a and b, previous Supplementary Fig. 10c and f) with the EMTMSA-\nbased electrolyte system (163.1 mAh g−1 at 0.05 C) is superior to that of the carbonate-based\nelectrolyte (161.5 mAh g−1 at 0.05 C). This confirms that EMTMSA, as a major solvent, does\nnot compromise Na+ transport kinetics or the initial utilization of active materials. Regarding\nCE (Fig. R8c), the EMTMSA-based electrolyte maintained an outstanding average CE of 99.94%\nafter long-term cycling (1000 cycles), which is comparable to the 99.89% achieved by the\ncarbonate-based electrolyte. Consequently, the EMTMSA-based electrolyte achieves a\nharmonious balance: its superior non-flammability greatly enhances battery safety, while its\npresence has no detrimental effect on electrochemical performance, and concurrently provides\nenhanced long-term operational stability.\nFig. R8 Electrochemical performance of cells with different electrolytes at room temperature.\nVoltage profiles of the Na||NFM cells with the (a) carbonate-based and (b) EMTMSA-based\nelectrolytes at 4.2 V cut-off voltage at 0.05 C. (c) CEs of the NFM||HC pouch cells with different\nelectrolytes at 4.2 V cut-off voltage upon 1000 cycles.\nNa\nChanges:\nMain text:\n“Compared to the carbonate-based electrolyte, the pouch cell with the EMTMSA-based electrolyte\nalso delivers improved average Coulombic efficiency (CE) at 4.2 V (~99.94%, Supplementary\nNa\n14\n)V(\negatloV\n4\n4\n3\n3\n2\n2\n.5\n.0\n.5\n.0\n.5\n.0\n0 5 0C\na p a c ity\n1 0\n(m\n1 5 0 2 0 0\na b\nc\n101.0\n4.2 V\nNa\n100.5\n100.0\n)%\n(\nE\n99.5\nC\n99.0\n~99.94%\n~99.89%\n98.5\nCarbonate\nEMTMSA\n98.0\n0 200 400 600 800 1000\nCycle\n)V(\negatloV\n4\n4\n3\n3\n2\n2\n.5\n.0\n.5\n.0\n.5\n.0\n0 5 0C\na p a c ity\n1 0\n(m\n1 5 0 2 0 0\nCarbonate-4.2 V EMTMSA-4.2 V\nFig. 14) and better rate performance (Supplementary Fig. 15).”\nSupplementary Information:\nUpdated Supplementary Fig. 14 Coulombic efficiencies of the pouch cells with different\nelectrolytes at 4.2 V cut-off voltage.\nNa\nComment 5: The safety of batteries is related to the self-generated heat initiation temperature\nT1, the thermal runaway trigger temperature T2, and the maximum thermal runaway\ntemperature T3. Whether the EMTMSA-based electrolyte can improve battery safety by\nchanging T1, T2, and T3? Please provide relevant evidence.\nResponse: We appreciate the reviewer’s comment on the cell-level safety, which is beyond the\nelectrolyte level. In order to evaluate the cell-level safety, thermal runaway evaluation of\nNFM||HC pouch cells are conducted using an accelerating rate calorimetry (ARC, BTC130,\nHEL, England). The pouch cells are charged to 4.2 V at 0.1 C before test. During “heat-wait-\nsearch” mode, the heating step is set as 5 °C per step and the detection limit is 0.03 °C min−1.\nThe thermal runaway point is set as 1 °C min−1.\nAs shown in Fig. R9, the EMTMSA-based electrolyte significantly elevates both the self-\ngenerated heat initiation temperature (T ) and the thermal runaway trigger temperature (T ),\n1 2\nrelative to the carbonate-based electrolyte, increasing T from 163 °C to 206 °C and T from\n1 2\n233 °C to 278 °C. This indicates that the thermal stability margin of the cell is effectively\nexpanded. Simultaneously, the maximum thermal runaway temperature (T ) of the EMTMSA-\n3\nbased electrolyte also showed a certain degree of reduction (decreasing from 471 °C to 468 °C).\nThe increase in T and T is primarily attributed to the high flame retardancy of EMTMSA and\n1 2\nthe formation of a stable CEI, which inhibits vigorous exothermic side reactions between the\nelectrolyte and the electrodes. The reduction in T indicates that the severity of the thermal\n3\nrunaway is also mitigated. These ARC data verify that the flame-retardant properties of the\nEMTMSA molecule are effectively propagated to the pouch-cell level, highlighting the value\nof molecular-level flame-retardant electrolyte design.\n15\n)%\n( EC\n1\n1\n1\n0\n0\n0\n9\n9\n9\n9\n1\n0\n0\n9\n9\n8\n8\n.0\n.5\n.0\n.5\n.0\n.5\n.0\n0\n4 .2 V\nN a\n2 0 0 4 0 0\nC y c le\n6 0 0\n~ 9 9\n~ 9 9 .9\n.8 9 %\n8 0 0\n4 %\nC\nE\naM rbT oM n a te\nS A\n1 0 0 0\nEMTMSA T=471 °C T=468 °C\n3 3\n)C\n°400\n(\ne\nru\nta\nre\np200\nm\ne\nT T=206 °C\nT=163 °C 1\n2\n=233 °C T\n2\n=278 °C\n0\n0 500 1000 1500 2000 2500\nTime (min)\nFig. R9 Temperature evolution monitored during the thermal runaway of fully charged pouch cells\nunder ARC testing. Where T represents the self-generated heat initiation temperature, T represents\n1 2\nthe thermal runaway trigger temperature, and T represents the maximum thermal runaway\n3\ntemperature. Compared to pouch cells with the carbonate-based electrolyte, these three key\nindicators are all reduced in pouch cells with EMTMSA-based electrolyte.\nChanges:\nMain text:\n“In addition to improved high-temperature cycling stability, the EMTMSA-based electrolyte also\ndelivers clear advantages under thermal-abuse conditions. Pouch cells charged to 100% state of\ncharge (4.2 V) were evaluated by accelerated rate calorimetry (ARC) to track key thermal-runaway\ntemperatures (Fig. 4f). The EMTMSA-based electrolyte markedly increases both the self-generated\nheat initiation temperature (T , from 163 °C to 206 °C) and the thermal-runaway trigger temperature\n1\n(T , from 233 °C to 278 °C), while showing a slight reduction in the maximum thermal-runaway\n2\ntemperature (T , from 471 °C to 468 °C). These ARC data verify that the flame-retardant properties\n3\nof the EMTMSA molecule are effectively propagated to the pouch-cell level, highlighting the value\nof molecular-level flame-retardant electrolyte design.”\na b c\n4.5 4.5\nCarbonate RT EMTMSA RT −60 °C overnight\n4.0 − − 2 4 0 0 ° ° C C 4.0 − − 2 4 0 0 ° ° C C\n)V 3.5 −60 °C )V 3.5 − − 6 7 0 0 ° ° C C\n( e g 3.0 ( e g 3.0 Frozen\na tlo 2.5 a tlo 2.5\nV V Carbonate\n2.0 2.0\n1.5 0.3% 14.8% 78.1% 1.5 69.8%80.6%93.8%\n42.3%\n1.0 1.0\n0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Liquid\nd Capacity (Ah) Capacity (Ah) EMTMSA\n0.8\nCharge/discharge both at −20 °C EMTMSA\n)h\nA\n0.6\n59.2%\ne\nCapacity retention at a specific temperature\n(\ny tic\na p a\n0.4\n37.0%\n4.2\n89.9% N 1 a A N\n6\nh\n.\np M\n8\no\n| u |H c C h NaN 42 F . M 3% ||HC\nC0.2 4.1 LMN C M o ‘T in ||HC 1 Ah-pouch\nCarbonate\n0.0\n0 100 200 300 400\nEMTMSA\n500\n)V\n(\ne g4.0\nf\nCycle atlov\nffo\nNaC\nC\n.\no\n0\ni\nM\nn\n%\n||ACNaN\n8\no\nF\n.\nu\nM\n9\nc\n%\n|\nh\n|HC\n( e ru ta r ) e C °400\nEMTMSA T3=471 °C T3=468 °C\ntu\nc\nre p\n-\np U\n3\n3\n.\n.\n9\n6\no F\n.\nu M\n5\nc\n%\n| h N |H a C N 7 P 9 o F . u M 2 c % | h |HCNaN 5 P 2 o F . u M 2 c % | h |HC NaN 5 P 6 o F . u M 4 c % | h |HC\npm200\n3.7\ne\nT T1=163 °C T1=206 °C\n3.6 NaNC\n0\n|\n.\n|\n%\nC-Coin\n0 500 1000 1500 2000 2500 3000 −10 −20 −30 −40 −50 −60 −70\nTime (min) Temperature (°C)\nelectrolytes at extreme conditions. Voltage profiles of pouch cells with the carbonate-based (a) and\nEMTMSA-based (b) electrolytes at 25 °C, −20 °C, −40 °C, −60 °C, and −70 °C. (c) Optical\nphotographs of different electrolytes after overnight storage at −60 ℃. The carbonate-based\nelectrolyte was frozen while the EMTMSA-based electrolyte remained liquid. (d) Cycling\nperformance of pouch cells with both charging and discharging at −20 ℃. (e) Comparison of\nelectrochemical performance between previous reports and this work, based on upper cut-off\nvoltages and capacity retentions at a specific low temperature (detailed corresponding references\nare provided in Supplementary Table 3). The cell with the EMTMSA-based electrolyte in this study\nenables superior high-voltage operation and excellent capacity retention at low temperature\noutperforming previous arts. (f) Temperature evolution monitored during the thermal runaway of\nfully charged pouch cells under ARC testing. Where T represents the self-generated heat initiation\n1\ntemperature, T represents the thermal runaway trigger temperature, and T represents the maximum\n2 3\nthermal runaway temperature. Compared to pouch cells with the carbonate-based electrolyte, these\nthree key indicators are all reduced in pouch cells with EMTMSA-based electrolyte.\nExperimental section:\n“Thermal runaway study of NFM||HC pouch cells are conducted using an accelerating rate\ncalorimetry (ARC, BTC130, HEL, England). The pouch cells are charged to 4.2 V at 0.1 C before\ntest. During “heat-wait-search” mode, the heating step is set as 5 °C per step and the detection limit\nis 0.03 °C min−1. The thermal runaway point is set as 1 °C min−1.”\nComment 6: In the battery cycle test depicted in Figure 3a and 3b, the airbag of the pouch cell\nis not removed. For the control group cell, gas is produced during the formation stage. If the\ngas is not completely removed, it will also have a negative impact on the battery cycle test,\nmaking it impossible to determine the effect of functional electrolyte components.\nResponse: We sincerely thank the reviewer for pointing out the effect of the gas pocket on the\ncycling performance.\nDuring the formation cycle, gas generation is expected due to the CEI/SEI formation. To\neliminate the influence of the gas on the cycling stability, pouch cells undergo a vacuum\ndegassing followed by final sealing (Fig. R10a and b) after formation. In this process, the edge\nof the gas pocket is gently punctured to release the accumulated gas under vacuum, and the\npouch is then hot-sealed under vacuum to ensure complete removal of residual gas (Fig. R10c,\nd). This process effectively eliminates the influence of formation-induced gas generation\non the subsequent cycling tests.\nA small gas pocket remains after final sealing (Fig. R10e), the pouch cells are placed in a\nmechanical fixture for cycling tests. In this configuration, any gas generated during the early\ncycles can be accommodated by the remaining gas pocket without affecting the stacking, thus\nminimizing its influence on cycling performance and enabling a more accurate evaluation of\nelectrolyte–electrode interfacial stability. At the same time, the gas pocket serves as a visible\nindicator that allows qualitative observation of gas evolution during long-term cycling.\nTherefore, it can be expected that for the reference carbonate-based electrolyte, which generates\nmore gas during cycling, the cycling performance would deteriorate even earlier if the gas\npocket were completely removed.\ncell final sealing and degassing processes before long-term cycling. (b) Photograph of the final\nsealing machine (equipped with a vacuum system and hot sealing function). (c) An enlarged\nphotograph of the pouch cell under the hot blade of the final sealing machine. During sealing, the\nupper platen descends as the chamber is evacuated by a vacuum pump. Under vacuum, the edge of\nthe gas pocket is gently punctured to release the accumulated gas, followed by hot-sealing of the\nresidual pocket. This procedure ensures complete removal of trapped gas generated during the\nformation process. (d) Photograph of the pouch cell after the final sealing, with excess gas pocket\nremoved. (e) Side-view (upper) and front-view (lower) photographs of the pouch cell mounted in\nthe mechanical fixture after cycling. A visibly swollen gas pocket is observed along the cell edge,\nindicating significant gas generation during long-term cycling.\nChanges:\nMain text:\n“The gas generated during the formation cycle was completely removed by a vacuum degassing\nprocess during the final sealing step after formation. A small gas pocket remains after final sealing,\nserving as a visible indicator for qualitative observation of gas evolution during long-term cycling,\nafter which the pouch cells are placed in a mechanical fixture for cycling tests (Supplementary Fig.\n10).”\nSupplementary Information:\n18\na\nE\nb\nle c tro ly te\nF\nin\nin a\nH o t s\nje c tio\nl s e a lin\ne a lin g\nF n\ng m a c h\no rm\nin e\na\ng\ntio\nG a s\ne n e ra tio\nn s ta g\nn\ne\nF in\ns e\na l h\na lin\nc\nd\nog t\ne\nD e g\nF in a l h o t s\nR e m o v e\nx c e s s g a s p o\na s s in g\nH\ne a lin\nc k e t\nR e m o v e\ne x c e s s g a s p o c k\no t s e a lin g\nm g\ne\ne t\ne c h\nFa ixn e d\nic a\nN o g a\nin\nl fix tu\ns\nC\nre\ny c lin g\nUpdated Supplementary Fig. 10 Final sealing process and the fixture for pouch cells. (a)\nSchematic illustration of the pouch cell final sealing and degassing processes before long-term\ncycling. (b) Photograph of the final sealing machine (equipped with a vacuum system and hot\nsealing function). (c) An enlarged photograph of the pouch cell under the hot blade of the final\nsealing machine. During sealing, the upper platen descends as the chamber is evacuated by a vacuum\npump. Under vacuum, the edge of the gas pocket is gently punctured to release the accumulated gas,\nfollowed by hot-sealing of the residual pocket. This procedure ensures complete removal of trapped\ngas generated during the formation process. (d) Photograph of the pouch cell after the final sealing,\nwith excess gas pocket removed. (e) Side-view (upper) and front-view (lower) photographs of the\npouch cell mounted in the mechanical fixture after cycling. A visibly swollen gas pocket is observed\nalong the cell edge, indicating significant gas generation during long-term cycling.\nExperimental section:\n“Following electrolyte filling and hot sealing in the glove box, the pouch cells were pre-cycled at\n40 °C at a low rate of 0.05 C for the formation step. After formation, the cells underwent a vacuum\ndegassing followed by final sealing (Supplementary Fig. 10). In this process, the edge of the gas\npocket is gently punctured to release the accumulated gas under vacuum, and the pouch is then hot-\nsealed under vacuum to ensure complete removal of residual gas. This process effectively eliminates\nthe influence of formation-induced gas generation on the subsequent cycling tests. A small gas\npocket remains after final sealing (Supplementary Fig. 10), the pouch cells are placed in a\nmechanical fixture for cycling tests.”\nComment 7: The use of EMTMSA-based electrolyte cell demonstrates excellent low-\ntemperature performance, but the high-temperature performance of the cell is not mentioned in\nthe article. How about the cycling performance at 45 °C and 60 °C?\nResponse: We appreciate the reviewer’s comments on high-temperature cycling stability of our\nelectrolyte. We evaluated the electrochemical performance of pouch cells using both the\ncarbonate-based and EMTMSA-based electrolytes at high temperatures (45 °C and 60 °C). As\nshown in Fig. R11, pouch cells with the EMTMSA-based electrolyte exhibit better cycling\n19\na\nE\nb\nle c tro ly te\nF\nin\nin a\nH o t s\nje c tio\nl s e a lin\ne a lin g\nF n\ng m a c h\no rm\nin e\na\ng\ntio\nG a s\ne n e ra tio\nn s ta g\nn\ne\nF in\ns e\na l h\na lin\nc\nd\nog t\ne\nD e g\nF in a l h o t s\nR e m o v e\nx c e s s g a s p o\na s s in g\nH\ne a lin\nc k e t\nR e m o v e\ne x c e s s g a s p o c k\no t s e a lin g\nm g\ne\ne t\ne c h\nFa ixn e d\nic a\nN o g a\nin\nl fix tu\ns\nC\nre\ny c lin g\nstability at elevated temperatures compared to those with the carbonate-based electrolyte.\nSpecifically, the capacity retention after 200 cycles at 45 °C is 75.7% for the carbonate-based\nelectrolyte versus 87.7% for the EMTMSA-based electrolyte; at 60 °C, the retention is 71.4%\nand 82.1%, respectively. This result highlights the advantage of our electrolyte for wide-\ntemperature operation.\nFig. R11 Cycling performance of pouch cells with the carbonate-based and EMTMSA-based\nelectrolytes at high temperatures with an upper cut-off voltage of 4.2 V at 0.5 C. (a) 45 °C. (b)\nNa\n60 °C.\nChanges:\nMain text:\n“Moreover, the high-temperature stability at 45 °C and 60 °C was evaluated in pouch cells at an\nupper charging voltage of 4.2 V . Pouch cells with the EMTMSA-based electrolyte exhibit better\nNa\ncycling stability compared with those employing the carbonate-based electrolyte, achieving 87.7%\ncapacity retention after 200 cycles at 45 °C (vs. 75.7%) and 82.1% retention at 60 °C (vs. 71.4%,\nSupplementary Fig. 20).”\nSupplementary Information:\n20\na\nb\n)hA(\nyticapaC\n)hA(\nyticapaC\n1\n1\n0\n0\n1\n1\n0\n0\n.5\n.0\n.5\n.0\n.5\n.0\n.5\n.0\n0\n0\n42E\n42E\n0 m\n0 m\n/C =\n0 m\n0 m\n/C =\ngg6\ngg6\nc\nc\ng\nc\nc\ng\nmm\nA\nmm\nA\n−2 N\n−2 H−1\nh\n−2 N\n−2 H−1\nh\nFC\nFC\nC u t-o ff v\nM (d o u b le -s id e )\n(d o u b le -s id e )\n5 0\nC u t-o ff v\nM (d o u b le -s id e )\n(d o u b le -s id e )\n5 0\no\no\nlta\nlta\ng\ng\ne\ne\n=\n=\n4\n4\n.2\n.2\nV\nV\nN\nN\n, C\na\nC\n, C\na\nC\nh a rg e\n1 0 0\ny c le\nh a rg e\n1 0 0\ny c le\n/d\n/d\nis\nis\nc\nc\nh\nh\na\na\nrg\nrg\ne\ne\na\na\nt 0\nt 0\n.5\n.5\nC\nC\n, 4 5 °C\n1 5 0\n, 6 0 °C\n1 5 0\nCE\nCE\naM\naM\n8 7 .7 %\n7 5 .7 %\nrb o n a te\nT M S A\n2\n8 2 .1 %\n7 1 .4 %\nrb o n a te\nT M S A\n2\n0\n0\n1\n9\n9\n8\n80\n1\n9\n9\n8\n80\n0\n5\n0\n5\n0\n0\n5\n0\n5\n0\n0\n0\n)%\n( EC\n)%\n( EC\nUpdated Supplementary Fig. 20 Cycling performance of pouch cells with the carbonate-based and\nEMTMSA-based electrolytes at high temperatures with an upper cut-off voltage of 4.2 V at 0.5 C.\nNa\n(a) 45 °C. (b) 60 °C.\nComment 8: For electrolyte solvents, physicochemical parameters such as dielectric constant,\nmelting and boiling points, flash point, and viscosity are crucial. Please provide the relevant\nparameters of EMTMSA to demonstrate its potential as a promising electrolyte solvent for\nsodium-ion batteries.\nResponse: We agree with the reviewer that the basic physicochemical parameters are important\nfor electrolyte. The key physicochemical parameters of the EMTMSA solvent were\nsummarized in Table R1. EMTMSA exhibits a low melting point (−86 °C), a high flash point\n(170 °C) and a high boiling point (155 °C). The dielectric constant of EMTMSA is 8.21, which\nfacilitates adequate dissolution of sodium salts, while its moderate viscosity (1.8 mPa s) ensures\ngood fluidity and adequate ionic transport rates. The combination of these key physicochemical\nparameters strongly demonstrates the promising potential of EMTMSA as an electrolyte solvent\nfor SIBs.\nTable R1 Physicochemical parameters of the EMTMSA solvent.\nMelting point Flash point Boiling point Dielectric constant Viscosity\nEMTMSA −86 °C 170 °C 155 °C 8.21 1.8 mPa s\nChanges:\nMain text:\n“This molecular asymmetry disrupts solid-state packing, resulting in extremely poor crystallinity\n21\n)hA(\nyticapaC\n1\n1\n0\n0\n.5\n.0\n.5\n.0\n0\n42E 0 m\n0 m\n/C =\n0 m\n0 m\n/C =\ngg6 c\nc\ng\nmm\nA\n−2 N\n−2 H−1\nh\n−2 N\n−2 H−1\nh\nFC\nC u t-o ff v\nM (d o u b le -s id e )\n(d o u b le -s id e )\n5 0\no lta g e = 4 .2 V\nN\n, C\na\nC\nh a rg e\n1 0 0\ny c le\n/d is c h a rg e a t 0 .5 C , 4 5\n1\n°C\n5 0\nCE aM\n8 7 .7 %\n7 5 .7 %\nrb o n a te\nT M S A\n2 0\n1\n9\n9\n8\n80\n0\n5\n0\n5\n0\n0\n)%\n( EC\na\nb\n1\n1\n0\n0\n.5\n.0\n.5\n.0\n5 0\nt-o ff v o lta g e = 4 .2 V\nN\n, C\nh\n1\na\n0\nrg\n0\ne /d is c h a rg e a t 0 .5 C , 6 0 °C\n1 5 0\nCE aM\n7 1 .4 %\nrb o n a te\nT M S A\n2 0 0\n)h\nA\n(\ny tic )%\na\np\n(\nE a C\nC\n42E 0 m\n0 m\n/C =\n0 m\n0 m\n/C =\ngg6 c\nc\ng\nmm\nA\n−2 N\n−2 H−1\nh\nFC M (d o u b\n(d o u b le\nle -s\n-s id\nid e\ne )\n)\nC y c le\n8 2 .1 %\n1\n9\n9\n8\n8\n0\n5\n0\n5\n0\n0\nand an ultralow melting point of −86 °C (Fig. 1d). In addition, its high flash and boiling points,\nalong with moderate viscosity and dielectric constant, render it a well-balanced and practically\nviable electrolyte solvent (Supplementary Table 1).”\nSupplementary information:\nUpdated Supplementary Table 1. Physicochemical parameters of the EMTMSA solvent.\nMelting point Flash point Boiling point Dielectric constant Viscosity\nEMTMSA −86 °C 170 °C 155 °C 8.21 1.8 mPa s\nExperimental section:\n“The flash point was measured using a closed-cup flash point tester (SYD-3536-1). The dielectric\nconstant was determined using a Wayne Kerr 6530B impedance analyzer. Viscosity was evaluated\nusing an AMETEK Brookfield viscometer at room temperature. The boiling point of EMTMSA was\nobtained experimentally during synthesis.”\nComment 9: The author investigated the unique effects of several components, including\nDMTMSA, DETMSA, and EMTMSA, in different battery systems. Can these components also\nexhibit similar functional characteristics in the same battery material system?\nResponse: To verify whether DMTMSA, DETMSA, and EMTMSA exhibit similar functional\ncharacteristics within the same battery material system (the NFM||HC pouch cell), we have\nsupplemented the cycling stability test results for the DMTMSA-based (Fig. R12a) and\nDETMSA-based (Fig. R12b) electrolytes (sulfonamide-based electrolytes were prepared by\nreplacing EMTMSA with DMTMSA or DETMSA) under identical test conditions. Our test\nresults show that the DMTMSA-based electrolyte retained 81.9% of its capacity after 1000\ncycles (Fig. R12a). This highly similar performance to EMTMSA-based electrolyte and\nsuperior performance significantly outperforms the carbonate-based electrolyte (Fig. R12c).\nHowever, although pouch cell with the DMTMSA-based electrolyte exhibited good cycling\nstability at high voltage, its discharge capacity dropped to zero at −60 °C (Fig. R12d), a\nperformance similar to, but slightly better than, that of the carbonate-based electrolyte at low\ntemperatures (Fig. R12e), which limits its broader application scope. Conversely, although the\nDETMSA has a lower T (−87 °C), indicating some potential for low-temperature applications,\nm\nthe DETMSA-based electrolyte exhibited noticeably poorer cycling stability under the same\n4.2 V high-voltage, retaining only 64.5% of its capacity after 1,000 cycles (Fig. R12b).\nNa\nOverall, only the EMTMSA-based electrolyte combines both high-voltage stability and low-\ntemperature application potential (Fig. R12c, f).\ncells with upper cut-off voltages of 4.2 V . Long-term cycling performance of the pouch cells with\nNa\nthe (a) DMTMSA-based, (b) DETMSA-based, (c) carbonate-based and EMTMSA-based\nelectrolytes. Discharge capacity at low temperatures of the pouch cells with the (d) DMTMSA-\nbased, (e) carbonate-based, (f) EMTMSA-based electrolytes.\nChanges:\nMain text:\n“More importantly, when evaluated at a higher cut-off voltage of 4.2 V (Fig. 3b), the pouch cell\nNa\nwith our EMTMSA-based electrolyte can still retain 81.6% of its initial capacity after 1,000 cycles\nat charging-discharging rate of 0.5 C (comparable with the pouch cell with the DMTMSA-based\nelectrolyte, Supplementary Fig. 12a), outperforming the carbonate-based and DETMSA-based\nelectrolytes (Supplementary Fig. 12b), which retain 65.6% and 64.5% of its original capacity for\n1000 cycles under the same conditions, respectively.”\n“Similarly, the capacity of the pouch cell using the DMTMSA-based electrolyte declines to nearly\nzero at −60 °C (Supplementary Fig. 17).”\nSupplementary Information:\n23\na\n)hA(\nyticapaC\nb\n)hA(\nyticapaC\nc\n)hA(\nyticapaC\n1 .2\n1 .0\n0 .8\n0 .6\n0 .4\n0 .2\n0 .0\n1 .2\n1 .0\n0 .8\n0 .6\n0 .4\n0 .2\n0 .0\n1 .2\n1 .0\n0 .8\n0 .6\n0 .4\n0 .2 0 .0\n0\n0\n−2 4 0 m g cm N F M (d o u b le -sid e)\n−2 2 0 m g cm H C (d o u b le -sid e)\n−1 E /C = 6 g A h\n2 0 0 4 0 0 6 0 0 8 0 0\nC ycle\nC u t-o ff vo lta g e= 4 .2 V C h a rg e /disch arge a t 0 .5 C Na\n−2 4 0 m g cm N F M (d o u b le -sid e)\n−2 2 0 m g cm H C (d o u b le -sid e) −1 E /C = 6 g A h\n2 0 0 4 0 0 6 0 0 8 0 0\nC ycle\nC u t-o ff vo lta g e= 4 .2 V C h a rg e /disch arge a t 0 .5 C Na\n−2 4 0 m g cm N F M (d o u b le -sid e)\n−2 2 0 m g cm H C (d o u b le -sid e) −1 E /C = 6 g A h\n2 0 0 4 0 0 6 0 0 8 0 0\nC ycle\n1 0 0\n9 5 81.9%\n)%\n( 9 0 EC\n8 5\nD M TM S A\n8 0\n1 0 0 0\n1 0 0\n9 5\n)% 64.5% ( 9 0 EC\n8 5\nD E TM S A\n8 0\n1 0 0 0\n1 0 0\n9 5 81.6%\n)%\n( 9 0 E 65.6% C\n8 5\nC arbonate E M TM S A 8 0\n1 0 0 0\nd\ne\nf\n)V(\negatloV\n)V( egatloV\n)V(\negatloV\n4.5\nD M TM SA R T\n−20 °C 4.0 −40 °C\n−60 °C 3.5\n3.0\n2.5\n2.0\n1.5\n79.8% 90.1% 0.2%\n1.00.0\n0.2 0.4 0.6 0.8 1.0\nC a p a city (A h )\n4.5 C arbonate R T\n−20 C° 4.0 −40 C°\n−60 C° 3.5\n3.0 2.5\n2.0\n1.5 0.3% 14.8% 78.1%\n1.0\n0.8 1.0 0.0 0.4 0.6 0.2\nC a p a city (A h )\n4.5 R T E M T M S A\n−20 C° 4.0 −40 C°\n−60 C° 3.5 −70 C°\n3.0\n2.5\n2.0\n1.5 80.6% 93.8% 69.8% 42.3% 1.0\n0.8 1.0 0.0 0.4 0.6 0.2\nC a p a city (A h )\nUpdated Supplementary Fig. 12 Long-term cycling performance of the pouch cells with upper\ncut-off voltages of 4.2 V . (a) DMTMSA-based electrolyte. (b) DETMSA-based electrolyte.\nNa\nSupplementary Fig. 17 Voltage profiles of the pouch cell with the DMTMSA-based electrolyte\nduring discharging at 25 °C, −20 °C, −40 °C, and −60 °C.\n24\n)V(\negatloV\na\nb\n)hA(\nyticapaC\n)hA(\nyticapaC\n4\n4\n3\n3\n2\n2\n1\n1\n1\n1\n0\n0\n0\n0\n0\n1\n1\n0\n0\n0\n0\n0\n.5\n.0\n.5\n.0\n.5\n.0\n.5\n.0\n.2\n.0\n.8\n.6\n.4\n.2\n.0\n.2\n.0\n.8\n.6\n.4\n.2\n.0\n−2 4 0 m g c m N F M (d o u b le -s id e )\n−2 2 0 m g c m H C (d o u b le -s id e )\n−1 E /C = 6 g A h\n0 2 0 0 4 0 0 6 0 0 8 0 0\nC y c le\nC u t-o ff v o lta g e = 4 .2 V C h a rg e /d is c h a rg e a t 0 .5 C\nN a\n−2 4 0 m g c m N F M (d o u b le -s id e )\n−2 2 0 m g c m H C (d o u b le -s id e )\n−1 E /C = 6 g A h\n0 2 0 0 4 0 0 6 0 0 8 0 0\nC y c le\nD M T M S A\n7 9 .8 0 .2 %\n.0 0 .2 0 .4 0 .6 0 .8\nC a p a c ity ( A h )\n%\nD\nD\nM\nE\n1 0 0\n9 5\n81.9%\n)%\n( 9 0 EC\n8 5\nT M S A\n8 0\n1 0 0 0\n1 0 0\n9 5\n)%\n64.5% ( 9 0 EC\n8 5\nT M S A\n8 0\n1 0 0 0\nR T\n− 2 0 ° C\n− 4 0 ° C\n− 6 0 ° C\n9 0 .1 %\n1 .0\nReviewer #3 (Remarks to the Author):\nThis manuscript introduces a novel non-flammable, low-melting-point sulfonamide molecule,\nEMTMSA, as a solvent for sodium-ion battery electrolytes. The system demonstrates fast\nsodium-ion transport and stable electrode–electrolyte interphase formation. The authors\nconducted extensive material characterizations and evaluated electrochemical performance in\nAh-level pouch cells, which showed promising high-voltage and low-temperature performance\nwith potential for practical applications. However, several conclusions and discussions require\nclarification or further justification. I therefore recommend a major revision before this\nmanuscript can be considered for publication. Specific comments are as follows:\nResponse: We appreciate the reviewer’s positive comments on our work.\nComment 1: What is the relative percentage of SSIP, CIP, and AGG in both EMTMSA- and\ncarbonate-based electrolytes? The manuscript states that AGG and CIP dominate in the\nEMTMSA-based electrolyte, yet the solvation structure in Fig. 2f does not show any FSI−\nanions.\nResponse: We thank the reviewer for of the comments on electrolyte solvation structure. The\npercentages of SSIP, CIP, and AGG were quantified by fitting the peak areas of the Raman\nspectra (Fig. R13a and b). While SSIP (56.4%) and CIP (43.6%) dominate the solvation\nenvironment in the carbonate-based electrolyte, the EMTMSA-based electrolyte is instead\ncharacterized by a much higher proportion of CIP (84.2%) and AGG (10.2%). The moderate\npolarity of the EMTMSA solvent facilitates partial dissociation of Na+ and FSI−, allowing a\nportion of FSI− to enter the first solvation sheath. We also adjust the schematic illustration of\nthe solvation structure of the EMTMSA-based electrolyte by including FSI− anion (Fig. R13c,\nd). Fig. R13 has been updated in the revised manuscript as updated Fig. 2, which has been\nhighlighted in yellow.\nFig. R13 Deconvolution of Na+–FSI− Raman peaks for the (a) carbonate-based and (b) EMTMSA-\nbased electrolytes with peak fits for different cluster types including SSIP, CIP, and AGG. Schematic\nfigures of the representative Na+ solvation structures in the (c) carbonate-based and (d) EMTMSA-\n25\nc\na\n6 0\nC\nC IP , 4\nS S IP , 5\n3 .6 %\n7 4 0\nP C\n.4\na\n%\nm a\n7 2 0\nn sh\nN a\nP C\nift (c m\n7 0\n−1)\nE\na\nM C\nP\n6 8\nC\nC\nd\nS\nb\nA G\n7 6 0\nN\nG\nC IP , 8 4 .2 %\nS S\n, 1 0 .2 %\n7 4 0\nR\nP C\n− F S I\nH\nIP , 5 .6 %\n7 2 0\na m a n sh ift (c\nN a\nE\nm\nM T M S A\n7 0 0\n−1)\nE M C\nE M\n6 8 0\nT M S A\nbased electrolytes.\nChanges:\nMain text:\n“While SSIP (56.4%) is the dominant cluster type in the carbonate-based electrolyte (Fig. 2a), AGG\n(10.2%) and CIP (84.2%) are the main species in the EMTMSA-based electrolyte (Fig. 2b),\nindicating that more FSI− anions participate in the first solvation sheath.”\na c g\nVT-NMR PC/EMC\nPC/EMC (EMC) (PC) Δδ=9.22 ppm\n−50 °C\nSSIP, 56.4%\nPC\nCIP, 43.6% 0.1 P C M / E N M a C FSI Δδ=2.28 ppm −20 °C\nEMC\nPC/EMC RT PC PC\n760 740 720 700 680 280 260 240 220 200 180 5.5 5.0 4.5 4.0\nRaman shift (cm−1) 17O chemical shift (ppm) 1H chemical shift (ppm)\nb d h VT-NMR EMTMSA/PC/EMC\nEMTMSA S=O\n(EMTMSA)\nEMT 1 M M S A N / a P F C S /E I MC Δδ=1.01 ppm −50 °C\n0.1 M NaFSI\nEMTMSA/PC/EMC Δδ=0.07 ppm −20 °C\nCIP, 84.2%\nAGG, 10.2% SSIP, 5.6% PC EMC\nEMTMSA/PC/EMC RT PC\n760 740 720 700 680 160 150 140 130 120 5.5 5.0 4.5 4.0\nRaman shift (cm−1) 17O chemical shift (ppm) 1H chemical shift (ppm)\ne f\nEMTMSA\nEMC\nPC Na PC Na\nFSI− EMC\nPC PC\nNa C O F S N H\nUpdated Fig. 2 The coordination interactions between Na+ and different solvent molecules in the\nEMTMSA-based and carbonate-based electrolytes by Raman spectroscopy and NMR techniques.\nDeconvolution of Na+–FSI− Raman peaks for the carbonate-based (a) and EMTMSA-based (b)\nelectrolytes with peak fits for different cluster types including SSIP, CIP, and AGG. 17O NMR\nspectra of the carbonate-based (c) and EMTMSA-based (d) electrolytes before and after\nincorporating 0.1 M and 1 M NaFSI into the solvent mixtures. The corresponding functional groups\nin different molecules and chemical shifts are denoted. Schematic figures of the representative Na+\nsolvation structures in the carbonate-based (e) and EMTMSA-based (f) electrolytes. 1H VT-NMR\nspectra of the PC/EMC (g) and EMTMSA/PC/EMC (h) solvent mixtures at 25 °C, −20 °C, and\n−50 °C. Peak broadening is observed for the carbonate mixtures upon cooling, corresponding to\nsignificantly decreased molecular rotation. In contrast, the peaks remain almost unchanged for the\nEMTMSA-carbonate mixture.\nComment 2: For low-temperature operation of SIBs, charging is usually much more difficult\nthan discharging. Since Figs. 4a and 4b only display LT discharge capacity, can the cells also\nbe charged under the same conditions? Long-term viability at extreme temperatures should also\nbe demonstrated.\nResponse: We agree with the reviewer that it is challenging for the cell to charge at the low\ntemperatures. To address this comment, we performed more tests at low temperatures as below.\n1. Fig. R14a and b show the voltage profiles for pouch cells using the carbonate-based and\n−40 °C, and −60 °C) charge–discharge conditions. The EMTMSA-based electrolyte\nlargely outperforms the carbonate-based one at low temperatures. Whereas the carbonate\nelectrolyte retains 60.3% of its room-temperature capacity at −20 °C, drops sharply to 8.1%\nat −40 °C, and becomes inactive at −60 °C, the EMTMSA-based electrolyte maintains 81.1%\nat −20 °C, 59.7% at −40 °C, and still delivers an impressive 41.9% retention even at −60 °C.\n2. The EMTMSA-based electrolyte also outperforms the carbonate-based system in cycling\nstability when both charging and discharging are carried out at −20 °C, as demonstrated by\nthe long-term cycling performance (Fig. R15). Under these conditions, the EMTMSA-\nbased cell retains 59.2% of its initial capacity after 500 cycles, whereas the carbonate-based\ncell maintains only 37.0%.\nFig. R14 Voltage profiles of pouch cells with the (a) carbonate-based and (b) EMTMSA-based\nelectrolytes during charge/discharge at a rate of 0.01 C, measured at 25 °C, −20 °C, −40 °C, and\n−60 °C.\nFig. R15 Long-term electrochemical performance of 1 Ah NFM||HC pouch cells with different\nelectrolytes at −20 °C, with both charging and discharging at 0.1 C.\n27\na\n)V(\negatloV\n4\n4\n3\n3\n2\n2\n1\n1\n1\n1\n0\n0\n0\n0\n0\n1\n1\n0\n0\n0\n0\n0\n.5\n.0\n.5\n.0\n.5\n.0\n.5\n.0\n.2\n.0\n.8\n.6\n.4\n.2\n.0\n.2\n.0\n.8\n.6\n.4\n.2\n.0\n−2 4 0 m g c m N F M (d o u b le -s id e )\n−2 2 0 m g c m H C (d o u b le -s id e )\n−1 E /C = 6 g A h\n0 2 0 0 4 0 0 6 0 0 8 0 0\nC y c le\nC u t-o ff v o lta g e = 4 .2 V C h a rg e /d is c h a rg e a t 0 .5 C\nN a\n−2 4 0 m g c m N F M (d o u b le -s id e )\n−2 2 0 m g c m H C (d o u b le -s id e )\n−1 E /C = 6 g A h\n0 2 0 0 4 0 0 6 0 0 8 0 0\nC y c le\nD M T M S A\n7 9 .8 0 .2 %\n.0 0 .2 0 .4 0 .6 0 .8\nC a p a c ity ( A h )\n%\nD\nD\nM\nE\n1 0 0\n9 5\n81.9%\n)%\n( 9 0 EC\n8 5\nT M S A\n8 0\n1 0 0 0\n1 0 0\n9 5\n)%\n64.5% ( 9 0 EC\n8 5\nT M S A\n8 0\n1 0 0 0\nR T\n− 2 0 ° C\n− 4 0 ° C\n− 6 0 ° C\n9 0 .1 %\n1 .0\nChanges:\nMain text:\n“The pouch cell with the EMTMSA-based electrolyte maintained excellent capacity retention, even\nunder simultaneous low-temperature charging and discharging, retaining 81.1%, 59.7%, and 41.9%\nof its room temperature capacity at −20 °C, −40 °C, and −60 °C, respectively (Supplementary Fig.\n18).”\n“Compared to the carbonate-based electrolyte, the pouch cell with the EMTMSA-based electrolyte\ndelivers improved cycling stability and CE over 500 cycles with both charging and discharging at\nat −20 °C (Fig. 4d; Supplementary Fig. 19).”\nUpdated Fig. 4 Electrochemical performance of 1 Ah NFM||HC pouch cells with different\nelectrolytes at extreme conditions. Voltage profiles of pouch cells with the carbonate-based (a) and\nEMTMSA-based (b) electrolytes at 25 °C, −20 °C, −40 °C, −60 °C, and −70 °C. (c) Optical\nphotographs of different electrolytes after overnight storage at −60 ℃. The carbonate-based\nelectrolyte was frozen while the EMTMSA-based electrolyte remained liquid. (d) Cycling\nperformance of pouch cells with both charging and discharging at −20 ℃. (e) Comparison of\nelectrochemical performance between previous reports and this work, based on upper cut-off\nvoltages and capacity retentions at a specific low temperature (detailed corresponding references\nare provided in Supplementary Table 3). The cell with the EMTMSA-based electrolyte in this study\nenables superior high-voltage operation and excellent capacity retention at low temperature\noutperforming previous arts. (f) Temperature evolution monitored during the thermal runaway of\nfully charged pouch cells under ARC testing. Where T represents the self-generated heat initiation\n1\ntemperature, T represents the thermal runaway trigger temperature, and T represents the maximum\n2 3\nthermal runaway temperature. Compared to pouch cells with the carbonate-based electrolyte, these\nthree key indicators are all reduced in pouch cells with EMTMSA-based electrolyte.\nSupplementary Information:\n28\na\n)V(\negatloV\nd\n)hA(\nyticapaC\nf\n)C°(\nerutarepmeT\n4.5\nC arbonate\n4.0\n3.5\n3.0\n2.5\n2.0\n1.5 0.3% 14.8%\n1.00.0\n0.4 0.2\nC a p a\n0.8\n0.6\n0.4\n0.2\n0.0\n0 100\nE M T M S A\n400\n200\n0\n0 500\n0.6\ncity (A\nb\n4.5\nR T\n−20 C° 4.0 −40 C°\n−60 C° 3.5 )V(\n3.0 ega\n2.5 tloV\n2.0\n1.5 78.1%\n1.00.0\n0.8 1.0\nh )\nC h a rg e /d isch a rg e b\nT =163 C° 1\n1500 1000\nT im e (m in )\nE\no\nc\nR T M T M S A\n−20 C° −40 C°\n−60 C° −70 C°\n80.6% 93.8% 69.8%\n42.3%\n0.8 1.0 0.4 0.6 0.2\nC a p a city (A h )\nth a t − 2 0 C°\ne 59.2%\n37.0%\nC arbonate\n) E M T M S A V(\ne 300 500 400 gatlov\nffo-tu\nT =471 C T =468 C ° ° c 3 3 reppU\nT =206 C° 1\nT =278 C° T =233 C° 2 2\n3000 2500 2000\n4.2\n4.1\n4.0\n3.9\n3.8\n3.7\n3.6\nE\n−10\n− 6 0 C o ve rn ig h t °\nF ro ze n\nC a rb o n a te\nL iq u id\nM T M S A\nE M T M S A\nCapacity retention at a specific temperature\n69.8%\nNaNFM ||HC\n42.3% 1 Ah-pouchNaNFM ||HC 89.9%\nLMNM‘T||HC 1 Ah-pouch Coin\n84.0% 58.9%\nNaCNFM||ACNaNFM||HC\nCoin Pouch\n76.5% NaNFM||HC Pouch\n56.4%\nNaNFM||HC Pouch\n79.2% 52.2% NaNFM||HCNaNFM||HC\nPouch Pouch\n70.4% NaNCM||HC-Coin\n−70 −60 −20 −30 −50 −40\nT em perature ( C ) °\na b\n4.5 4.5\nCarbonate EMTMSA\n4.0 4.0\n3.5 RT 3.5 RT\n)V −20 °C )V −20 °C\n(\ne g\n3.0 −\n−\n4\n6\n0\n0\n°\n°\nC\nC\n(\ne g\n3.0 −\n−\n4\n6\n0\n0\n°\n°\nC\nC\na tlo 2.5 a tlo 2.5\nV V\n2.0 2.0\n1.5 1.5\n0.2%8.1% 60.3% 41.9% 59.7% 81.1%\n1.0 1.0\n0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0\nCapacity (Ah) Capacity (Ah)\nUpdated Supplementary Fig. 18 Voltage profiles of pouch cells with the (a) carbonate-based and\n(b) EMTMSA-based electrolytes during charge/discharge at a rate of 0.01 C, measured at 25 °C,\n−20 °C, −40 °C, and −60 °C.\nComment 3: The authors mentioned that the suppressed Na precipitation observed in the\nEMTMSA-based electrolyte can be attributed to its lower cell impedance at low temperatures.\nHowever, the evolution in cell impendence at LT is not provided.\nResponse: We agree with the reviewer that the impedance evolution of the cells at low\ntemperature is important to support our argument. In situ EIS-DRT analysis for the pouch cells\nusing both the EMTMSA-based and carbonate-based electrolytes at −20 °C was performed (Fig.\nR16). At −20 °C, the EMTMSA-based cell exhibits much lower R (electrolyte resistance) and\ne\nR (SEI and CEI resistance) compared to the carbonate electrolyte during the entire charge\nSEI/CEI\nand discharge process. This indicates that the EMTMSA-based electrolyte enables the\nformation of interphases with faster Na+ transport and facilitates more efficient Na+ desolvation\nat low temperatures.\nγ(ohm)\n1.5 V 3.0\nCarbonate, −20 °C EMTMSA, −20 °C\ne\ng\nra\nh\nc s 2.0\niD\n4.2 V\nR R R W R R R W\ne SEI/CEI ct diff e SEI/CEI ct diff 1.0\ne\ng\nra\nh\nC\n0.0\n1.5 V\n−5 −4 −3 −2 −1 0 1 2 −5 −4 −3 −2 −1 0 1 2\nlogτ(s) logτ(s)\nFig. R16 In situ distribution of relaxation times data of the pouch cells with carbonate-based (left)\nand EMTMSA-based (right) electrolytes at −20 °C upon charging and discharging between 1.5 V\nto 4.2 V.\nChanges:\nMain text:\n“The suppressed Na0 precipitation observed in the EMTMSA-based electrolyte can be attributed to\nits lower cell impedance at low temperatures (Supplementary Fig. 22), as well as the formation of a\nmore ionic conductive and stable solid–electrolyte interphase (SEI) on the HC anode.”\nUpdated Supplementary Fig. 22 In situ distribution of relaxation times data of the pouch cells\nwith carbonate-based (left) and EMTMSA-based (right) electrolytes at −20 °C upon charging and\ndischarging between 1.5 V to 4.2 V. At −20 °C, the EMTMSA-based cell exhibits much lower Re\n(electrolyte resistance) and RSEI/CEI (SEI and CEI resistance) compared to the carbonate\nelectrolyte during the entire charge and discharge process.\nComment 4: How to interpret the “negative” capacity losses associated with EMTMSA\nelectrolyte in Fig. 5f.\nResponse: We thank the reviewer for their attention to the results of the capacity loss test\nfollowing various pause times in Fig. 5f. For the carbonate-based electrolyte, a partial positive\ncapacity loss was observed after experiencing different pause times. This clearly indicates that\nthe corresponding SEI underwent partial dissolution and reorganization during the rest period,\nsuggesting its poor ability to suppress side reactions. In contrast, in the EMTMSA-based\nelectrolyte, we observed “negative capacity losses”. This confirms that no SEI dissolution\noccurred; rather, the capacity change reflects a natural capacity variation following the rest\nperiod. This clearly reflects that the lower polarity of the EMTMSA-based electrolyte helps\npreserve SEI integrity and suppress excessive thickening. Notably, this “negative capacity\nlosses” phenomenon has also been reported in Adv. Mater. e15562 (2025).\nChanges:\nMain text:\n“In contrast, the lower polarity of the EMTMSA-based electrolyte helps preserve SEI integrity and\nsuppress excessive thickening48.”\nReference:\n“48. Xu, L. et al. 4.5‐V‐class safe lithium‐ion batteries with silicon‐majority‐graphite anodes\nenabled by self‐limiting interphase. Adv. Mater. e15562 (2025).”\nComment 5: SEI formed on hard carbon and Cu substrates can differ in chemical composition\nand thickness. It is recommended to include SEI characterization of hard carbon anodes after\ncycling.\nResponse: We thank the reviewer for this suggestion. We have conducted surface chemistry\nanalysis on the cycled HC anodes by XPS (Fig. R17). The XPS C1s and O1s data reveal that\nthe SEI formed in the carbonate-based electrolyte contains more organic species (C ratio: 37.4%,\n30\n1 .5 V\negrahcsiD\n4 .2 V\negrahC\n1 .5 V\n− 5\nC a r b\nR\n− 4\no\ne\nn\n−\na\n3\nte , − 2 0\nR\nS E I/C\n− 2lo\ng\nC °\nE I\n− 1(\ns τ )\nR\n0\nc t\nW\n1\nd iff\n2 −\nE\n−\nT\nR\ne\nS\n−\nT\nR\n−\nS\n−\n2\nI/C E I\n− 1\nlo g τ ( s )\nR\n0\nc t\nW\n1\nd iff\n2\nγ (o h m )\n3 .0\n2 .0\n1 .0\n0 .0\nsuch as C–O and C=O), while it is significantly reduced in the EMTMSA-based electrolyte,\nlowering to 25.2% (Fig. R17a and b, calculated based on the ratio of C element associated\nsignals to all detected elements). The C1s data of the SEI formed in the carbonate-based\nelectrolyte shows a Na C peak (Fig. R17a), corresponding to the formation of Na precipitation\nx\nobserved in the SEM images of the HC anode extracted from the cycled pouch cell. Furthermore,\nthe XPS F1s data shows more NaF species are present in the EMTMSA-derived SEI (Fig. R17c).\nThese results demonstrate that the SEI formed by the EMTMSA-based electrolyte is more\ninorganic with NaF-rich components, consistent with our TOF-SIMS data obtained from the\nCu substrate.\nFig. R17 has been updated as Fig. 5g and Supplementary Fig. 23, and related discussion\nhas been incorporated, which are both highlighted in yellow in the revision.\nFig. R17 XPS analysis for the SEIs formed on HC with different electrolytes. (a) C1s spectra of the\nSEIs formed on HC. (b) O1s spectra of the SEIs formed on HC. (c) F1s spectra of the SEIs formed\non HC.\nChanges:\nMain text:\n“X-ray photoelectron spectroscopy (XPS) results show that the SEI formed on HC in the\nETMTMSA-based electrolyte contains a higher proportion of NaF species and less organic species\nwithout detectable Na C (corresponding to the detrimental sodium precipitation), compared to that\nx\nformed in the carbonate-based electrolyte (Fig. 5g, Supplementary Fig. 23).”\n31\na\n2 9\nC\n2\n1s\n2− C O 3\n2− C O 3\n2 9 0\nB in\nC =O\nC =O\n2 8 8\nd in g e\nC –O\nC –O\n2 8 6\nn e rg y (e V\nC a rb o n a te\nC –C\nN a Cx\nE M T M S A\nC –C\n2 8 4 2\n)\n8 2\nb\n5 4\nO\n0\n1s\nN a A uger\nN a A uger\n5 3 8 5 3 6\nB in\nC –O\nC –O\n5 3 4\nd in g e n\n5 3 2\ne rg y\nC\nC =O\nC =O\n5 3 0\n(e V )\na rb o n a te\nE M T M S A\n5 2 8 5 2 6\nc\n6 9\nF\n2\n1s\n9 0\nS O Fx y\nFx y\n6 8 8\nB in d in g e n\n6 8 6\ne rg y\nN\n(e\nC a rb o n a te\naF\nE M T M S A\nN aF\n6 8 4 6 8 2\nV )\nUpdated Fig. 5 Characterizations of SEIs formed on HC anodes with different electrolytes. Optical\n(a) and SEM and EDS elemental mapping (b) images of the HC anodes extracted from the pouch\ncells in different electrolytes at −20 ℃ for 200 cycles. Obvious Na0 precipitation was observed from\nthe one cycled in the carbonate-based electrolyte (b). Scale bars 10 mm (a) and 5 μm (b). High-\nresolution TEM images of the HC particles cycled in the carbonate-based (c) and ETMTMSA-based (d)\nelectrolytes. Scale bars, 10 nm. (e) Evolution of the capacity contributed by electrolyte reduction\nbefore and after each pause as a function of cycle number by Na||Cu cells with different electrolytes.\nThe grey-shaded regions with arbitrary width indicate the periods during which the cells were\npaused. (f) Capacity loss associated with different pause durations for cells using carbonate-based\nand EMTMSA-based electrolytes. Each shaded curve represents an individual cell, while the solid\nlines track the average values from three parallel cells. (g) F1s XPS spectra of the SEIs formed in\nboth electrolytes on HC. TOF-SIMS depth profiles and 3D reconstructed images of the interested\nspecies NaF −, CHO −, and Cu− signals for SEIs formed in the carbonate-based (h) and EMTMSA-\n2 2\nbased (i) electrolytes. Blue and red colors correspond to the minimum and maximum intensities\nrespectively. F1s XPS spectra of SEIs formed in both electrolytes on HC.\nSupplementary Information:\n32\nb\ng\n6\na\nF\n9 2\nN a precipitation\nC a rb o n a te\nN a p re c ip ita tio\nC a rb o n a te 1 s\nS O Fx y\nN aF\nE M T M S A\nN aF\nS O Fx y\n6 9 0 6 8 8 6 8 6 6 8 4 6 8 2\nB in d in g e n e rg y (e V )\nE M\nn\nh\ni\nT\nytisnetni\ndezilamroN\nytisnetni\ndezilamroN\nM S A\nC a\n0\nrb o\nT M\nN i\nN a\nn a te\n2 0\nS A\n2 0\n0\n0\nc\nd\nC a rb o n a te\n8 – 1 0 n m\n10 nm 10 nm E M T M S A\n2 – 4 n m\n10 nm 10 nm\n4 0 0\nS p u tte rin g\n4 0 0\nS p u tte rin g\n6\ntim\n6\ntim\n0 0e\n(s\n0 0e\n(s\n)\n)\n8\n8\n0\n0\n0\n0\ne\nPause tim e: h 50 3 0 )hAμ(\n2 0 ytica\n1 0 paC\n0\n0 5\nf )h\n0 .6 Aμ(\n0 .3 sso\n0 .0 l yt\n− 0 .3 icap\n− 0 .6 aC\nC a rb o n a\n− N aF 2\n− C H O 2 − C u\n− N aF 2\n1 0 0 0\n− N aF 2\n− C H O 2\n− C u\n− N aF 2\n1 0 0 0\nte\n1\n30\n0C\n15\n1 5\ny c le\nE M\n− C H O 2\n− C H O 2\nEM TM SA\n2 0\nPause tim e:\n50 h\n30 h\n15 h 5 h\nT M S A\n− C u\n− C u\nb\nUpdated Supplementary Fig. 23 XPS analysis for the SEIs formed on HC with different\nelectrolytes. (a) C1s spectra of the SEIs formed on HC. (b) O1s spectra of the SEIs formed on HC.\nComment 6: What sputtering rate was used in TOF-SIMS? Some experimental details are\nmissing in the Supporting Information and should be clarified.\nResponse: The size of the sputtering beam and sputtering rate is 250×250 µm2 and 0.1 nm s−1,\nrespectively. These parameters have been included in the experimental section, which has been\nhighlighted in yellow.\nChanges:\nExperimental Section:\n“Surface chemistry of the cycled electrodes was analyzed by X-ray photoelectron spectroscopy\n(XPS, Thermo Fisher Escalab Xi+) and time of flight secondary ion mass spectrometer (TOF-SIMS,\nION-TOF M6, sputtering beam 250×250 µm2, sputtering rate 0.1 nm s−1).”\nComment 7: Given the structural similarity between NaFSI and EMTMSA, how can one\ndistinguish whether the inorganic SEI/CEI components originate from salt or EMTMSA\ndecomposition? Does this imply that EMTMSA also contributes to interfacial stability via\nsacrificial decomposition? A DFT analysis of HOMO–LUMO levels may help identify\ndecomposition preference.\nResponse: We highly appreciate this comment regarding the decomposition contributions of\nthe NaFSI salt and EMTMSA solvent. Following the reviewer’s suggestion, we calculated the\nLUMO and HOMO energy levels by DFT calculations for the FSI−, NaFSI, EMTMSA, and\nNa+–EMTMSA (Fig. R18).\nAnodic stability: bare EMTMSA exhibits the highest HOMO level (−7.8983 eV), indicating it\nis intrinsically the easiest species to oxidize. However, upon coordination with Na+, its HOMO\nis greatly lowered to −11.6280 eV. This substantial stabilization suggests that Na+–EMTMSA\nbecomes much more resistant to oxidation and is therefore more anodically stable than the FSI⁻\n33\n)Ve(\nlevel\nygrenE\n−\n−\n−\n−\n−\n−\n−\n1\n1\n1\n2\n0\n2\n4\n6\n8\n0\n2\nH\nU\nO\nM O\n− 2 .4 8 6 5\n− 1 0 .4 9 0 2\nM O\n− F S I\n−\n−\n2\n.1\n.5\na\n7\n0\n6\n−\n−\nE\n0\n.3\n.8\nT\n3\n9\n8\n1\n− 5 .2 3 1 9\n− 1 1 .6 2 8 0\n+ – E M T M S A\nanion (−10.4902 eV). This trend implies that, under such solvation conditions, oxidative\ndecomposition would preferentially occur on FSI⁻ rather than on Na⁺–EMTMSA, consistent\nwith the formation of F-containing CEI species and the enhanced oxidative stability of the\nEMTMSA-based electrolyte.\nCathodic stability: the LUMO levels follow the order EMTMSA (−0.3391 eV) > NaFSI\n(−2.1706 eV) > FSI− (−2.4865 eV) > Na⁺–EMTMSA (−5.2319 eV). The relatively high LUMO\nof bare EMTMSA suggests that the bare solvent molecule is intrinsically the most reduction-\nstable species. Coordination with Na+ markedly lowers its LUMO, making the Na+–EMTMSA\ncomplex substantially more prone to reduction than FSI⁻. This trend implies that, under such\nsolvation conditions, reductive decomposition would preferentially occur on Na+–EMTMSA,\nconsistent with the inorganic-rich SEI/CEI composition.\nFig. R18 has been updated as Supplementary Fig. 25, and related discussion has been\nincorporated, which are both highlighted in yellow in the revision.\nFig. R18 The LUMO and HOMO energy levels of the FSI−, NaFSI, EMTMSA, and Na+–EMTMSA\ncalculated by DFT calculations. The trends in energy levels imply that under such solvation\nconditions, oxidative decomposition preferentially occurs on FSI− while reductive decomposition is\nmore likely on Na+–EMTMSA, collectively leading to the formation of inorganic- and F-containing\ninterphases in the EMTMSA-based electrolyte.\nChanges:\nMain text:\n“This reflects the advantage of our EMTMSA molecule with a sulfonimide-salt-like molecular\nstructure that can deduce similar decomposition products as the sulfonimide salt51 to effectively\npassivate the high-voltage aggressive cathode surface (Supplementary Fig. 25).”\nSupplementary Information:\n34\n)Ve(\nlevel\nygrenE\n−\n−\n−\n−\n−\n−\n−\n1\n1\n1\n2\n0\n2\n4\n6\n8\n0\n2\nH\nU\nO\nM O\n− 2 .4 8 6 5\n− 1 0 .4 9 0 2\nM O\n− F S I\n−\n−\n2\n.1\n.5\na\n7\n0\n6\n−\n−\nE\n0\n.3\n.8\nT\n3\n9\n8\n1\n− 5 .2 3 1 9\n− 1 1 .6 2 8 0\n+ – E M T M S A\nUpdated Supplementary Fig. 25 The LUMO and HOMO energy levels of the FSI−, NaFSI,\nEMTMSA, and Na+–EMTMSA calculated by DFT calculations. The trends in energy levels imply\nthat under such solvation conditions, oxidative decomposition preferentially occurs on FSI− while\nreductive decomposition is more likely on Na+–EMTMSA, collectively leading to the formation of\ninorganic- and F-containing interphases in the EMTMSA-based electrolyte.\nExperimental section:\n“The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO)\nenergy levels of the FSI−, NaFSI, DMTMSA, EMTMSA, DETMSA, and Na+–EMTMSA were\ncalculated using the density functional theory (DFT) in Gaussian 09 W with B3LYP8–10/6-311+g (d,\np)11,12 optimization).”\n35\n)Ve(\nlevel\nygrenE\n−\n−\n−\n−\n−\n−\n−\n1\n1\n1\n2\n0\n2\n4\n6\n8\n0\n2\nH\nU\nO\nM O\n− 2 .4 8 6 5\n− 1 0 .4 9 0 2\nM O\n− F S I\n−\n−\n2\n.1\n.5\na\n7\n0\n6\n−\n−\nE\n0\n.3\n.8\nT\n3\n9\n8\n1\n− 5 .2 3 1 9\n− 1 1 .6 2 8 0\n+ – E M T M S A\nReviewer #4 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This\nis part of the Nature Communications initiative to facilitate training in peer review and to\nprovide appropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse: We sincerely appreciate your contribution as a co-reviewer of our manuscript. Your\ntime and expertise have greatly contributed to strengthening our work, and we are grateful for\nyour insights alongside those of the primary reviewer.\n36"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have satisfactorily addressed the reviewer’s comments, and the revisions have strengthened the manuscript overall.", "reviewer_2": "The manuscript has been revised according to reviewers' comments and suggestion.", "reviewer_3": "The authors have addressed my questions.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": ""}]

Decision letter