Rational electrolyte solvent screening for high-energy lithium metal batteries at low temperatures

Domain: physics
Published: 2025-12-09
Editorial outcome: minor_revision

Abstract
Weakening the solvation ability has shown great potential in electrolyte solvent design to promote the cycling performance of Li-metal batteries, yet suffers from sluggish kinetics and uncontrolled Li dendrite formation, particularly at low temperatures. Herein, we propose a guideline for rational electrolyte solvent screening, where the restrained electrostatic potential of the coordinated O is proposed as one descriptor to quantify the solvation effect, followed by introducing the dipole moment as a correction descriptor to optimize solvents’ sensitivity to the electric fields and interphase stability. Under the guidance of this screening principle, a class of asymmetric fluorinated ethers is rationally designed, of which the 3,3,3-trifluoropropyl-1-methyl ether with moderate restrained electrostatic potential of the coordinated O and dipole moment is identified to mediate a stable six-membered chelating structure with Li+, significantly promoting Li kinetics and reversibility from 30 to −60 °C. Coupled with its good anodic stability, this single-salt single-solvent electrolyte enables the 50 μm Li||4.0 mAh cm−2 LiNi0.8Mn0.1Co0.1O2 coin-cells maintaining >90% capacity after 200 cycles at benign and low temperatures. A practical Li-metal pouch-cell delivers a high specific energy (based on the mass of all components) of 345.3 Wh kg−1 over 40 cycles at −40 °C.

Introduction
Li-metal batteries (LMBs) hold great promise to push the battery specific energy over 400 Wh kg−1 due to the high theoretical capacity (3860 mAh g−1), low density (0.534 g cm−3), and the lowest reduction potential (−3.04 V vs standard hydrogen electrode, SHE) of Li-metal negative electrodes1–4. Nevertheless, the high reactivity of Li-metal coupled with large volume change during charging/discharging inevitably induces the formation of unstable solid electrolyte interphase (SEI) and uneven deposition of Li, leading to the growth of Li dendrites and formation of dead Li4,5, which yields low Coulombic efficiency (CE) and largely restricts the cycling life of LMBs. These issues will be further exacerbated when operating the LMBs at low temperatures (<–30 °C) owing to the sluggish kinetics6,7, which howbeit is eagerly needed for subsea operations, polar expedition, and border patrol applications.

Electrolyte engineering offers a cost-effective and efficient approach to mitigate the above issues8. To fulfill the demand for high-energy and low-temperature LMBs, the following critical requirements for electrolytes should be met simultaneously, (i) the ability to form robust and conductive SEI to enable rapid Li+ migration and highly reversible Li plating/stripping; (ii) fast Li+ de-solvation kinetics while maintaining sufficient Li salt dissociation; (iii) high anodic stability to match the high-voltage and high-capacity layered positive electrodes such as Ni-rich LiNi0.8Mn0.1Co0.1O2 (NMC811). Numerous electrolyte design strategies have been proposed, including functional additives9,10, high concentration electrolytes (HCEs)11–15, localized high-concentration electrolytes (LHCEs)16–18, and weakly solvated electrolytes (WSEs)19–24.

Among the various electrolyte design strategies, weakening the solvation ability of the solvent through steric hindrance, changing chelation sites or structures, and incorporating fluorination represent the most promising approach. The reduced Li+–solvent coordination will not only promote the de-solvation process but also intensify the Li+–anion pairing to facilitate the formation of anion-derived inorganic-rich SEI at a regular concentration, which enhances the electrochemical kinetics and reversibility simultaneously. The well-explored 1,2-dimethoxyethane (DME) solvent featuring ethylene glycol backbone (i.e., bidentate) tends to chelate with Li+ in a stable five-membered ring configuration, yet suffers from high de-solvation energy. By substituting the methoxy groups on DME with larger-sized ethoxy groups, Chen et al. reported a 1,2-diethoxyethane (DEOE) electrolyte with weakened Li+ solvation and enhanced Li CE15. Li et al. further methylated the inner α-H of DEOE to form 1,2-diethoxypropane (DEP), which further reduces its solvation power and thus improving the Li CE to ~99.5%23. In addition, Yu et al. rationally tuned the ethoxy group of DEOE with partially or fully fluorinated moieties, which yields high Li CEs of >99.5% and enables the stable operation of high-voltage NMC811 cells20. Recently, Choi et al. proposed the asymmetric molecular design strategy to ether with only one side being fluorinated, the resulting asymmetric 1-(2,2,2-trifluoro)-ethoxy-2-methoxyethane (F3EME) bestows fast redox kinetics for high-power LMBs23.

Alternative to the well-explored DME series solvent with bidentate sites, the monodentate ether possesses much lower solvation power that offers great potential for operating the LMBs at low temperatures. Holoubek et al. employed diethyl ether (DEE) to tailor the electrolyte solvation structure, which enables the Li||sulfurized polyacrylonitrile (SPAN) coin cells to retain 84% and 76% of its room temperature capacity when cycled at −40 and −60 °C, respectively7. Zhang et al. chose a binary solvent consisting of DEE and DME solvents (DEE: DME = 9:1 vol%) with the aid of 0.1 M perfluoroalkylsulfonyl quaternary ammonium nitrate (PQA-NO3), the developed electrolyte demonstrated stable cycling of Li||NMC811 coin cell at −60 °C10. Wei et al. reported a 2,2-difluoroethyl acetate (EA-2F) solvent with local dipole to enable the operation of Li||LiNi0.6Mn0.2Co0.2O2 (NMC622) pouch cell with a high specific energy of 350 Wh kg−1 for 75 cycles at −20 °C25. Despite the encouraging progress has been made, achieving stable cycling of LMBs with practically high specific energy beyond 300 Wh kg−1 under low temperatures (<–30 °C) is unattainable for the current reported electrolytes. Moreover, excessively weakening the solvation ability will result in low ionic conductivity, there by hindering the Li+ transport in bulk electrolyte. Therefore, the structure of the solvent must be regulated carefully to balance the kinetics of ion transport, de-solvation, and Li+ migration through SEI.

In this work, we design a series of asymmetric ether solvents (Fig. 1a) and quantify their solvation ability. As shown in Fig. 1b, the appropriate ion-dipole interactions were established by introducing dual descriptors of the restrained electrostatic potential (RESP)26–29 of the coordinated O (denoted as RESPO) and dipole moment (μ), facilitating rational solvent screening for LMBs electrolytes. Applying this RESPO–μ selection principle, we identify 3,3,3-trifluoropropyl-1-methyl ether (TFPM) as a promising solvent to enable high-energy LMBs at low temperatures. The TFPM-based electrolyte induces a stable six-membered chelating solvation structure through Li-F and Li-O bi-coordination (Fig. 1c), which is expected to enhance the ionic conductivity, de-solvation kinetics, and interfacial stability simultaneously, thus enabling highly reversible Li plating/stripping and uniform deposition morphologies from 30 to −60 °C. Furthermore, 50 μm Li||4.0 mAh cm−2 NMC811 practical cells using TFPM electrolyte demonstrate >200 cycles at both 30 °C and −40 °C. Notably, a prototype Li||LiNi0.9Mn0.05Co0.05O2 (NMC9055) pouch cell demonstrates a high specific energy of 345.3 Wh kg−1 at −40 °C and can stably operate for 40 cycles with 90.8% capacity retention.

Results
Solvent design rationale and screening principle
Ether solvents demonstrate good stability towards Li-metal, of which their solvation ability plays a critical role in governing Li+–anion interactions, solubility, ionic conductivity, and viscosity. The solvent with weak solvation ability could enhance Li+–anion coordination to promote anion-derived inorganic-rich SEI, while it often compromises the solubility and ionic conductivity. The monodentate DEE solvent demonstrates weak solvation ability with good Li-metal compatibility at low temperatures, yet suffers from poor rate capability and anodic stability7,30. Based on our prior study31 as well as recent publications32,33 that validates the efficacy of asymmetric ether design strategy in improving Li redox reversibility and kinetics, we initiate the asymmetric design to the monodentate ether solvent by retaining methoxy groups to ensure adequate Li+ coordination while leveraging alkyl groups with different steric hindrance to regulate solvation ability, yielding a series of asymmetric ethers, namely, ethyl methyl ether (EM), n-propyl methyl ether (PM), and n-butyl methyl ether (BM). To enhance the oxidation stability and promote the inorganic SEI, fluorination was further introduced to generate a series of asymmetric fluorinated ethers, including 2,2,2-trifluoroethyl methyl ether (TFEM), 3,3,3-trifluoropropyl methyl ether (TFPM), and 4,4,4-trifluorobutyl methyl ether (TFBM). Notably, none of the asymmetric fluorinated ethers designed here are commercially available. A two-step reaction has been successfully used for the synthesis of all fluorinated ethers (Supplementary Fig. 1). The structures were confirmed by nuclear magnetic resonance (NMR) spectra, including 1H NMR, 13C NMR, and 19F NMR (Supplementary Figs. 2–4). The general physicochemical properties of all solvents are shown in the Supplementary Table 1. Due to impractical low boiling point of EM (~7 °C) and poor solubility of TFEM [<0.1 M for lithium bis(fluorosulfonyl)imide (LiFSI)], they were excluded from further analysis with only structural differences being briefly discussed. According to the calculation results based on density functional theory (DFT), all fluorinated ethers exhibit lower highest occupied molecular orbitals (HOMOs) compared to their non-fluorinated counterparts (Supplementary Fig. 5 and Supplementary Data 1–6), suggesting improved oxidation stabilities after fluorination.

μ is a measure of the polarity of the solvent molecule, which will not only significantly affect the coordination and solvation of Li+ but also reflect the response of solvents to electric fields within electric double layer (EDL), where de-solvation, SEI formation, and adsorption of solvents on the electrode surface occur. Many studies have shown that ion-dipole interaction plays a critical role in solvation and de-solvation, interphase regulation and protection25,34–36. Generally, molecules with low μ exhibit poor sensitivity to the electric fields within EDL, resulting in sluggish de-solvation. Nevertheless, molecules with excessively high μ may lead to strong dissolution of interphase components37. Therefore, it is necessary to highlight molecules with moderate μ (1.5~3.5 Debye) holding great potential for facile de-solvation and interphase regulation. Despite having desirable μ values, there are some solvents [e.g., 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoroprpyl ether (TTE)] that can barely dissociate the LiFSI salt, suggesting that other parameter besides μ value need to be considered. Usually, the oxygen atom of ether is the primary site to coordinate with Li+, and its coordination ability determines the solvation ability of ether solvent. To quantify the coordination ability of oxygen atoms, RESPO was proposed as the other descriptor. RESPO is a modified ESP method that incorporates physical constraints (e.g., atomic electronegativity equilibration and bond symmetry constraints) to optimize charge distributions, ensuring further improvement of the rationality of the charge model, which lead to more efficient and accurate distinction between ethers with different solvation abilities. Specifically, as the RESPO value increases (less negative), the solvation ability of oxygen decreases. The electrolyte solvents of LMBs are desired to have relatively weak solvation ability while providing sufficient solubility, where the solvents with moderately high RESPO values (−0.46 ~ −0.36 eV) can be screened out.

As shown in Fig. 1b, under the guidance of RESPO–μ dual descriptors, TFPM, bis(2-fluoroethyl) ether (BFE), tetrahydrofuran (THF), and tetrahydropyran (THP) emerged as optimal solvents with good Li reversibility and kinetics among the designed solvents and other reported solvents7,31,38–41. Unfortunately, the poor oxidation stability of THF and THP solvents has hindered their application with high-voltage positive electrodes. BFE, a recently reported monofluoride ether with Li-F and Li-O tridentate coordination chemistries, demonstrates good compatibility with both high-voltage positive electrodes and Li-metal in a wide temperature range, validating the reliability of our solvent screening principle. Driven by weakened coordination ability of O and moderate μ, TFPM tends to form a stable six-membered chelating structure through Li-F and Li-O coordination (Fig. 1c), which is expected to bestow sufficient salt solubility, fast kinetics, and high interfacial stability even at low temperatures.

Fluorine coordination chemistry mediated chelating solvation structure
Prior to experimental validation, comprehensive theoretical investigations were conducted to explore the rationale of the dual descriptors-based selection principle. DFT calculations were initially employed to elucidate the binding geometries between diverse solvent molecules and Li+. As shown in Fig. 2a–d, Supplementary Fig. 6, and Supplementary Data 7–12, the Li-O distance in TFPM is relatively large (1.853 Å), which is only slightly lower than that in TFEM (1.889 Å), reflecting the weak coordination effect of the O atom. Notably, TFPM exhibits the strongest Li-F interaction with the shortest Li-F distance of 1.845 Å, indicating the strong coordination tendency of F atom (on -CF3) with Li+, which lead to the formation of a stable six-membered chelating structure in Li+–TFPM complexes with the most negative binding energy (−2.16 eV). Although the TFEM and TFBM molecules have higher μ that can induce stronger ion-dipole interactions, the larger tension of the five-membered and seven-membered rings leads to a decrease in the stability of the chelating structures, as indicated by less negative binding energies (Fig. 2d and Supplementary Fig. 6b). The synergistic O and F chelation is anticipated to boost the Li salt dissociation and mitigate the ionic conductivity reduction typically associated with fluorination.

NMR measurements were carried out to verify the coordination effect between solvent and Li+. In 7Li NMR spectra, higher chemical shift represents less shielding effect of Li nuclei, suggesting weak coordination of the surrounding ligands. As shown in 7Li NMR in Fig. 2e, LiFSI/PM and LiFSI/BM electrolytes have larger chemical shifts compared to LiFSI/TFPM and LiFSI/TFBM electrolytes, which indicates a weaker solvation ability of PM and BM solvents. However, considering the synergistic coordination effect of O and F, further differentiation of coordination interaction was revealed through 17O NMR (Fig. 2f) and 19F NMR (Fig. 2g). For the LiFSI/TFPM electrolyte, minimal shift was observed in 17O spectrum, while maximum shift was observed in the 19F spectrum, indicating the weakest coordination ability of O and the strongest coordination ability of F in the TFPM solvent, which is well consistent with the DFT calculations (Fig. 2a–d). This Li-F and Li-O bidentate coordination effect significantly promotes the solvation ability of TFPM, leading to more shielding effect and lower chemical shift in 7Li NMR spectrum.

The Li+ solvation structure dominates the ionic transport, de-solvation, and SEI compositions, which was first investigated by molecular dynamics (MD) simulations (Supplementary Fig. 7 and Supplementary Data 13–16). According to the statistic results from MD, all electrolytes including 2 M LiFSI/PM, 2 M LiFSI/BM, 2 M LiFSI/TFPM, and 2 M LiFSI/TFBM possess an ion-rich solvation structure, of which more than 95% first solvation sheath was dominated by two or more FSI− anions (Fig. 3a–d), with the most probable solvation structure being 3 anions and 1 solvent molecule coordinated with Li+ (Supplementary Fig. 8 and Supplementary Data 17–20). To quantify the local environment around the Li+ in various electrolytes, the radial distribution functions (RDFs) of both the anion and solvent with respect to Li+ were calculated (Fig. 3e–h). Specifically, the average coordination number of the FSI− anion and solvent in the first solvation sheath (within 3 Å from Li+) was calculated to be Li+(FSI−)2.86(PM)1.16, Li+(FSI−)2.68(BM)1.33, Li+(FSI−)3.03(TFPM)0.99, and Li+(FSI−)2.93(TFBM)1.08 for the 2 M LiFSI/PM, 2 M LiFSI/BM, 2 M LiFSI/TFPM, and 2 M LiFSI/TFBM electrolytes, respectively, of which the minimal coordination number of O in TFPM is consistent with the screening results of RESPO. As shown in the Fig. 3i, the 2 M LiFSI/TFPM electrolyte exhibits a much higher FSI−/solvent ratio (i.e., 3.06) than that of 2 M LiFSI/PM (i.e., 2.47), 2 M LiFSI/BM (i.e., 2.02), and 2 M LiFSI/TFBM (i.e., 2.71) in the primary solvation sheath.

The solvation structure in different electrolytes was further verified by Raman spectroscopy. The Raman band located between 690–780 cm−1 is corresponding to the stretching vibration peak of the S-N-S bond on FSI−. As the position of this peak gradually upshifts, the coordination state of FSI− anion transforms from solvent-separated ion pairs (SSIPs) to contact ion pairs (CIPs), and then to aggregates (AGGs)42. As shown in Fig. 3j, the peak positions of S-N-S bond in four electrolytes are all greater than 745.0 cm−1, reflecting the solvation structures of all electrolytes are dominated by AGGs, which is in good agreement with MD results. The peaks are centered at 745.6 cm−1 for PM and BM electrolytes, which upshifts to 752.2 cm−1 for TFBM electrolyte and further to 755.5 cm−1 for TFPM electrolyte, indicating enhanced anion coordination, which is favorable for promoting the formation of inorganic-rich SEI. Furthermore, the peak at about 804 cm−1 is assigned to the vibration of C–F bond of TFPM. This peak is broadened after dissolving LiFSI salt with the full width at half maxima (FWHM) increased from 21.0 cm−1 to 32.1 cm−1, validating that partial F atoms on TFPM has been coordinated with Li+, while no obvious FWHM change is observed for TFBM after dissolving salt.

According to the differential scanning calorimetry (DSC) curves, there is no phase change for all electrolytes even when the temperature was decreased to below −100 °C (Supplementary Fig. 9), indicating that the electrolytes will not be frozen, which is a prerequisite for operating the LMBs under low temperatures. Among the four investigated electrolytes, the 2 M LiFSI/TFPM electrolyte exhibits moderate ionic conductivities and viscosities at all measured temperatures range from −40 to 40 °C (Supplementary Figs. 10, 11). In addition, the average de-solvation energies (Avg. ΔEdsv) of four electrolytes were calculated based on the solvation structures derived from MD results. As revealed in Fig. 3k, the 2 M LiFSI/TFPM electrolyte exhibits the lowest Avg. ΔEdsv of 2.14 eV, outperforming that of 2 M LiFSI/PM (2.25 eV), 2 M LiFSI/TFBM (2.29 eV), and 2 M LiFSI/BM (2.62 eV), which is beneficial for facile de-solvation and homogeneous Li deposition. Therefore, the Li-F and Li-O bidentate coordination chemistries offered by TFPM solvent induces the formation of six-membered chelating structure in LiFSI/TFPM electrolyte, leading to the sufficient salt dissociation, reduced solvent number and enhanced Li+–FSI− pairing in the solvation sheath, which not only facilitates ion transport and de-solvation but also will contribute to formation of highly conductive inorganic-rich SEI.

Highly reversible Li plating/stripping from 30 to −60 °C
The compatibility of the TFPM electrolyte was systematically evaluated in Li||Cu asymmetric cells. As shown in Fig. 4a, with the modified Aurbach method43, the TFPM electrolyte can offer a high CE up to 99.6%, which is better than that of PM (99.3%), BM (99.3%), and TFBM (99.1%) electrolytes at a current density of 1.0 mA cm−2 for 1.0 mAh cm−2 at 30 °C. To examine the Li deposition morphology in various electrolytes, 5.0 mAh cm−2 of Li metal was plated on the bare Cu foils. As shown in the scanning electron microscopy (SEM) images in Supplementary Fig. 12, particle-like Li deposits with many pores are observed in PM, BM, and TFBM electrolytes, which may induce the over-decomposition of the electrolyte and result in the accumulation of thick SEI. In sharp contrast, highly compact and smooth Li deposition was achieved in TFPM electrolyte, which is beneficial for minimizing the parasitic reaction and inhibiting the formation of dead Li during long-term cycling.

Long-term cycling of Li||Cu cells further highlights the advantages of TFPM electrolyte. As shown in Fig. 4b, the cells using PM, BM, and TFBM electrolytes exhibit fluctuating CE during cycling, leading to cells failure within 440 cycles. Among these three electrolytes, the PM electrolyte delivers a relatively high average CE of 99.2% within 400 cycles, which can be attributed to its high ionic conductivity (Supplementary Fig. 10) and relatively low de-solvation energy (Fig. 3k). Notably, TFPM electrolyte demonstrates stable cycling for over 850 cycles with a high average CE of 99.3%, representing the pioneering work for “single salt single solvent” electrolytes reported thus far at a relatively high current density of 1.0 mA cm−2 (Supplementary Table 2), which is not only attributed to its low de-solvation energy but also the moderate μ of TFPM that holds sufficient sensitivity to the electric fields while maintaining the interphase integrity simultaneously. As shown in Supplementary Fig. 13, owing to its rapid electrochemical kinetics and enhanced interfacial stability, the Li plating/stripping profiles of Li||Cu cell using TFPM electrolyte deliver the lowest initial overpotential and smoothest evolution during cycling.

The operational advantages of TFPM electrolyte become more pronounced at low temperatures. As shown in Fig. 4c, when the temperature was reduced to −40 °C, the CE of Li||Cu cells using PM, BM, and TFBM electrolytes sharply drops to 93.3%, 95.5%, and 97.0%, respectively, with highly porous Li deposit morphologies (Fig. 4e). Whereas, the TFPM electrolyte still retains a high CE of 98.7% even at a high current density of 1.0 mA cm−2, which is competitive compared to state-of-the-art reported electrolytes under such high current densities (Supplementary Table 3). The SEM image and photograph reveal that the TFPM electrolyte still bestows highly dense and smooth Li deposits with a shiny silver appearance (Fig. 4e). When the temperature was further dropped to −60 °C, the cells using PM and BM electrolytes exhibit abnormal voltage profiles during Li plating/stripping (Fig. 4d), and the Li metal was barely deposited on Cu foils with obvious dendrites formation (Fig. 4f). For the cell using TFBM, despite delivering smooth Li plating/stripping profiles, it was found that CE dropped dramatically to 91.7% with dark and non-uniform Li deposits (Fig. 4f). Extraordinarily, the cell using the TFPM electrolyte still maintains a high CE of 96.1% (Fig. 4d) and yields uniform Li deposition morphology with silver appearance (Fig. 4f), demonstrating the effectiveness of TFPM electrolyte in enabling highly reversible Li plating/stripping at low temperatures.

Fast electrochemical kinetics and high interfacial stability
In addition to the ionic conductivity and de-solvation process, the compositions of SEI play a vital role for the Li+ migration and subsequent Li deposition behavior. Thus after 10 cycles of Li plating/stripping in Li||Cu cells with different electrolytes, X-ray photoelectron spectroscopy (XPS) analysis with etching depth profiles was applied to study the SEI components and their distributions on Li metal in various electrolytes. As shown in Fig. 5a and Supplementary Fig. 14, the atomic ratio of C for PM (63.5%) and BM (60.7%) electrolytes is much higher than TFPM (43.0%) and TFBM (37.3%) electrolytes, which remains consistently high for PM and BM electrolytes even after 240 s of Ar+ sputtering, but is significantly reduced for TFPM and TFBM electrolytes. The C 1s spectra with depth profiles also show much higher peak intensities for the PM and BM electrolytes (Supplementary Fig. 15), which is an indication of rich inorganic components for the SEI. On the other hand, inorganic components represented by the atomic ratio of F ranks as TFBM (8.4%) > TFPM (7.7%) > PM (6.4%) > BM (3.7%). While in the F 1s spectra (Fig. 5b), the LiF peak intensity, reflecting the absolute F content, is much higher for TFPM than TFBM electrolyte. This indicates that the SEI formed in the TFBM electrolyte may be dissolved out by the TFBM solvent with high μ value, leading to less SEI residues on the surface of Li metal, as demonstrated by previous publications36,37, which may cause continuous electrolyte decomposition during prolonged cycling. According to the N 1s, S 2p, and O 1s spectra shown in Supplementary Figs. 16–18, more FSI− anions were decomposed in TFPM and TFBM electrolytes to generate inorganic SEI components. It is worth noting that more LiNxOy and LiSx, the thorough decomposition products of FSI− anion, were detected for TFPM electrolyte, building up inorganic-rich SEI that is highly stable with low resistance.

To clarify the vital impact of μ on the interphase stability, the electrochemical quartz crystal microbalance (EQCM) was employed to monitor the gravimetric variation of SEI in-situ44–46. As shown in the Supplementary Fig. 19, after 300 s of discharge, a high mass of 13,170 ng cm−2, corresponding to the plated Li with SEI layer, is generated in the BM electrolyte. During the subsequent resting process, the SEI formed by BM electrolyte slowly dissolves at a rate of 6.6 ng s−1 cm−2 due to the low dissolution ability of BM solvent (low μ). The initial plated Li with SEI layers formed by TFPM and TFBM electrolytes show similar mass. However, the SEI dissolves rapidly in the TFBM electrolyte at a high rate of 20.9 ng s−1 cm−2 due to the high μ of TFBM, whereas a moderate dissolution rate of 9.3 ng s−1 cm−2 was found for the TFPM electrolyte, facilitating the formation of a robust inorganic-rich SEI. Moreover, the TFPM electrolyte also shows substantially higher exchange current density compared to the other three electrolytes at both 30 °C and −40 °C (Supplementary Figs. 20, 21), demonstrating its high sensitivity to the electric fields that promotes Li plating/stripping kinetics. This validates that the TFPM solvent with moderate μ could achieve a good balance between sensitivity to the electric fields within EDL and interphase stability.

To reveal the effect of different solvents on Li plating behavior, the electrochemical impedance spectra (EIS) were recorded for Li||Cu cells after 10 cycles. The impendence can be divided into bulk resistance (Rb), charge transfer resistance (Rct), and SEI resistance (RSEI), which describe the processes of Li+ transport in bulk electrolyte, de-solvation of solvated Li+, and migration of Li+ through SEI, respectively. As shown in Fig. 5c, at 30 °C, the Rb trend of various cells is well-accorded with the ionic conductivities of various electrolytes. Looking into the semi-cycle region of EIS spectra, it is found that TFPM electrolyte with moderate μ exhibits much lower total resistance of RSEI and Rct compared to the other three electrolytes, validating its much lower SEI resistance and de-solvation energy, which is highly desirable for high-rate operation of LMBs. When the temperature decreases sequentially from 30 °C to −20 °C, −40 °C and −60 °C (Fig. 5d, e, Supplementary Fig. 22), the advantages of TFPM electrolyte are further amplified with substantially lower total resistance (Rb + RSEI + Rct) than the other three, holding great potential for operating LMBs at low temperatures.

Based on the above discussions, it should be noted that two asymmetric fluorinated ethers (i.e., TFPM and TFBM), with only one methylene group difference in chemical structure, exhibit dramatic difference in Li deposition behavior. As schematically illustrated in Fig. 5f, the TFPM electrolyte is better than the TFBM electrolyte in the following aspects: (1) due to the weakened coordination ability of O and F-mediated coordination effect, TFPM solvent induces a stable six-membered chelating solvation structure with highly rich in anion, resulting in low de-solvation energy while maintaining sufficient salt dissociation; (2) the TFBM electrolyte exhibits lower ionic conductivity and higher viscosity compared to the TFPM electrolyte, hindering efficient ion transport, especially under high-rate and low-temperature conditions; (3) the TFBM solvent with high μ tends to adsorb on the surface of Li-metal strongly, leading to the aggressive dissolution of interphase components, while the TFPM solvent with moderate μ could achieve a balance between sensitivity to the electric fields within EDL and interphase protection; (4) because of the strong dissolution ability of TFBM solvent, FSI− anions do not have enough time to decompose before being dissolved out, resulting in insufficient anion reduction; (5) since the “interphase dissolution–electrolyte decomposition” process is constantly going on in the TFBM electrolyte, the interphase is in a highly dynamic evolution, which yields highly unstable SEI with large resistance. While the TFPM with moderate μ enables sufficient reduction of FSI− anions, leading to the formation of highly robust inorganic-rich SEI with low resistance. These merits significantly enhance the electrochemical kinetics and interfacial stability for Li-metal electrode, which are beneficial for enabling high-energy and low-temperature LMBs.

Full cell and pouch cell evaluation at benign and low temperatures
To build high-energy LMBs, the electrolyte must possess high anodic stability to match the high-voltage positive electrodes such as NMC811 or NMC9055. Linear sweep voltammetry (LSV) with Al as working electrode was performed to evaluate the anodic stability of four electrolytes. As shown in Supplementary Fig. 23, the current quickly rise to 3 µA cm−2 at 3.7 V for the BM electrolyte, which maybe ascribed to the Al corrosion and/or oxidation decomposition of the BM solvent. The current leakage was postponed to 4.5 V for PM electrolyte, which was attributed to its weakened solvation ability that barely solvate Al3+, thereby inhibiting the Al corrosion to a certain extent. While for two fluorinated electrolytes (i.e., TFPM and TFBM), no obvious current leakage was observed until 5.5 V, demonstrating their enhanced anodic stability. The chronoamperometry (CA) test of Li||Al cells further validates this phenomena, high leakage current was found for BM electrolyte when holding at 4.5 V (Supplementary Fig. 24a). Furthermore, the SEM image shows obvious corrosion spots on Al surface after 24 h of holding (Supplementary Fig. 24b). In contrast, the PM, TFPM, and TFBM electrolytes show no Al corrosion and negligible leakage current with the TFPM electrolyte being the lowest. Moreover, to further testify the anodic stability of the electrolytes in practical batteries, the electrochemical floating test of Li||NMC811 cells using different electrolytes were carried out. As shown in Fig. 6a, the cell using BM electrolyte exhibit obvious increase in leakage current when elevating the voltage to 4.4 V, of which the leakage current further exceeds 100 µA cm−2 at 4.5 V, indicating severe parasitic reaction occurs. This suggests that the BM electrolyte is inapplicable for high-voltage positive electrodes, which was excluded for practical battery evaluation. Similarly, the TFPM electrolyte demonstrates the highest oxidation stability with negligible leakage current, validating its great feasibility for high-voltage operation.

Owing to its advantages in Li-metal compatibility, electrochemical kinetics, and high-voltage tolerance, the TFPM electrolyte was applied to the high-voltage LMBs with Ni-rich NMC positive electrodes. As shown in Fig. 6b and Supplementary Fig. 25, under a high cut-off voltage of 4.4 V, the Li||NMC811 cell using the TFPM electrolyte shows good rate performance from 0.1 C to 20 C at 30 °C, which still delivers a high specific capacity of 127.1 mAh g−1 with low voltage polarization even at 20 C, significantly outperforming that of PM (96.3 mAh g−1) and TFBM (34.6 mAh g−1) electrolytes. This is attributed to the rapid de-solvation process and low ΔEdsv offered by TFPM electrolyte. Long-term cycling performance of Li||NMC811 cells were first evaluated at 2.0 C. As shown in Fig. 6c and Supplementary Fig. 26, the Li||NMC811 cell using TFPM exhibits very stable cycling performance with a high specific capacity of 83.3% and a high average CE of 99.8% after 600 cycles. Considering its good rate capability, we then further test the cell at a high rate of 5.0 C, of which the high-power output is desirable for electric vertical take-off and landing (eVTOL) scenarios. As shown in Fig. 6d and Supplementary Fig. 27, the Li||NMC811 cell with TFPM electrolyte demonstrates a stable cycling performance with 81.0% capacity retention and a high average CE of 99.9% after 900 cycles. This can be attributed to the fast electrochemical kinetics and interphase protection ability provided by TFPM, which not only effectively promotes the Li+ transport, de-solvation, and Li deposition, but also inhibits the side reaction during cycling. Because of the high anodic stability and relatively rich inorganic components in SEI, the TFBM electrolyte can also support the operation 900 cycles, but with a lower capacity retention of 68.0%. More importantly, the TFPM electrolyte also enables the stable cycling of even aggressive Li||NMC9055 cell for more than 750 cycles at 2.0 C (Supplementary Figs. 28, 29).

To mimic the realistic assembly of LMBs for practical application, 50 μm thin Li foil and high areal capacity NMC811 positive electrode (4.0 mAh cm−2) were adopted to assemble the Li||NMC811 coin cells, leading to a practically low negative to positive electrode capacity (N/P) ratio of 2.6. As presented in Fig. 6e, at 30 °C, the cell with PM electrolyte suffers obvious short circuit with a short life-span of only 80 cycles (Supplementary Fig. 30a), which is ascribed to the uneven Li deposition that lead to the growth of Li dendrites. While the cell using TFBM electrolyte exhibits stable cycling during the first 120 cycles and then undergoes rapid capacity decay, which is due to the highly dynamic evolution of the SEI caused by the TFBM solvent with high μ, leading to the depletion of the limited electrolyte and Li metal. In stark contrast, the cell using TFPM electrolyte demonstrates a high capacity retention of 93.7% after 200 cycles with a high average CE of 99.8%. In addition, the charge-discharge profiles do not show obvious increase in voltage polarization during cycling, indicating its high interfacial stability (Supplementary Fig. 30b).

To further highlight the advantages of TFPM electrolyte, the cell was then tested at low temperatures. As shown in Fig. 6f and Supplementary Fig. 31, when the temperature was dropped to −40 °C, the cell using the TFPM electrolyte still displays a high specific capacity of 143.7 mAh g−1 and enables stable cycling at 0.1 C for 200 cycles, with a capacity retention of 94.1% and a high average CE of 99.9%. In contrast, due to the poor ionic conductivity and high viscosity of the TFBM electrolyte that lead to severe concentration polarization, the cell using TFBM electrolyte only delivers an initial specific capacity of 116.1 mAh g−1. Furthermore, its unstable and dynamic interphase also results in rapid capacity decay with only 32.2% capacity retained after 200 cycles. Furthermore, owing to the high conductivity, low viscosity, and low de-solvation energy of PM electrolyte, the cell using PM electrolyte could release higher capacity than that using TFBM, but the organic-rich SEI results in the formation of dendritic and dead Li, making it unable to support long-term cycling. Considering thin Li-metal (50 μm) and high loading positive electrode (4.0 mAh cm−2), the cycling life advantage for TFPM electrolyte is very obvious at low-temperature (≤ −20 °C) in comparison with other reported electrolytes (Supplementary Table 4). Besides, the cell using TFPM electrolyte also demonstrates good rate performance even at −40 °C (Supplementary Fig. 32). Notably, the TFPM electrolyte is still able to sustain the stable cycling of cell for more than 150 cycles at a very low temperature of −60 °C with an initial specific capacity of 116.5 mAh g−1 (Supplementary Fig. 33).

The advantages of the TFPM electrolyte were eventually subjected to the test of Li-metal pouch cells. To achieve a high specific energy of ~500 Wh kg−1, all parameters of the battery need to be strictly controlled. To reach this demand, a ~6.2 Ah level Li||NMC9055 pouch cell with lean electrolyte (1.91 g Ah−1) and a low N/P ratio of 1.0 was assembled. The detailed parameters are shown in Supplementary Table 5, and the specific energy is calculated based on the mass of all components in the pouch cell. As shown in Fig. 7a, the pouch cell delivers the highest discharge capacity of 6.22 Ah at the third formation cycle, corresponding to a high specific energy of 488.0 Wh kg−1. Furthermore, the pouch cell is able to retain over 80% capacity after 82 cycles with low voltage polarization (Fig. 7b).

Given the good performance of the Li-metal coin cells at low temperatures (Fig. 6 and Supplementary Fig. 33), we further evaluate the electrochemical performance of pouch cell at −40 °C. Another pouch cell with initial capacity of 6.29 Ah (491.1 Wh kg−1, based on the mass of all components with detailed parameters shown in Supplementary Table 6) at room temperature is able to deliver a capacity of 4.40 Ah at −40 °C, corresponding to 70% capacity at room temperature (Fig. 7c and Supplementary Fig. 34). This yields a high specific energy of 345.3 Wh kg−1 (based on the mass of all components) at such low temperature (i.e., −40 °C). What is more encouraging is that the pouch cell can be stably cycled for more than 40 cycles with a high capacity retention of 90.8% (Fig. 7c). In comparison with some relevant reported low-temperature pouch cells25,40,47–51, our study stands out in terms of the highest specific energy and longest cycles at such low temperature (Fig. 7d and Supplementary Table 7).

Methods
Materials
N-propyl methyl ether (PM, >95%) and n-butyl methyl ether (BM, >98%) were purchased from Tokyo Chemical Industry Co., Ltd. 2,2,2-trifluoroethanol (99.5%), 3,3,3-trifluoro-1-propanol (98%), 4,4,4-trifluoro-1-butanol (97%), iodomethane (MeI, 99.5%), and sodium hydride (NaH, 60% in mineral oil) were purchased from Energy Chemical. LiNi0.8Mn0.1Co0.1O2 (NMC811) was kindly provided by Guangzhou Tinci Materials Technology Co. Ltd. Lithium bis(fluorosulfonyl)imide (LiFSI, >99.9%) was purchased from Nippon Shokubai Co., Ltd. The separator (Celgard 2500, 25 μm thick, 55% porosity, and 0.064 μm average pore size) was purchased from Celgard, LLC. Li foil (99.95%, 500 μm thick), composite metal strip (50 μm Li foil + 8 μm Cu foil), Cu foil, Al current collector, and CR 2032-type coin cell cases were purchased from Guangdong Canrd New Energy Technology Co. Ltd. LiNi0.9Mn0.05Co0.05O2 (NMC9055) powders and the Li-metal pouch cells (6.2 Ah) without injecting electrolyte were purchased from Guangzhou Science Technology Co. Ltd.

Synthesis
The synthesis route is shown in the Supplementary Fig. 1. As a typical procedure, NaH (12.0 g, 60 wt.% in mineral oil, 0.30 mol) and 240 mL anhydrous DMF were added to a 500 mL two-necked round-bottom flask, the mixture was cooled to 0 °C. Then, 0.20 mol 2,2,2-trifluoroethanol (or 3,3,3-trifluoro-1-propanol or 4,4,4-trifluoro-1-butanol) was slowly added. Thereafter, the reaction was returned to room temperature for 30 min. Later, MeI (27.6 g, 0.24 mol) was slowly added at 0 °C, and the reaction was returned again to room temperature for 16 h. After reaction, 30 mL deionized water was slowly added to quench the reaction. The crude product was obtained by distilling the mixture under proper temperature. Then, the crude product was washed twice with 1.0 mol/L NaOH aqueous solution, saturated NaCl aqueous solution, and 1.0 mol/L HCl aqueous solution in sequence, of which the organic phase was retained after each wash. The obtained organic phase was distilled triple to yield the final product. Yield: ~70%. TFEM: 1H NMR (600 MHz, CDCl3, δ/ppm): 3.77 (q, 2H), 3.52 (s, 3H). 19F NMR (564 MHz, CDCl3, δ/ppm): −74.20 (t, 3F). 13C NMR (150 MHz, CDCl3, δ/ppm): 126.78-121.23, 69.91-69.24, 59.81. TFPM: 1H NMR (600 MHz, CDCl3, δ/ppm): 3.60 (t, 2H), 3.37 (s, 3H), 2.43-2.36 (m, 2H). 19F NMR (564 MHz, CDCl3, δ/ppm): −64.90 (t, 3F). 13C NMR (150 MHz, CDCl3, δ/ppm): 128.87-123.38, 65.22-65.16, 58.40, 34.34-33.78. TFBM: 1H NMR (600 MHz, CDCl3, δ/ppm): 3.42 (t, 2H), 3.34 (s, 3H), 2.22-2.14 (m, 2H), 1.84-1.80 (m, 2H). 19F NMR (564 MHz, CDCl3, δ/ppm): −66.47 (t, 3F). 13C NMR (150 MHz, CDCl3, δ/ppm): 129.99-124.51, 70.57, 58.19, 30.82-30.25, 22.22-22.17.

Electrolyte preparation
Before preparing the electrolyte, all solvents were thoroughly dried by sequential treatment with anhydrous Na2SO4, Na-metal, and molecular sieves (4 Å) prior to use. The electrolyte mentioned in this manuscript was prepared by dissolving 2.0 mmol (for NMR, 1.0 mmol) LiFSI in 1.0 mL solvent of PM, BM, TFPM, or TFBM in an Ar filled glovebox (25 °C, O2 < 0.01 ppm, H2O < 0.01 ppm). For TFEM, 0.1 mmol LiFSI and 1.0 mL solvent were mixed together, but LiFSI cannot be completely dissolved, thus the supernatant was taken for analysis.

Electrode preparation
The NMC powder, acetylene black (Li400, Denka Black), and poly(vinylidene fluoride) (PVDF, HSV900, Kynar) were mixed in N-methyl-2-pyrrolidone (NMP, 99.9%) using a Thinky (AR-100) mixer at a weight ratio of 80:12:8 for 10 h. Then the slurry was evenly coated on an Al foil and dried under vacuum at 120 °C for 10 h. The active mass loading of NMC positive electrode was controlled at ~3.5 mg cm−2 or ~19 mg cm−2 for high-loading test.

Materials characterization
Nuclear magnetic resonance (NMR) measurements were performed on an AVANCE NEO 600 MHz instrument (Bruker). The Raman spectrum of the electrolyte was tested using a QE Pro spectrometer (Ocean Optics) with an emission wavelength of 532 nm. Differential scanning calorimetry (DSC) measurement was performanced in the temperatures range from 30 to −150 °C at a scanning speed of 5 °C min−1 with a DSC 214 polyma (NETZSCH). The density and viscosity of electrolyte were measured by a SVM3001 viscometer (Anton Paar) with the temperatures range from 30 to −40 °C. Deposition morphology of Li was observed using a MAIA3 scanning electron microscopy (SEM, TESCAN Brno, s.r.o.). The chemical components on the Li-metal negative electrode surface were analyzed by XPS (PerkinElmer PHI 1600 ESCA). The cell was disassembled in the Ar filled glovebox, of which the electrode was rinsed with the corresponding electrolyte solvent three times and dried under vacuum. The washed electrode was transferred from the glovebox to the XPS chamber directly without exposure to air for the XPS test. The Ar+ sputtering for the XPS depth-profiling was carried out at a beam energy of 500 eV.

Electrochemical measurement
All the coin-cells were assembled in a standard 2032-configuration in the Ar filled glovebox. Linear sweep voltammetry (LSV), chronoamperometry (CA), electrochemical floating test, and electrochemical impedance spectroscopy (EIS) were carried out using a Potentiostat (VMP3, Bio-Logic). The ionic conductivity was measured by EIS method using a Pt||electrolyte||Pt parallel cell with the temperatures range from 40 to −40 °C. The EIS spectrum was collected within a frequency range of 1 MHz to 10 mHz with a voltage amplitude of 10 mV. The measurements were recorded with 6 data points per decade of frequency to ensure sufficient resolution. CA test was conducted by holding Li||Al cell (40 μL electrolyte) at 4.5 V for 24 h. LSV was tested in a three-electrode system with Al as the working electrode, Li as both the counter and reference electrodes from open circuit voltage (OCV) to 7.0 V at a scanning rate of 1.0 mV s−1. Tafel curve was obtained from LSV at a scan rate of 0.5 mV s−1 on a Li||Li symmetric cell (40 μL electrolyte). The value of the exchange current density was calculated by the Tafel equation:

η=a+blogi (1)

The cycling tests of Li||Cu and Li||NMC coin cells, and Li||NMC pouch cells were performed using a LAND CT2001A instrument (Wuhan Jinnuo Electronic Co. Ltd.) at different temperatures. The Li||NMC coin cells were cycled at different rates within the voltage range of 2.8–4.4 V. Three formation cycles at 0.1 C were performed for all coin cells at 30 °C before long-term cycling. The NMC positive electrode was cut into circular pieces with a diameter of 12 mm, and each coin cell used 80 μL electrolyte. The pouch cells were cycled in the voltage range of 2.8–4.4 V at 500 mA for the initial three formation cycles, and then at 1200 mA (25 °C) or 310 mA (−40 °C) for the subsequent cycles. The cycling of the pouch cell was tested under a fixing device to provide 100 kPa external pressure. For the coin cell with a 3.5 mg cm−2 NMC811-positive electrode, the current density is 0.74 mA cm−2 at 1 C. For the coin cell with a 19 mg cm−2 NMC811-positive electrode, the current density is 4.0 mA cm−2 at 1 C. For the coin cell with a 3.5 mg cm−2 NMC9055-positive electrode, the current density is 0.75 mA cm−2 at 1 C.

For Li||Cu cells, 500 μm thick Li foil with diameter of 8 mm, one layer of 19 mm Celgard 2500 separator, 14 mm diameter Cu foil, and 40 µL electrolyte were used. For cycling test, after three activation cycles (plating 1 h with a current density of 0.05 mA cm−2 and then stripping to 1.0 V), 1.0 mAh cm−2 of Li was plated on the Cu electrode, followed by stripping to 1.0 V. For the Aurbach method43 tested at 30 °C and −40 °C, with a current density of 1.0 mA cm−2, 5.0 mAh cm−2 of Li was firstly plated on the Cu electrode and stripped fully. Then a Li reservoir of 5.0 mAh cm−2 was plated on Cu electrode, followed by 10 cycles of Li plating and stripping with a current density of 1.0 mA cm−2 and a capacity of 1.0 mAh cm−2. Lastly, all deposition Li was stripped completely to 1.0 V. When tested at −60 °C, the current density and corresponding capacity decreased to 0.1 mA cm−2 and 0.1 mAh cm−2, respectively.

The cell for the electrochemical quartz crystal microbalance (EQCM) test consists of a gold-coated quartz crystal (9 MHz AT-cut quartz, 5 mm diameter Au)45,46, as the working electrode, a Li metal as the counter electrode, and 3.0 mL electrolyte in the Ar-filled glovebox. The mass change of working electrode can be calculated from the collected frequency of quartz crystal by QCM922 quartz crystal microbalance with following equation:

Δf= (2f0^2 / A ρQ Q μQ) Δm (2)

where Δf is the frequency change of the gold-coated quartz crystal electrode, f0 is the resonant frequency of the quartz crystal, A is the active crystal surface, ρQ is the density of quartz (2.648 g cm−3) μQ is the shear modulus of quartz (2.947×1011 g cm−1 s−2), and Δm is the mass change of the electrochemical process in working electrode. The process was performed at a constant current of 0.1 mA cm−2 for 300 s by VersaSTAT3 Potentiostat/Galvanostat, followed by stood still for 400 s and concurrently maintained the frequency measurement to obtain the dissolution mass.

Theoretical calculations
Density functional theory (DFT) calculations were performed using the Gaussian 16 package. The molecular geometries of the ground states were optimized, and the frequency analysis was carried out to ensure the optimized stationary point at the B3LYP/6-311+G(d,p) level. The restrained electrostatic potentials (RESP) were calculated by Multiwfn 3.8 programs28. The binding energy between Li+ ions and solvent molecules solvent was calculated as: Eb = Etotal − (ELi+ + Esolvent), where Etotal is the total energy of the Li+–solvent complexes, ELi+ and Esolvent are the energy of the individual Li+ ion and solvent molecule, respectively.

Molecular dynamics (MD) simulations were performed by employing the GROMACS 2018 program52. The optimized potentials for liquid simulations all-atom (OPLS-AA) force field were adopted to described molecules and ions. 30 LiFSI and 145 PM molecules, or 30 LiFSI and 125 BM molecules, or 30 LiFSI and 129 TFPM molecules, or 30 LiFSI and 110 TFBM molecules were put into the simulation box according to the molar ratio in electrolytes, respectively. Firstly, the steepest descent method was employed to minimize the initial energy generated for each system. Then 8 ns simulation was carried out in the canonical ensemble (NVT) under Nosé-Hoover thermostat. Another 40 ns simulation was performed in the isothermal-isobaric ensemble (NPT) by Parrinello-Rahman barostat. Finally, the final 30 ns simulation for analysis to obtain coordination numbers and radial distribution functions (RDF).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this paper, the authors evaluate the electrochemical properties, solvation environment, and performance of asymmetric fluorinated solvents for lithium-ion batteries. The primary focus is to improve the low temperature operation of liquid electrolytes for lithium metal batteries through fluorinated solvents. They also performed theoretical calculations to offer possible explanations for the observed electrochemical trends and behavior. They identified 3,3,3-trifluoropropyl-1-methyl ether (TFPM) as the top performing solvent for low temperature operation with a 2M LiFSI salt.\nMy primary concern with this paper is the lack of novelty for their research approach. There are countless papers and reviews describing the enhanced low temperature performance of fluorinated and/or asymmetric solvents for lithium ion batteries in this and similar journals. These reports use a similar approach to this work by understanding how to control the solvation/coordination of lithium with fluorinated solvents to impact factors such as ionic conductivity, kinetics, oxidation stability, and stable SEI formation. This also includes prior work by the authors from 2023 (Angew. Chem. Int. Ed. 2023, 62, e202218151) in which they look at a structurally similar 1,1,2,2-tetrafluoro-3-methoxypropane (TFMP) solvent. They use a 7:1 mix of TFMP with DME with 1 m LiFSI. They showed cycling at -40 oC with NMC811 for 100 cycles with the electrolyte in coin cell format. This work and general conclusions are very similar to this current study.\nIn the current work, the authors also present a thorough characterization of the system across multiple time and length scales with a variety of analytical techniques as they previously did for TFMP from 2023. They show improvement in coin cell cycling at -40 oC with NMC811 for 200 cycles with minimal loss in capacity for the TFPM solvent compared to the TFMP solvent. They also expand the practical use of the TFPM solvent by demonstration of 6.0 Ah pouch cells that can operate at low temperature (-40 oC) with good capacity retention and coulombic efficiency. They also demonstrate -60 oC operation in a coin cell with good performance for 150 cycles. The use of lean electrolyte conditions and low N/P ratio in these cells and comparison of its ambient and low temperature performance with current literature highlights the potential utility of the electrolyte solvent. However, is this enough to overcome the lack of novelty in their approach compared to prior work from their group and others which should be addressed? If addressed, it could be accepted. Aside from this issue, the following points should also be considered:\n1. Figure 2 only shows the coordination environment for a single lithium cation and solvent molecule. While this highlights the point the authors are trying to make regarding the differences in energies and bond distances for Li-O and F-O, it does not provide a real picture of the solvation environment. How do the bond distances and coordination environment change as the number of solvent molecules is increased? Additionally, where is the anion in the solvation structure? This can be extracted from the MD simulations and further optimized via DFT to show the “true” Li+ coordination in the solvent. This will provide a much more accurate representation of the bonding energies and distances in the electrolyte system.\n2. Why was LiFSI chosen rather than other electrolyte salts (i.e. LiTFSI, LiClO, LiPF6)? Comment on if you would expect similar or different behavior for other common Li salts with these solvents.\n3. Was the impact of LiFSI concentration on any of the electrochemical properties evaluated since it appears that only 2M solution was evaluated?\n4. While the radar chart is a visually nice figure (7f), it does not appear that there was a true qualitative methodology for assessing the designed electrolytes since 4 out of the 5 aspects show the electrolyte has the ideal properties. This makes it appear to readers as if these aspects have been solved, which I do not think that is the case since it is only a qualitative assessment. This could be deceiving to readers and should have some qualitative figures of merit for the rankings. If not, it should be removed from the main text.\n5. I think there I a typo in line 275, should it be “PM, TFPM, and TFBM” rather than “BM, TFPM, and TFBM” since the BM shows Al corrosion in Figure S21b while the PM, TFPM, and TFBM electrolytes do not.\n6. There are also multiple typos and grammar errors in the paper with the following words misused: line 40 “demanding”; line 46 “wakening”; line 88 “enhances”; line 111 “facial”; lines117-118 sentence is very difficult to understand; and line 349 “slighter”", "reviewer_2": "Zehao et al. introduced a promising approach for screening electrolyte solvents for lithium-metal batteries operating under ultra-low temperatures and high voltages using the RESPO–μ principle. As a result, the fluorinated ethers TFPM and TFBM demonstrate superior electrochemical performance compared to their non-fluorinated counterparts, PM and BM. However, it should be noted that a conceptually similar strategy involving rational solvent tuning was previously proposed in a diethyl ether (DEE)-based system by Zhiao et al. (Nat. Energy 7, 94–106, 2022), where improvements in oxidation stability and low-temperature performance were achieved by manipulating the solvation structure. While the present manuscript is thorough, methodically executed, and well-organized, the central concept remains insufficiently substantiated. How solvents with moderate dipole moment (μ) contribute to interphase stability? This mechanism warrants further clarification through either direct or indirect experimental validation. Given the high standards of Nature Communications, the authors are encouraged to address the following concerns before the manuscript can be considered for publication.\nSome detailed comments are attached below.\n1. In the 7Li NMR spectra, the observed downfield shift (de-shielding) of Li+ in LiFSI/BM and LiFSI/PM suggests weaker solvation compared to LiFSI/TFPM and LiFSI/TFBM. However, molecular dynamics (MD) simulations indicate that TFPM and TFBM electrolytes exhibit highly similar solvation structures, as stated in the manuscript: Li+(FSI−)3.03(TFPM)0.99, and Li+(FSI−)2.93(TFBM)1.08 . How can the significantly lower de-solvation energy observed for LiFSI/TFPM be rationalized relative to the other systems?\n2. In both ¹⁷O and ¹⁹F NMR spectra, the addition of Li salt results in an upfield shift for oxygen and a downfield shift for fluorine. This suggests that fluorine acts as an electron donor, whereas oxygen functions as an electron acceptor. The author should consider the distinct electronic roles of oxygen and fluorine in the FSI⁻ anion. Particularly since LiFSI accounts for approximately 75% of the coordination environment.\n3. In Figure 3a-d, the TFBM has significantly more AGGs/CIPs compared to TFPM. Why does it not yield better Echem performance?\n4. 99.3% overall Coulombic efficiency (CE) in a Li/Cu cell is not unprecedented for LiFSI-based systems. For example, values of 99.4% have been reported in 1 M LiFSI in BTFE/DME (Energy Environ. Sci., 2022, 15, 1647–1658), and up to 99.7% in 1.2 M LiFSI/F5DEE (Nat. Energy, 2022, 7, 94–106). At a minimum, the authors should provide a fair comparison with the current state-of-the-art.\n5. If dissolution is indeed an issue in the TFBM system, can the difference in SEI composition be identified via XPS? Based on the current data (Figure 5a and S14), the SEI appears similar in both the TFBM and TFPM systems. How, then, can the authors substantiate their claim that dissolution leads to the growth of redundant SEI?\n6. The high-voltage stability of fluorinated solvents is not surprising and has also been reported in mentioned Nature Energy paper. Fluorination lowers the HOMO energy level of the molecules, thereby enhancing their oxidative stability. However, how does solvent modification help prevent Al corrosion typically induced by LiFSI?\n7. What is the extent of gas evolution in TFBM and TFPM cells during cycling at 4.4 V?", "reviewer_3": "Electrolyte solvent engineering could tune both the SEI structure and chemistry, which has been considered as a critical and pragmatic approach to enable high performance Li-metal batteries. Unfortunately, the lack of solvent screening principle has hindered their efficient development. In this work, the authors innovatively proposed a solvent screening principle based on unique RESPO-μ dual-descriptors to enable high-energy LMBs at ultra-low temperatures. Furthermore, the authors design and synthesize a new class of weakly solvating fluorinated ethers to validate the effectiveness of this RESPO–μ screening principle. The work provides valuable insights into electrolyte design to enhances electrochemical kinetics and reversibility simultaneously, achieving an unprecedented high energy density of 345.3 Wh kg−1 at −40°C. Therefore, from my perspective, this manuscript is very interesting and should be very helpful to the field that is rapidly evolving recently. I would enthusiastically recommend publication of this work after the following minor issues are properly addressed.\n1. In the research of solvent descriptors, researchers have proposed numbers of parameters as criteria for solvent screening, among which electrostatic potential is also a very important screening parameter. What are the advantages of the RESPO parameter proposed by the authors compared to electrostatic potential?\n2. The designed TFPM electrolyte presented good capacity retention and cycling stability at both room temperature and low temperature. Has the author explored the rate performance at low temperatures with TFPM electrolyte?\n3. Among the series of fluorinated ether molecules designed, the formation of a hexagonal coordination structure can significantly improve the performance of batteries, especially at ultra-low temperature. What influence does this configuration have on the lithium deposition and kinetics at low temperatures?\n4. The solvation of TFPM and TFBM is very similar, and the composition of SEI is almost the same at room temperature. However, there is a significant difference in impedance between these two at low temperatures（−40 and −60°C）, and there is also a significant difference in battery performance. What is the main affecting factor?", "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": "Dear editor and reviewers,\nWe have carefully considered all the comments and have fully addressed them point-\nby-point in “response to reviewer’s comments” below. To facilitate reading of this letter,\nwe have, (i) systematically reported the editor and reviewer comments in italics and in\nblue followed by our responses in red, and (ii) handled each reviewer separately. The\ncorresponding changes made to both “Manuscript” and “Supplementary Information”\nwere highlighted in yellow.\nWe once again thank the editor and reviewers for their constructive comments that\nhelp in improving and strengthening our paper. Having done our best to satisfactorily\nanswer the editor and reviewers’ comments as well as their requests for extra data, we\nhope that this revised version could meet the high standards of Nature Communications.\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nIn this paper, the authors evaluate the electrochemical properties, solvation\nenvironment, and performance of asymmetric fluorinated solvents for lithium-ion\nbatteries. The primary focus is to improve the low temperature operation of liquid\nelectrolytes for lithium metal batteries through fluorinated solvents. They also\nperformed theoretical calculations to offer possible explanations for the observed\nelectrochemical trends and behavior. They identified 3,3,3-trifluoropropyl-1-methyl\nether (TFPM) as the top performing solvent for low temperature operation with a 2 M\nLiFSI salt.\nMy primary concern with this paper is the lack of novelty for their research approach.\nThere are countless papers and reviews describing the enhanced low temperature\nperformance of fluorinated and/or asymmetric solvents for lithium-ion batteries in this\nand similar journals. These reports use a similar approach to this work by\nunderstanding how to control the solvation/coordination of lithium with fluorinated\nsolvents to impact factors such as ionic conductivity, kinetics, oxidation stability, and\nstable SEI formation. This also includes prior work by the authors from 2023 (Angew.\nChem. Int. Ed. 2023, 62, e202218151) in which they look at a structurally similar\n1,1,2,2-tetrafluoro-3-methoxypropane (TFMP) solvent. They use a 7:1 mix of TFMP\nwith DME with 1 m LiFSI. They showed cycling at −40 °C with NMC811 for 100 cycles\nwith the electrolyte in coin cell format. This work and general conclusions are very\nsimilar to this current study.\nIn the current work, the authors also present a thorough characterization of the system\nacross multiple time and length scales with a variety of analytical techniques as they\npreviously did for TFMP from 2023. They show improvement in coin cell cycling at\n−40 °C with NMC811 for 200 cycles with minimal loss in capacity for the TFPM solvent\ncompared to the TFMP solvent. They also expand the practical use of the TFPM solvent\nby demonstration of 6.0 Ah pouch cells that can operate at low temperature (−40 °C)\nwith good capacity retention and coulombic efficiency. They also demonstrate −60 °C\noperation in a coin cell with good performance for 150 cycles. The use of lean\nelectrolyte conditions and low N/P ratio in these cells and comparison of its ambient\nand low temperature performance with current literature highlights the potential utility\nof the electrolyte solvent. However, is this enough to overcome the lack of novelty in\ntheir approach compared to prior work from their group and others which should be\naddressed? If addressed, it could be accepted. Aside from this issue, the following points\nshould also be considered:\nAuthors’ Response: We would like to thank the reviewer for his/her useful comments\nand suggestions to further improve our manuscript. But we dissent from the reviewer\nthat a work lacks novelty if something related to it has been studied, regardless of what\nit achieves and how important it is. However, please be aware that, for the LiCO\n2\ncathode material alone (discovered in 1980), there are many papers published on the\nNature, Science and Nature sister journals in the past 10 years. Similarly, fluoroethylene\ncarbonate (FEC) was first identified as solvent for graphite anode at 1999, they are\nmany works investigated its effect on other types of anodes, such as silicon and lithium\nmetal anodes, many of which have been published on Nature sister journals, Angew.\nChem., JACS and PANS. Therefore, we certainly can’t conclude that a work lacks\nnovelty if something related to it has been studied, as a matter of fact, we should\nlook into what it achieves and how important it is.\nFor reviewer’s concern on the novelty of our work, we would like to emphasize the\nnovelty in the following several aspects:\nFirst, as pointed out by the Reviewer, there are indeed many papers describing the\nenhanced low temperature performance of fluorinated and/or asymmetric solvents for\nlithium-ion batteries, which was realized by controlling the solvation/coordination of\nLi+ with fluorinated solvents to promote ionic conductivity, kinetics, oxidation stability,\nand stable SEI formation, including our previous study (Angew. Chem. Int. Ed., 2023,\n62, e202218151). However, most of these studies were realized by employing multiple\nsalts/solvents to compensate their individual drawbacks. Achieving fast kinetics,\nsuperior interphase stability, and uniform Li deposition simultaneously in a single-\nsalt single-solvent system is extremely challenging, with only very few reports up\nto now. To be specific, there is one very recent paper reported the operation of Li-metal\npouch-cell at low temperature of −20 °C using single-salt single-solvent electrolyte\n(Joule, 2025, 9, 101823). While in our study, a Li-metal pouch-cell was demonstrated\nto deliver an unprecedentedly high energy density of 345.3 Wh kg−1 over 40 cycles at\n−40 °C. Based on the dramatically different electrolyte system, namely multiple-\nsolvent vs. single-solvent, one should NOT conclude that our work lacks novelty.\nSecond, recent electrolyte solvent design based on weakening the solvation ability has\nshown great potential, however, the lack of solvent screening principle has hindered\ntheir efficient development. In this study, we innovatively proposed a guideline for\nrational electrolyte solvent screening, where the restrained electrostatic potential of the\ncoordinated O (RESP ) was proposed as one descriptor to quantify the solvation effect,\nO\nfollowed by introducing the dipole moment (μ) as correction descriptor to optimize\nsolvents’ sensitivity to the electric fields and interphase stability. Under the guidance of\nthis RESP –μ screening principle, a new class of asymmetric fluorinated ethers was\nO\nrationally designed, of which the 3,3,3-trifluoropropyl-1-methyl ether (TFPM) with\nmoderate RESP and μ was identified to mediate a stable six-membered chelating\nO\nstructure with Li+, significantly promoting Li kinetics and reversibility from 30 to\n−60 °C. This study provides a highly efficient principle for rational electrolyte solvent\nscreening, opens up new frontiers in electrolyte design towards practical high-energy\nLMBs under extreme temperatures, demonstrating that our study has both scientific\nnovelty and importance.\nThird, as commented by the Reviewer, the screened TFPM electrolyte exhibits much\nsuperior performance compared to previous literatures. Specifically, (1) For the Li||Cu\ncell, TFPM electrolyte demonstrates ultra-stable cycling for over 850 cycles with a high\naverage CE of 99.3%, representing the pioneering work for “single salt single solvent”\nelectrolytes reported thus far at relatively high current densities of 1.0 mA cm−2\n(Supplementary Table 2); (2) for Li-metal coin-cells with high areal capacity and low\nN/P ratio, the cycling life advantage for TFPM electrolyte is very obvious at low-\ntemperature (≤ −20 °C) in comparison with other reported electrolytes (Supplementary\nTable 4); (3) for the Li-metal pouch-cells, the cell using TFPM electrolyte delivers an\nunprecedentedly high energy density of 345.3 Wh kg−1 over 40 cycles at −40 °C, which\nstands out in terms of the highest energy density and longest cycles in comparison with\nthe reported studies at low temperatures (≤ −20 °C) (Supplementary Table 7), validating\nthe superiority of our study.\nIn summary, our study overcomes the long-standing challenges of Li compatibility\nand electrochemical kinetics, and provides a highly efficient principle for rational\nelectrolyte solvent screening. This leads to an unprecedentedly high energy density of\n345.3 Wh kg−1 over 40 cycles at −40 °C for Li-metal pouch-cell, which is unattainable\nfor previous electrolyte design and can be expanded to other metal batteries. Therefore,\nour work has both scientific novelty and importance, and will be of significant interest\nto the broad readership of Nat. Commun. in both scientific and industrial arena. The\nsignificant progress, as well as the importance of high-energy and low-temperature\nbatteries in our society, warrants the publication of our manuscript in Nat. Commun.\nComment #1: Figure 2 only shows the coordination environment for a single lithium\ncation and solvent molecule. While this highlights the point the authors are trying to\nmake regarding the differences in energies and bond distances for Li-O and F-O, it does\nnot provide a real picture of the solvation environment. How do the bond distances and\ncoordination environment change as the number of solvent molecules is increased?\nAdditionally, where is the anion in the solvation structure? This can be extracted from\nthe MD simulations and further optimized via DFT to show the “true” Li+ coordination\nin the solvent. This will provide a much more accurate representation of the bonding\nenergies and distances in the electrolyte system.\nAuthors’ Response: We thank the reviewer for this insightful comment. We agree with\nthe Reviewer that the solvation structure extracted from the MD simulation can more\naccurately reflect the binding energies and distances in the electrolyte system. Thus, per\nyour suggestion, the most probable solvation structures (3 anions and 1 solvent) of four\nelectrolytes were extracted and further optimized via DFT. As shown in the following\nFigure R1, the TFPM electrolyte exhibits the largest Li-O distance of 2.056 Å,\nindicating the weakest coordination ability of O atom in TFPM, which is consistent\nwith the “single Li+ and single solvent molecule” calculated results shown in Figure 2a-\nd in Manuscript. Furthermore, the binding energies (i.e., negative value of de-solvation\nenergy) were also calculated according to the solvation structure and their distributions\nextracted from MD simulation. As shown in the following Figure R2, the 2 M\nLiFSI/TFPM electrolyte exhibits the lowest Avg. ΔE of 2.14 eV, outperforming that\ndsv\nof 2 M LiFSI/PM (2.25 eV), 2 M LiFSI/TFBM (2.29 eV), and 2 M LiFSI/BM (2.62\neV), which is beneficial for facile de-solvation and homogeneous Li deposition.\nThe relevant discussion was added to the Revised Manuscript and Supplementary\nInformation.\na Li-O : 2.017 Å b Li-O : 2.002 Å c Li-O : 2.056 Å d Li-O : 2.001 Å\nsol. sol. sol. sol.\nFigure R1. The most probable solvation structures of (a) 2 M LiFSI/PM, (b) 2 M\nLiFSI/BM, (c) 2 M LiFSI/TFPM, and (d) 2 M LiFSI/TFBM electrolytes extracted from\nMD simulations.\n2.8\n2.4\n2.0\n1.6\nFigure R2. Average de-solvation energy (Avg. ΔE ) of different electrolytes\ndsv\ncalculated according to the solvation structure and their distributions extracted from\nMD simulation.\nAuthors’ modification to the Manuscript and Supplementary Information:\n(Page 5 in Manuscript):\nAccording to the statistic results from MD, all electrolytes including 2 M LiFSI/PM, 2\nM LiFSI/BM, 2 M LiFSI/TFPM, and 2 M LiFSI/TFBM possess anion-rich solvation\nstructure, of which more than 95% first solvation sheath was dominated by two or more\nFSI− anions (Figure 3a-d), with the most probable solvation structure being 3 anions\nand 1 solvent molecule coordinated with Li+ (Supplementary Figure 8).\n(Page 8 in Supplementary Information):\nThe most probable solvation structures of four electrolytes were added to the\nSupplementary Information as newly added Supplementary Figure 8.\nComment #2: Why was LiFSI chosen rather than other electrolyte salts (i.e. LiTFSI,\n)Ve(\nEΔ\n.gvA\nvsd\n2.62\n2.29\n2.25\n2.14\nLiClO , LiPF )? Comment on if you would expect similar or different behavior for other\n4 6\ncommon Li salts with these solvents.\nAuthors’ Response: Thanks for your insightful comment. Li salt not only dominates\nthe ion transport behavior in the bulk electrolyte, but also largely affects the solid\nelectrolyte interphase (SEI) components on the anode, which may result in vastly\ndifferent electrochemical performance of LMBs. LiFSI exhibits superior hydrolysis\nresistant, high conductivity, and good solubility in ether solvents. While combined with\nweakly solvating ether solvents, the reduced Li+–solvent coordination will not only\npromote the de-solvation process but also intensify the Li+–FSI− pairing to facilitate the\nformation of anion-derived inorganic-rich SEI at a regular concentration, which\nenhances the electrochemical kinetics and reversibility simultaneously, especially for\nLMBs. Therefore, the combination of LiFSI-ether electrolytes has been widely adopted\nand studied for high-energy density LMBs.[1-5]\nAs for the other Li salts mentioned by the Reviewer, (1) LiTFSI has a similar structure\nand elemental compositions to the LiFSI, which possesses even better thermal and\nmoisture stability than LiFSI. Nevertheless, the larger volume of TFSI− anion and lower\nconductivity of LiTFSI limits its kinetics at high-rate conditions or ultra-low\ntemperatures; (2) LiClO does NOT contain F element and fails to contribute inorganic\nreversible Li plating/stripping. Furthermore, the high oxidation state of Cl (+7) raises\nserious safety concerns; (3) LiPF is the most widely used and mature Li salt in\n6\ncommercial Li-ion batteries. However, it suffers from poor thermal and moisture\nstability as well as limited solubility in ether solvents.\nTo comprehensively address the Reviewer’s comments, we extended our study to assess\nthe properties of TFPM with different Li salts. First, as shown in the following Figure\nR3, LiFSI presents a higher max solubility (i.e., 5.9 M) than that of LiTFSI (i.e., 5.1\nM), LiClO (i.e., 3.8 M), and LiPF (i.e., 1.6 M) in TFPM solvent, which contributes to\n4 6\nthe sufficient ionization of LiFSI at the applied concentration (2 M) and avoid salt\nprecipitation at ultra-low temperatures. Second, the physical properties of TFPM\nsolvent with different Li salts at 2 M (expect 1.5 M for LiPF ) were also measured\n6\n(Figure R4). Among these four TFPM electrolytes, 2 M LiFSI/TFPM electrolyte\nemerges as the optimal candidate for practical application due to its highest ionic\nconductivity, relatively lower density and viscosity. Third, to show the universality of\nTFPM solvent with different salts in improving the electrochemical performance, 50\nμm Li||4.0 mAh cm−2 NMC811 cell using 2 M LiTFSI/TFPM electrolyte was also\nevaluated (Figure R5), which exhibits similarly good performance as that using 2 M\nLiFSI/TFPM electrolyte. However, the 2 M LiTFSI/TFPM electrolyte delivers inferior\nrate capability and low-temperature performance.\nTherefore, the LiFSI salt remains the best choice in this study.\n8\n6\n4\n2\nFigure R4. Physical properties of TFPM with different Li salts.\n)M(\nytilibulos\nxaM\n5.9\n5.1\n3.8\n1.6\nLiPF LiClO LiFSI LiTFSI\n6 4\nSalt / TFPM\n2.0\n1.6\n1.2\n0.8\n0.4\n0.0\n)1−mc\nSm(\nσ\n4.5\n4.0\n3.5\n3.0\n2.5\n2.0\n)s\naPm(\nytisocsiV\n1.40\n1.36\n1.32\n1.28\n1.24\n1.20\n)3−mc\ng(\nytisneD\n1.5 M LiPF 2 M LiClO 2 M LiFSI 2 M LiTFSI\n6 4\nx M Salt / TFPM\n4.5\n4.0\n3.5\n3.0\n0 60 120 180 240 300\nFigure R5. (a) Cycling performance of the 50 μm Li||4.0 mAh cm−2 NMC811 cells.\nThe corresponding charge-discharge profiles of the cell with (b) 2 M LiFSI/TFPM and\n(c) 2 M LiTFSI/TFPM electrolytes at 30 °C.\nComment #3: Was the impact of LiFSI concentration on any of the electrochemical\nproperties evaluated since it appears that only 2 M solution was evaluated?\nAuthors’ Response: Thanks for your comment. Prior to conducting the comprehensive\nstudy, we actually conduct preliminary evaluations of the electrolytes with different\nconcentrations. For the conventional Li-ion batteries, the concentration of the\nelectrolyte was normally controlled at ~ 1 M to achieve a balance between ionic\nconductivity, viscosity, and interfacial stability. Increasing the electrolyte salt\nconcentration could effectively reduce the solvent molecules while extensively recruit\nanions within the Li+ inner solvation sheath, leading to a high anodic stability as well\nas an anion-derived inorganic-rich SEI with better Li cyclability, but at a cost of high\nviscosity and cost.\nTherefore, we first evaluated the Li||Cu cells using four electrolytes at a conventional\nLiFSI concentration of 1 M. As shown in the following Figure R6, the cell using four\nelectrolytes all demonstrate high CEs of ≥ 98.5% with the 1 M LiFSI/TFPM electrolyte\nbeing the highest of 99.5%. Next, to optimize the electrolyte performance, we further\n)V(\negatloV\n4.5\n4.0\n3.5\n3.0\n0 60 120 180 240 300\nCapacity (mAh g−1)\n)V(\negatloV\n300\n240\n180\n120\n60\n0\n0 25 50 75 100\nCycle number\nCapacity (mAh g−1)\n)1−g\nhAm(\nyticapaC\n100\n95\n90\n85\n80\n)%(\nycneiciffe\ncibmoluoC\na\n50 μmLi||4.0 mAhcm−2 NMC811\n4.4 V,0.2 C / 0.5 C, N/P=2.6, 30 °C\nb c\n100th\n1st\n1st\n100th\n25th\n50th\n25th 50th\nstudied the ionic conductivity of TFPM system at different LiFSI concentration (Figure\nR7). The ionic conductivity of LiFSI/TFPM electrolyte demonstrated a linear\ncorrelation with concentration at low concentrations, followed by a saturation turning\npoint upon exceeding 2 M. Furthermore, the 2 M LiFSI/TFPM electrolyte also\ndemonstrates better low temperature Li||Cu CE (Figure R8) and Li-metal cycling\nperformance (Figure R9) than 1 M LiFSI/TFPM electrolyte.\nTherefore, the concentration of the electrolyte was optimized at 2 M in this study with\na balance of ionic conductivity, viscosity, de-solvation kinetics, interfacial stability,\nand battery performance.\n1.0\n0.5\n0.0\n0 10 20 30 40\nFigure R6. Modified Aurbach measurement of Li||Cu cells using electrolytes with 1 M\nLiFSI at a current density of 1.0 mA cm−2 for 1.0 mAh cm−2 at 30 °C.\nFigure R7. Concentration dependent ionic conductivity of LiFSI/TFPM electrolyte.\n)V(egatloV\n30 °C Avg. CE\n1mAcm−2, 1mAhcm−2\nTime (h)\n2.4\n2.0\n1.6\n1.2\n0.8\n0.4\n0.0\n0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8\n)1−mc\nSm(\nσ\nx M LiFSI/TFPM\nConcentration (M)\n1.0\n0.5\n0.0\n-0.5\n0 10 20 30\n)V(egatloV\n−60 °C Avg. CE\n0.1mAcm−2, 0.1mAhcm−2\nTime (h)\nFigure R8. Modified Aurbach measurement of Li||Cu cells using 1 M and 2M\nLiFSI/TFPM electrolytes at −60 °C with a current density of 0.1 mA cm−2 for 0.1 mAh\ncm−2.\n100\n95\n90\n85\nLiFSI/TFPM electrolytes at 1.0 C and 30 °C.\nComment #4: While the radar chart is a visually nice figure (7f), it does not appear\nthat there was a true qualitative methodology for assessing the designed electrolytes\nsince 4 out of the 5 aspects show the electrolyte has the ideal properties. This makes it\nappear to readers as if these aspects have been solved, which I do not think that is the\ncase since it is only a qualitative assessment. This could be deceiving to readers and\nshould have some qualitative figures of merit for the rankings. If not, it should be\nremoved from the main text.\nAuthors’ Response: Thank you for this valuable suggestion. We agree with the\nReviewer that the original radar chart may be misleading since it is a qualitative\nassessment. To better reflect the real case, following the methodology described in Nat.\nEnergy, 2022, 7, 94. (Yu et al.) and Nat. Energy, 2025, 10, 365. (Choi et al.), we\ncarefully listed all relevant data respects to anodic stability, Li-metal compatibility, de-\nsolvation, ionic conduction, and interphase stability in the following Table R1. Based\non these quantitative data, we reassigned the ratings to these aspects and modified the\nradar plot carefully (Figure R10). The relevant data and discussion have been updated\nin Manuscript and Supplementary Information.\n)%(\nycneiciffe\ncibmoluoC\n280\n210\n140\n70\n0\n0 100 200 300 400 500 600 700\nCycle number\n)1−g\nhAm(\nyticapaC\nLi||NMC811,4.4 V, 1 C, 30 °C\nPM TFPM\nBM TFBM\nAnodic stability\n10\nstability 4 compatibility\n2\nFigure R10. Radar plot with five aspects to evaluate the designed electrolytes. Ratings\nof 0 to 10 are given to each electrolyte (10 stands for ideal properties).\nTable R1. Rationale for radar plot.\nAspect Reference data & ratings\nPM BM TFPM TFBM\nLeakage current in CA test\nAnodic of Li||Al cells after 24 h 1.48 2.22 0.19 0.34\nstability (μA cm−2)\nRatings 6 4 10 9\nAvg. CE (%) and cycle\nLi-metal 99.2@400 99.1@260 99.3@850 99.0@380\nnumber of Li||Cu cells\ncompatibility\nRatings 8 7 9 7\nAvg. ΔE (eV) 2.25 2.62 2.14 2.29\ndsv\nDe-solvation\nRatings 8 7 9 8\nIonic conductivity at 30 °C\nIonic 4.12 2.69 1.71 1.26\n(mS cm−1)\nconduction\nRatings 9 8 7 6\nThe S and F ratios (%) in\nXPS after 120 s of Ar+\n4.84 2.51 11.44 8.38\nInterphase sputtering, indicative of\n9.48 6.05 12.60 8.85\nstability robust inorganic\ncomponents in the SEI.\nRatings 7 5 9 7\nAuthors’ modification to the Manuscript and Supplementary Information:\n(Page 10 in Manuscript):\nUnder the guidance of RESP –μ dual descriptors, despite possessing slightly lower\nO\nionic conductivity, the TFPM electrolyte stands out in the aspects of interphase stability,\nde-solvation, Li-metal compatibility, and anodic stability (Figure 7f and Supplementary\nTable 8), which enables the stable operation of high-energy LMBs at ultra-low\ntemperatures.\n(Page 18 in Manuscript):\nThe original Figure 7f was updated based on the quantitative data in the Manuscript.\n(Page 44 in Supplementary Information):\nTable R1 was added to Supplementary Information as newly added Supplementary\nTable 8.\nComment #5: I think there is a typo in line 275, should it be “PM, TFPM, and TFBM”\nrather than “BM, TFPM, and TFBM” since the BM shows Al corrosion in Figure S21b\nwhile the PM, TFPM, and TFBM electrolytes do not.\nAuthors’ Response: Thank you for catching this error. We have carefully checked and\nmodified the description in the manuscript.\nAuthors’ modification to the Manuscript:\n(Page 8 in Manuscript):\nIn contrast, the PM, TFPM, and TFBM electrolytes show no Al corrosion and negligible\nleakage current with the TFPM electrolyte being the lowest.\nComment #6: There are also multiple typos and grammar errors in the paper with the\nfollowing words misused: line 40 “demanding”; line 46 “wakening”; line 88\n“enhances”; line 111 “facial”; lines117-118 sentence is very difficult to understand;\nand line 349 “slighter”.\nAuthors’ Response: Thank you for catching these errors. We have proof-read and\npolished the English carefully in the revised manuscript.\nAuthors’ modification to the Manuscript and Supplementary Information:\n(Page 2 in Manuscript):\nTo fulfill the demand for high-energy and ultra-low temperature LMBs, the following\ncritical requirements for electrolytes should be met simultaneously, …\n(Page 2 in Manuscript):\nAmong the various electrolyte design strategies, weakening the solvation ability of the\nsolvent through steric hindrance, changing chelation sites or structures, and\nincorporating fluorination represents the most promising approach.\n(Page 4 in Manuscript):\nThe solvent with weak solvation ability could enhance Li+–anion coordination to\npromotes anion-derived inorganic-rich SEIs, while it often compromises the solubility\nand ionic conductivity.\n(Page 4 in Manuscript):\nTherefore, it is necessary to highlight molecules with moderate μ (1.5~3.5 Debye)\nholding great potential for facile de-solvation and interphase regulation.\n(Page 4 in Manuscript):\nRESP is a modified ESP method that incorporates physical constraints (e.g., atomic\nelectronegativity equilibration and bond symmetry constraints) to optimize charge\ndistributions, ensuring further improvement of the rationality of the charge model,\nwhich leads to more efficient and accurate to distinguish the ethers with different\nsolvation ability.\n(Page 10 in Manuscript):\nUnder the guidance of RESP –μ dual descriptors, despite possessing slightly lower\nO\nionic conductivity, the TFPM electrolyte stands out in the aspects of interphase stability,\nde-solvation, Li-metal compatibility, and anodic stability (Figure 7f and Supplementary\nTable 8), which enables the stable operation of high-energy LMBs at ultra-low\ntemperatures.\nReviewer #2 (Remarks to the Author):\nZehang et al. introduced a promising approach for screening electrolyte solvents for\nlithium-metal batteries operating under ultra-low temperatures and high voltages using\nthe RESP –μ principle. As a result, the fluorinated ethers TFPM and TFBM\nO\ndemonstrate superior electrochemical performance compared to their non-fluorinated\ncounterparts, PM and BM. However, it should be noted that a conceptually similar\nstrategy involving rational solvent tuning was previously proposed in a diethyl ether\n(DEE)-based system by Zhiao et al. (Nat. Energy 2022, 7, 94–106), where\nimprovements in oxidation stability and low-temperature performance were achieved\nby manipulating the solvation structure. While the present manuscript is thorough,\nmethodically executed, and well-organized, the central concept remains insufficiently\nsubstantiated. How solvents with moderate dipole moment (μ) contribute to interphase\nstability? This mechanism warrants further clarification through either direct or\nindirect experimental validation. Given the high standards of Nature Communications,\nthe authors are encouraged to address the following concerns before the manuscript\ncan be considered for publication.\nAuthors’ Response: We would like to thank the reviewer for his/her comments and\ninsightful suggestions on our manuscript. We have addressed all comments below and\nrevised the manuscript accordingly.\nDipole moment (μ) reflects the unevenness of molecular charge distribution and is an\nimportant parameter for studying the behavior of molecule under the electric field. Li\net al. compared a series of solvents with different μ and found that solvents with higher\nμ tend to preferentially adsorb on the surface of Li-metal anode, thereby affecting the\nelectrochemical performance of the SEI.[6] Liu et al. reported an asymmetric Li salt,\nlithium 1,1,1-trifluoro-N-[2-[2-(2-methoxyethoxy)ethoxy)]ethyl] methanesulfonamide\n(LiFEA) that possesses a pseudo-crown ether-like anion, with a large molecular dipole,\nwhich allows for self-cleaning of unfavorable organic components in the SEI during\ncycling (Nat. Energy, 2023, 8, 934).[7] However, excessively high molecular dipole\ncould also dissolve out the desired inorganic components, thus reducing the stability of\nSEI. Later on, they carefully designed and synthesized another Li salt, lithium 2-[2-(2-\nmethoxy ethoxy)ethoxy]ethanesulfonyl(trifluoromethanesulfonyl) imide (LiETFSI)\nwith suitable molecular dipole, which rationalized this self-cleaning behavior and\nfurther improved its compatibility towards Li-metal anode.[8] Therefore, the μ of solvent\nmust be regulated carefully to balance its ability to absorb and stabilize the interphase\nas well as its solubility to the SEI.\nTo further clarify the vital impact of μ on the interphase stability, we first combined\nquartz crystal microbalance analysis with electrochemical methods (EQCM) to\nmeasure the in-situ gravimetric variation of SEI during initial Li plating and subsequent\nresting processes (Figure R11).[9-11] After 300 seconds of discharge, a high mass of\n13170 ng cm−2, corresponding to plated Li with SEI layer, was generated in the BM\nelectrolyte (Figure R11a). During the subsequent resting process, the SEI formed by\nBM electrolyte slowly dissolved at a rate of 6.6 ng s−1 cm−2 due to the low dissolution\nability of BM solvent (low μ). The initial plated Li with SEI layers formed by TFPM\nand TFBM electrolytes showed similar mass (Figure R11b-c). However, the SEI\ndissolved rapidly in the TFBM electrolyte at a high rate of 20.9 ng s−1 cm−2 due to the\nhigh μ of TFBM, whereas a moderate dissolution rate of 9.3 ng s−1 cm−2 was found for\nthe TFPM electrolyte. This balanced dissolution process facilitated the formation of a\nrobust inorganic-rich SEI.\nNext, the sensitivity of different solvents to the electric field was evaluated through\nTafel plots (Figure R12-13).[12] Due to the higher μ, TFPM (i.e. 2.41 mA cm−2) and\nTFBM (i.e. 1.26 mA cm−2) electrolytes showed higher exchange current density (j )\n0\nthan PM (i.e. 0.98 mA cm−2) and BM (i.e. 0.93 mA cm−2) electrolytes at 30 °C. However,\nTFBM electrolyte provided a lower j than TFPM electrolyte, which may be attributed\n0\nto the high viscosity of TFBM electrolyte. Among these four electrolytes, TFPM\nelectrolyte exhibited the highest j at both 30 °C and −40 °C, which enables fast charge\n0\ntransfer and rapid de-solvation, thereby achieving excellent rate and low-temperature\nperformance.\nBased on the above thorough discussions, we can draw the conclusion that the solvent\nwith moderate μ could achieve a good balance between interphase stability and\nsensitivity to the electric fields. The relevant data and discussions were added to the\nManuscript and Supplementary Information.\n4\n3\n2\n1\n0\n0 100 200 300 400 500 600 700\nTime (s)\nFigure R11. In-situ EQCM results of mass change in (a) 2 M LiFSI/BM (low μ), (b) 2\nM LiFSI/TFPM (moderate μ), and (c) 2 M LiFSI/TFBM (high μ) electrolytes during\ninitial Li plating at a current density of 0.1 mA cm−2 and subsequent resting process.\n)V(\negatloV\n14000\n12000\n10000\n8000\n6000\n4000\n2000\n0\n)2−mc\ngn(\nssaM\n4\n3\n2\n1\n0\n0 100 200 300 400 500 600 700\nTime (s)\n)V(\negatloV\n14000\n12000\n10000\n8000\n6000\n4000\n2000\n0\n)2−mc\ngn(\nssaM\n4\n3\n2\n1\n0\n0 100 200 300 400 500 600 700\nTime (s)\n)V(\negatloV\n14000\n12000\n10000\n8000\n6000\n4000\n2000\n0\n)2−mc\ngn(\nssaM\na\nSEI formation Dissolution\n0.1 mA cm−2\nRate: 6.6 ng s−1cm−2\nResting\nb\nSEI formation Dissolution\n0.1 mA cm−2\nRate:9.3 ng s−1cm−2\nResting\nc\nSEI formation Dissolution\n0.1 mA cm−2\nRate:\n20.9 ng s−1cm−2\nResting\n4\n2\n0\n-2\n-4\n-0.2 -0.1 0.0 0.1 0.2\n)2−mc\nAm(\nytisned\ntnerruC\ngoL\nPM 0.98 mA cm−2\nBM 0.93 mA cm−2\nTFPM 2.41 mA cm−2\nTFBM 1.26 mA cm−2\n30 °C\nVoltage (V)\nFigure R12. Tafel plots of Li plating/stripping in four electrolytes at 30 °C.\n1\n-1\n-3\n-5\n-7\n-0.2 -0.1 0.0 0.1 0.2\nFigure R13. Tafel plots of Li plating/stripping in four electrolytes at −40 °C.\nAuthors’ modification to the Manuscript and Supplementary Information:\n(Page 7 in Manuscript):\nTo clarify the vital impact of μ on the interphase stability, the electrochemical quartz\ncrystal microbalance (EQCM) was employed to monitor the gravimetric variation of\nSEI in-situ. As shown in the Supplementary Figure 19, after 300 seconds of discharge,\na high mass of 13170 ng cm−2, corresponding to the plated Li with SEI layer, is\ngenerated in the BM electrolyte. During the subsequent resting process, the SEI formed\nby BM electrolyte slowly dissolves at a rate of 6.6 ng s−1 cm−2 due to the low dissolution\nability of BM solvent (low μ). The initial plated Li with SEI layers formed by TFPM\nand TFBM electrolytes show similar mass. However, the SEI dissolves rapidly in the\nTFBM electrolyte at a high rate of 20.9 ng s−1 cm−2 due to the high μ of TFBM, whereas\na moderate dissolution rate of 9.3 ng s−1 cm−2 was found for the TFPM electrolyte,\nfacilitating the formation of a robust inorganic-rich SEI. Moreover, the TFPM\nelectrolyte also shows substantially higher exchange current density compared to the\nother three electrolytes at both 30 °C and −40 °C (Supplementary Figure 20-21),\ndemonstrating its superior sensitivity to the electric field that promotes Li\nplating/stripping kinetics. This validates that the TFPM solvent with moderate μ could\nachieve a good balance between sensitivity to the electric fields within EDL and\ninterphase stability.\n(Page 19 in Supplementary Information):\nIn-situ EQCM results was added to the Supplementary Information as newly added\n)2−mc\nAm(\nytisned\ntnerruC\ngoL\nPM 5.02×10−4mA cm−2\nBM 6.29×10−4mA cm−2\nTFPM 1.01×10−3mA cm−2\nTFBM 8.48×10−4mA cm−2\n−40 °C\nVoltage (V)\nSupplementary Figure 19.\n(Page 20 in Supplementary Information):\nTafel plots at 30 °C was added to the Supplementary Information as newly added\nSupplementary Figure 20.\n(Page 21 in Supplementary Information):\nTafel plots at −40 °C was added to the Supplementary Information as newly added\nSupplementary Figure 21.\nComment #1: In the 7Li NMR spectra, the observed downfield shift (de-shielding) of\nLi+ in LiFSI/BM and LiFSI/PM suggests weaker solvation compared to LiFSI/TFPM\nand LiFSI/TFBM. However, molecular dynamics (MD) simulations indicate that TFPM\nand TFBM electrolytes exhibit highly similar solvation structures, as stated in the\nmanuscript: Li+(FSI−) (TFPM) , and Li+(FSI−) (TFBM) . How can the\n3.03 0.99 2.93 1.08\nsignificantly lower de-solvation energy observed for LiFSI/TFPM be rationalized\nrelative to the other systems?\nAuthors’ Response: Thanks for your insightful comment. We agree with the reviewer\nthat the downfield shift of Li+ in the 7Li NMR suggests weaker solvation of LiFSI/BM\nand LiFSI/PM electrolytes compared to LiFSI/TFPM and LiFSI/TFBM electrolytes.\nHowever, in TFPM and TFBM electrolytes, the shielding effect of Li nuclei is\nsignificantly enhanced because of the coordination of F with Li+, resulting in high\nchemical shifts. To further clarify the coordination ability of different atoms, 17O and\n19F NMR spectra were also collected in the original Manuscript (Figure 2f-g), which\nreveals weakest coordination ability of O and strongest coordination ability of F for the\nTFPM solvent. The Li-F and Li-O bidentate coordination in TFPM electrolyte leads to\nthe lowest coordination number of solvent (i.e., 0.99) in the Li+ primary solvation\nsheath, according to the MD simulations.\nIndeed, as commented by the Reviewer, TFPM and TFBM electrolytes exhibit similar\nsolvation structures, namely Li+(FSI−) (TFPM) and Li+(FSI−) (TFBM) . The\n3.03 0.99 2.93 1.08\nde-solvation energies of four electrolytes were then carefully calculated according to\nthe solvation structures and their distributions extracted from MD simulation. As shown\nin the following Figure R14, the 2 M LiFSI/TFPM electrolyte exhibits the lowest Avg.\nΔE of 2.14 eV, outperforming that of 2 M LiFSI/PM (2.25 eV), 2 M LiFSI/TFBM\ndsv\n(2.29 eV), and 2 M LiFSI/BM (2.62 eV), which is beneficial for facile de-solvation and\nhomogeneous Li deposition. We are fully aware that de-solvation energy for TFPM\nelectrolyte is only slightly lower than the other three (rather than being significantly\nlower than). We sincerely appreciate the reviewer for identifying the inaccuracies in our\nManuscript. Based on your valuable comment, we have carefully revised the relevant\ndescription to improve the accuracy of our statements.\n2.8\n2.4\n2.0\n1.6\nFigure R14. Average de-solvation energy (Avg. ΔE ) of different electrolytes\ndsv\ncalculated according to the solvation structures and their distributions extracted from\nMD simulation.\nAuthors’ modification to the Manuscript and Supplementary Information:\n(Page 8 in Manuscript):\n(1) due to the weaken coordination ability of O and F-mediated coordination effect,\nTFPM solvent induces a stable six-membered chelating solvation structure with highly\nrich in anion, resulting in very low de-solvation energy while maintaining sufficient salt\ndissociation;\nComment #2: In both ¹⁷O and ¹⁹F NMR spectra, the addition of Li salt results in an\nupfield shift for oxygen and a downfield shift for fluorine. This suggests that fluorine\nacts as an electron donor, whereas oxygen functions as an electron acceptor. The author\nshould consider the distinct electronic roles of oxygen and fluorine in the FSI⁻ anion.\nParticularly since LiFSI accounts for approximately 75% of the coordination environment.\n)Ve(\nEΔ\n.gvA\nvsd\n2.62\n2.29\n2.25\n2.14\nAuthors’ Response: Thanks for your insightful comment. The difference in chemical\nshifts for O and F observed in NMR spectra predominantly arises from different\ncoordination effects. Generally, regardless of solvent or anion, the O atom possesses a\nmuch more negative electrostatic potential (ESP) than F atom. As a result, the Li–O\nsol.\ncoordination results in the O nucleus being shielded by the Li nucleus due to the strong\nelectrostatic interactions, rendering the O atom electron-accepting. In contrast, Li–F\nsol.\ncoordination is significantly weaker than Li–O coordination, leading to electron\nsol.\nlocalization at the F nucleus (de-shielding effect) and rendering F atoms electron-\ndonating.\nAs mentioned by the Reviewer, all electrolytes possess anion-rich solvation structure\naccording to the statistic results from MD simulations. However, FSI− anion always\ncoordinates with Li+ through O atom rather than F atom, which has been widely\nobserved in previous publications.[4,12-14] To better address the Reviewer’s concern, the\nmost probable solvation structures (3 anions and 1 solvent) of four electrolytes were\nextracted from MD and further optimized via DFT. As shown in the following Figure\nR15, the FSI− anions only coordinate with Li+ through O atom in all four solvates.\na b c d\nFigure R15. The most probable solvation structures of (a) 2 M LiFSI/PM, (b) 2 M\nLiFSI/BM, (c) 2 M LiFSI/TFPM, and (d) 2 M LiFSI/TFBM electrolytes extracted from\nMD simulations.\nComment #3: In Figure 3a-d, the TFBM has significantly more AGGs/CIPs compared\nto TFPM. Why does it not yield better Echem performance?\nAuthors’ Response: We thank the reviewer for this insightful comment, which enables\nus to better clarify the differences between TFPM and TFBM. Actually, all solvents\nreported in this study are weakly solvating ethers, which bestows all four electrolytes\nrich in CIPs and AGGs according to MD simulations and Raman spectra (Figure 3).\nThis yields very decent Li||Cu CEs in all four electrolytes. However, according to the\nEQCM results shown in Figure R11, the TFBM solvent with high μ exhibits a high SEI\ndissolution rate of 20.9 ng s−1 cm−2, resulting in highly unstable SEI. Furthermore, as\nshown in Supplementary Figure 10-11, TFBM electrolyte exhibits the lowest\nconductivity and highest viscosity among these four electrolytes, hindering efficient ion\ntransport under high-rate and low-temperature conditions, which leads to inferior\nbattery performance, especially under ultra-low temperature (Figure 6f).\nComment #4: 99.3% overall Coulombic efficiency (CE) in a Li/Cu cell is not\nunprecedented for LiFSI-based systems. For example, values of 99.4% have been\nreported in 1 M LiFSI in BTFE/DME (Energy Environ. Sci., 2022, 15, 1647–1658), and\nup to 99.7% in 1.2 M LiFSI/F5DEE (Nat. Energy, 2022, 7, 94–106). At a minimum, the\nauthors should provide a fair comparison with the current state-of-the-art.\nAuthors’ Response: Thanks for your comment. As commented by the Reviewer, there\nare indeed some previous studies reports higher Li||Cu CEs than ours. However, most\nof which are multi salt or multi solvent systems, and it would be overly complex to\ninclude them all for comparison. Moreover, multi salt and/or multi solvent systems can\nachieve synergistic effects to promote electrochemical performance by selecting\nsalts/solvents with desired characteristics, whereas achieving high CE and long cycle\nlife remains a great challenge in a “single-salt single-solvent” system. In addition, the\nLi||Cu CEs highly depend on the testing conditions, such as current density. Generally,\ntesting at low current densities would lead to high CEs, therefore, it is not reasonable\nto compare the CEs under different current densities. Based on the above considerations,\nwe have explicitly specified in the title of Supplementary Table 2 that we only compare\nthe state-of-the-art reported single-salt single-solvent electrolytes at a current\ndensity of 1.0 mA cm−2.\nThe first study mentioned by the Reviewer (Energy Environment. Sci., 2022, 15, 1647-\n1658) reported a CE value of 99.4% with the modified Aurbach method at a current\ndensity of 0.5 mA cm−2 using dual solvent of BTFE/DME (our work: 99.5% at 1.0 mA\ncm−2 with a single-salt single-solvent formulation); The second paper (Nat. Energy,\n2022, 7, 94–106) reported long cycling CE values of up to 99.7%, however, the testing\ncurrent density was also 0.5 mA cm−2, thus these work were excluded from our\ncomparisons. As discussed in our Manuscript (page 6): “TFPM electrolyte\ndemonstrates ultra-stable cycling for over 850 cycles with a high average CE of 99.3%,\nrepresenting the pioneering work for “single salt single solvent” electrolytes reported\nthus far at a relatively high current density of 1.0 mA cm−2 (Supplementary Table 2),”\ndemonstrating the superiority of our TFPM electrolyte.\nComment #5: If dissolution is indeed an issue in the TFBM system, can the difference\nin SEI composition be identified via XPS? Based on the current data (Figure 5a and\nS14), the SEI appears similar in both the TFBM and TFPM systems. How, then, can the\nauthors substantiate their claim that dissolution leads to the growth of redundant SEI?\nAuthors’ Response: Thanks for your insightful comment. At first glance, the SEI\ncomponents appear to be similar in TFPM and TFBM electrolytes. Take a closer look,\nit is found that the LiF peak intensity, reflecting the absolute F content, is much higher\nfor TFPM than TFBM electrolyte (Figure 5b). Besides, more LiN O and Li S , the\nx y 2 x\nthoroughly decomposition products of FSI− anion, were detected for TFPM electrolyte.\nThis indicates that the SEI formed in the TFBM electrolyte maybe dissolved out by the\nTFBM solvent with high μ value. To verify this hypothesis, the electrochemical quartz\ncrystal microbalance (EQCM) was newly employed to monitor the gravimetric\nvariation of SEI in-situ. As shown in the Figure R11, the SEI dissolves rapidly in the\nTFBM electrolyte at a high rate of 20.9 ng s−1 cm−2 due to the high μ of TFBM, whereas\na moderate dissolution rate of 9.3 ng s−1 cm−2 was found for the TFPM electrolyte.\nAs originally summarized in our Manuscript (page 8): “the TFPM electrolyte is superior\nto the TFBM electrolyte in the following aspects: (1) due to the weaken coordination\nability of O and F-mediated coordination effect, TFPM solvent induces a stable six-\nmembered chelating solvation structure with highly rich in anion, resulting in low de-\nsolvation energy while maintaining sufficient salt dissociation; (2) the TFBM\nelectrolyte exhibits lower ionic conductivity and higher viscosity compared to the\nTFPM electrolyte, hindering efficient ion transport, especially under high-rate and low-\ntemperature conditions; (3) the TFBM solvent with high μ tends to absorb on the surface\nof Li-metal strongly, leading to the aggressive dissolution of interphase components,\nwhile the TFPM solvent with moderate μ could achieve a balance between sensitivity\nto the electric fields within EDL and interphase protection; (4) because of the strong\ndissolution ability of TFBM solvent, FSI− anions do not have enough time to\ndecompose before being dissolved out, resulting in insufficient anion reduction; (5)\nsince the “interphase dissolution–electrolyte decomposition” process is constantly\ngoing on in the TFBM electrolyte, the interphase is in a highly dynamic evolution,\nwhich yields highly unstable SEI with large resistance. While the TFPM with moderate\nμ enables sufficient reduction of FSI− anions, leading to the formation of highly robust\ninorganic-rich SEI with low resistance. These merits significantly enhance the\nelectrochemical kinetics and interfacial stability for Li-metal anodes, which is\nextremely beneficial for enabling high-energy and low-temperature LMBs.”\nAuthors’ modification to the Manuscript and Supplementary Information:\n(Page 7 in Manuscript):\nTo clarify the vital impact of μ on the interphase stability, the electrochemical quartz\ncrystal microbalance (EQCM) was employed to monitor the gravimetric variation of\nSEI in-situ. As shown in the Supplementary Figure 19, after 300 seconds of discharge,\na high mass of 13170 ng cm−2, corresponding to the plated Li with SEI layer, is\ngenerated in the BM electrolyte. During the subsequent resting process, the SEI formed\nby BM electrolyte slowly dissolves at a rate of 6.6 ng s−1 cm−2 due to the low dissolution\nability of BM solvent (low μ). The initial plated Li with SEI layers formed by TFPM\nand TFBM electrolytes show similar mass. However, the SEI dissolves rapidly in the\nTFBM electrolyte at a high rate of 20.9 ng s−1 cm−2 due to the high μ of TFBM, whereas\na moderate dissolution rate of 9.3 ng s−1 cm−2 was found for the TFPM electrolyte,\nfacilitating the formation of a robust inorganic-rich SEI. Moreover, the TFPM\nelectrolyte also shows substantially higher exchange current density compared to the\nother three electrolytes at both 30 °C and −40 °C (Supplementary Figure 20-21),\ndemonstrating its superior sensitivity to the electric field that promotes Li\nplating/stripping kinetics. This validates that the TFPM solvent with moderate μ could\nachieve a good balance between sensitivity to the electric fields within EDL and\ninterphase stability.\n(Page 19 in Supplementary Information):\nIn-situ EQCM results was added to the Supplementary Information as newly added\nSupplementary Figure 19.\n(Page 20 in Supplementary Information):\nTafel plots at 30 °C was added to the Supplementary Information as newly added\nSupplementary Figure 20.\n(Page 21 in Supplementary Information):\nTafel plots at −40 °C was added to the Supplementary Information as newly added\nSupplementary Figure 21.\nComment #6: The high-voltage stability of fluorinated solvents is not surprising and\nhas also been reported in mentioned Nature Energy paper. Fluorination lowers the\nHOMO energy level of the molecules, thereby enhancing their oxidative stability.\nHowever, how does solvent modification help prevent Al corrosion typically induced by\nLiFSI?\nAuthors’ Response: Thanks for your insightful comment. Indeed, the Al corrosion is a\ncritical issue associated with imide salt (e.g., LiFSI)-based electrolyte. There are many\nprevious studies have demonstrated that by modifying the solvent structure to weaken\nits solvation ability, the resulting weakly solvating solvent can barely solvate the Al3+,\nthereby accumulating a passivation layer of Al(FSI) complexes on Al surface to\nx\nsuppress the Al corrosion kinetically.[15,16] Furthermore, the weakly solvating\nelectrolytes could also promote the decomposition of FSI− anion, which facilitates the\nformation of a LiF-rich passivation layer.[2,16] To confirm this point, XPS analysis was\napplied to study the components on Al surface after CA test. As shown in the following\nFigure R16, a high content of LiF was detected on the Al electrode in TFPM and TFBM\nelectrolytes, which aids in the prevention of Al corrosion.\nF 1s\nPM BM TFPM TFBM\n0 nm FSI LiF FSI LiF FSI LiF FSI LiF\n2 nm\n692 688 684 692 688 684 692 688 684 692 688 684\nBinding energy (eV) Binding energy (eV) Binding energy (eV) Binding energy (eV)\nFigure R16. F 1s XPS spectra of Al electrode after CA test before and after Ar+\nsputtering.\nComment #7: What is the extent of gas evolution in TFBM and TFPM cells during\ncycling at 4.4 V?\nAuthors’ Response: Thanks for your comment. Per your suggestion, differential\nelectrochemical mass spectroscopy (DEMS) was carried out to evaluate the gas\nevolution during the initial charging-discharging of Li||NMC9055 cells (Figure R17).\nThere is almost no gas generated for both TFPM and TFBM electrolytes during the\ncharging process, demonstrating the excellent stability of TFPM and TFBM electrolytes\neven at high cut-off voltage of 4.4 V. When discharging to below 3.9 V, a small amount\nof CO , C H , and H (ppm level) was detected. Specifically, more gases were released\n2 3 6 2\nin the TFBM than that in the TFPM electrolyte, which can be attributed to the highly\nrobust interphases derived from TFPM electrolyte, minimizing the parasitic reaction.\nBesides, no obvious cell swelling was observed for the pouch cell after cycling (Figure\n7), indicating that the gas evolution is very mild in TFPM electrolyte.\n4.5 4.5\n4.0 4.0\n3.5 3.5\n3.0 3.0\n0 6 12 18 24 0 6 12 18 24\nFigure R17. In-suit DEMS analysis of gas evolution for the Li||NMC9055 cells using\n(a) 2 M LiFSI/TFPM and (b) 2 M LiFSI/TFBM electrolytes during the first cycle.\nmpp 0.5 0.5\nmpp 0.5 0.5\nmpp\n0.5 0.5\nmpp\n5 5\n)V(\negatloV\na b\nCO CO\n2 2\nCH CH\n3 6 3 6\nCH CH\n2 6 2 6\nH H\n2 2\nLi||NMC9055,2.8-4.4 V Li||NMC9055,2.8-4.4 V\nTime (h) Time (h)\nThe comments from Reviewer #3\nElectrolyte solvent engineering could tune both the SEI structure and chemistry, which\nhas been considered as a critical and pragmatic approach to enable high performance\nLi-metal batteries. Unfortunately, the lack of solvent screening principle has hindered\ntheir efficient development. In this work, the authors innovatively proposed a solvent\nscreening principle based on unique RESP -μ dual-descriptors to enable high-energy\nO\nLMBs at ultra-low temperatures. Furthermore, the authors design and synthesize a new\nclass of weakly solvating fluorinated ethers to validate the effectiveness of this RESP –\nO\nμ screening principle. The work provides valuable insights into electrolyte design to\nenhances electrochemical kinetics and reversibility simultaneously, achieving an\nunprecedented high energy density of 345.3 Wh kg−1 at −40 °C. Therefore, from my\nperspective, this manuscript is very interesting and should be very helpful to the field\nthat is rapidly evolving recently. I would enthusiastically recommend publication of this\nwork after the following minor issues are properly addressed.\nAuthors’ Response: We would like to thank the reviewer for his/her useful comments\nand suggestions to further improve our manuscript. We have revised our manuscript\ncarefully based on the reviewer’s comments.\nComment #1: In the research of solvent descriptors, researchers have proposed\nnumbers of parameters as criteria for solvent screening, among which electrostatic\npotential is also a very important screening parameter. What are the advantages of the\nRESP parameter proposed by the authors compared to electrostatic potential?\nO\nAuthors’ Response: Thanks for your insightful comment, which allows us to better\nclarify the advantages of the RESP parameter. The electrostatic potential (ESP)\nO\nreflects the charge distribution on the surface of solvent molecules rather than the\nintrinsic electronegativity of solvent atoms. When a strongly electronegative F atom is\nclose to O atom, the ESP of O atom will be significantly reduced, resulting in an ESP\nvalue that does not match the solvent’s solubility and solvation ability, which reduces\nthe accuracy of ESP as a screening parameter. The restrained electrostatic potential\n(RESP) is a method for calculating atomic charges and also a modified ESP method\nthat incorporates physical constraints (e.g., atomic electronegativity equilibration and\nbond symmetry constraints) to optimize charge distributions, ensuring further\nimprovement of the rationality of the charge distribution model. RESP method can\nO\neliminate the interference of F atoms and accurately describe the charge distribution of\nO atoms, leading to more efficient and accurate to distinguish the fluorinated ethers\nwith different coordination ability of O atoms. The relevant discussion has been added\nto the Manuscript.\nAuthors’ modification to the Manuscript:\n(Page 4 in Manuscript):\nRESP is a modified ESP method that incorporates physical constraints (e.g., atomic\nelectronegativity equilibration and bond symmetry constraints) to optimize charge\ndistributions, ensuring further improvement of the rationality of the charge model,\nwhich leads to more efficient and accurate to distinguish the ethers with different\nsolvation ability.\nComment #2: The designed TFPM electrolyte presented good capacity retention and\ncycling stability at both room temperature and low temperature. Has the author\nexplored the rate performance at low temperatures with TFPM electrolyte?\nAuthors’ Response: Per your suggestion, the rate performance of Li||NMC811 cells\nwith different electrolytes at −40 °C was also tested. As shown in the following Figure\nR18, the Li||NMC811 cell with 2 M LiFSI/TFPM electrolyte exhibits very decent rate\nperformance from 0.1 C to 0.5 C even at −40 °C, which still delivers a high capacity of\n109.5 mAh g−1 at 0.5 C.\n180\n120\n60\n0\n0 5 10 15 20\nFigure R18. Rate performance of the Li||NMC811 cells at −40 °C.\nAuthors’ modification to the Manuscript and Supplementary Information:\n(Page 9 in Manuscript):\n)1−g\nhAm(\nyticapaC\n0.1 C TFPM 0.1 C\n0.2 C\n0.5 C\nTFBM PM\nLi||NMC811,4.4 V, −40 °C\nCycle number\nBesides, the cell using TFPM electrolyte also demonstrates good rate performance even\nat −40 °C (Supplementary Figure 32).\n(Page 32 in Supplementary Information):\nRate performance of the Li||NMC811 cells at −40 °C was added to the Supplementary\nInformation as newly added Supplementary Figure 32.\nComment #3: Among the series of fluorinated ether molecules designed, the formation\nof a hexagonal coordination structure can significantly improve the performance of\nbatteries, especially at ultra-low temperature. What influence does this configuration\nhave on the lithium deposition and kinetics at low temperatures?\nAuthors’ Response: We thank the reviewer for this insightful comment, which allows\nus to better elucidate the correlation between the six-membered chelating solvation\nstructure and its excellent Li compatibility at ultra-low temperatures. First, as shown in\nFigure 2a-d, the six-membered ring formed by Li-TFPM complexes exhibit the most\nnegative binding energy, indicating a more stable structure. The synergistic O and F\nchelation is anticipated to enhance the Li salt dissociation while mitigating the ionic\nconductivity reduction typically associated with fluorination, thereby preventing salt\nprecipitation and enabling fast ion transport at ultra-low temperatures. Second, at low\ntemperatures, the de-solvation step becomes the rate-determining step. The stable six-\nmembered chelating structure with pronounced spatial effects will inhibit further\ncoordination of free solvents and promote anion coordination, leading to anion-rich\ncoordination structure of Li+(FSI−) (TFPM) with an extremely high FSI−/solvent\n3.03 0.99\nratio of 3.06. This consequently bestows the lowest de-solvation energy of 2.14 eV for\nTFPM electrolyte, enabling uniform Li deposition with fast kinetics at low temperatures.\nComment #4: The solvation of TFPM and TFBM is very similar, and the composition\nof SEI is almost the same at room temperature. However, there is a significant difference\nin impedance between these two at low temperatures（−40 and −60 °C）, and there is\nalso a significant difference in battery performance. What is the main affecting factor?\nAuthors’ Response: Thanks for your insightful comment. Indeed, as commented by\nthe Reviewer, the solvation structure of TFPM and TFBM electrolyte is very similar.\nAt first glance, the SEI components appear to be similar in TFPM and TFBM\nelectrolytes. Take a closer look, it is found that the LiF peak intensity, reflecting the\nabsolute F content, is much higher for TFPM than TFBM electrolyte (Figure 5b).\nBesides, more LiN O and Li S , the thoroughly decomposition products of FSI− anion,\nx y 2 x\nwere detected for TFPM electrolyte. This indicates that the SEI formed in the TFBM\nelectrolyte maybe dissolved out by the TFBM solvent with high μ value. To verify this\nhypothesis, the electrochemical quartz crystal microbalance (EQCM) was newly\nemployed to monitor the gravimetric variation of SEI in-situ. As shown in the Figure\nR19, the SEI dissolves rapidly in the TFBM electrolyte at a high rate of 20.9 ng s−1\ncm−2 due to the high μ of TFBM, whereas a moderate dissolution rate of 9.3 ng s−1 cm−2\nwas found for the TFPM electrolyte. Moreover, the TFPM electrolyte also shows\nsubstantially higher exchange current density compared to the TFBM electrolyte at both\n30 °C and −40 °C (Figure R20-21), demonstrating its superior sensitivity to the electric\nfield that promotes Li plating/stripping kinetics.\nTherefore, as summarized in our Manuscript (page 8): “the TFPM electrolyte is superior\nto the TFBM electrolyte in the following aspects: (1) due to the weaken coordination\nability of O and F-mediated coordination effect, TFPM solvent induces a stable six-\nmembered chelating solvation structure with highly rich in anion, resulting in low de-\nsolvation energy while maintaining sufficient salt dissociation; (2) the TFBM\nelectrolyte exhibits lower ionic conductivity and higher viscosity compared to the\nTFPM electrolyte, hindering efficient ion transport, especially under high-rate and low-\ntemperature conditions; (3) the TFBM solvent with high μ tends to absorb on the surface\nof Li-metal strongly, leading to the aggressive dissolution of interphase components,\nwhile the TFPM solvent with moderate μ could achieve a balance between sensitivity\nto the electric fields within EDL and interphase protection; (4) because of the strong\ndissolution ability of TFBM solvent, FSI− anions do not have enough time to\ndecompose before being dissolved out, resulting in insufficient anion reduction; (5)\nsince the “interphase dissolution–electrolyte decomposition” process is constantly\ngoing on in the TFBM electrolyte, the interphase is in a highly dynamic evolution,\nwhich yields highly unstable SEI with large resistance. While the TFPM with moderate\nμ enables sufficient reduction of FSI− anions, leading to the formation of highly robust\ninorganic-rich SEI with low resistance. These merits significantly enhance the\nelectrochemical kinetics and interfacial stability for Li-metal anodes, which is\nextremely beneficial for enabling high-energy and low-temperature LMBs.”\n4\n3\n2\n1\n0\n0 100 200 300 400 500 600 700\nTime (s)\nFigure R19. In-situ EQCM results of mass change in (a) 2 M LiFSI/BM (low μ), (b) 2\nM LiFSI/TFPM (moderate μ), and (c) 2 M LiFSI/TFBM (high μ) electrolytes during\ninitial Li plating at a current density of 0.1 mA cm−2 and subsequent resting process.\n)V(\negatloV\n14000\n12000\n10000\n8000\n6000\n4000\n2000\n0\n)2−mc\ngn(\nssaM\n4\n3\n2\n1\n0\n0 100 200 300 400 500 600 700\nTime (s)\n)V(\negatloV\n14000\n12000\n10000\n8000\n6000\n4000\n2000\n0\n)2−mc\ngn(\nssaM\n4\n3\n2\n1\n0\n0 100 200 300 400 500 600 700\nTime (s)\n)V(\negatloV\n14000\n12000\n10000\n8000\n6000\n4000\n2000\n0\n)2−mc\ngn(\nssaM\na\nSEI formation Dissolution\n0.1 mA cm−2\nRate: 6.6 ng s−1cm−2\nResting\nb\nSEI formation Dissolution\n0.1 mA cm−2\nRate:9.3 ng s−1cm−2\nResting\nc\nSEI formation Dissolution\n0.1 mA cm−2\nRate:\n20.9 ng s−1cm−2\nResting\n4\n2\n0\n-2\n-4\n-0.2 -0.1 0.0 0.1 0.2\nFigure R20. Tafel plots of Li plating/stripping in four electrolytes at 30 °C.\nFigure R21. Tafel plots of Li plating/stripping in four electrolytes at −40 °C.\nAuthors’ modification to the Manuscript and Supplementary Information:\n(Page 7 in Manuscript):\nTo clarify the vital impact of μ on the interphase stability, the electrochemical quartz\ncrystal microbalance (EQCM) was employed to monitor the gravimetric variation of\nSEI in-situ. As shown in the Supplementary Figure 19, after 300 seconds of discharge,\na high mass of 13170 ng cm−2, corresponding to the plated Li with SEI layer, is\ngenerated in the BM electrolyte. During the subsequent resting process, the SEI formed\nby BM electrolyte slowly dissolves at a rate of 6.6 ng s−1 cm−2 due to the low dissolution\nability of BM solvent (low μ). The initial plated Li with SEI layers formed by TFPM\nand TFBM electrolytes show similar mass. However, the SEI dissolves rapidly in the\nTFBM electrolyte at a high rate of 20.9 ng s−1 cm−2 due to the high μ of TFBM, whereas\na moderate dissolution rate of 9.3 ng s−1 cm−2 was found for the TFPM electrolyte,\nfacilitating the formation of a robust inorganic-rich SEI. Moreover, the TFPM\nelectrolyte also shows substantially higher exchange current density compared to the\nother three electrolytes at both 30 °C and −40 °C (Supplementary Figure 20-21),\ndemonstrating its superior sensitivity to the electric field that promotes Li\nplating/stripping kinetics. This validates that the TFPM solvent with moderate μ could\nachieve a good balance between sensitivity to the electric fields within EDL and\ninterphase stability.\n(Page 19 in Supplementary Information):\nIn-situ EQCM results was added to the Supplementary Information as newly added\nSupplementary Figure 19.\n(Page 20 in Supplementary Information):\nTafel plots at 30 °C was added to the Supplementary Information as newly added\nSupplementary Figure 20.\n(Page 21 in Supplementary Information):\nTafel plots at −40 °C was added to the Supplementary Information as newly added\nSupplementary Figure 21.\nThe comments from Reviewer #4\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nAuthors’ Response: We sincerely appreciate the reviewer for his/her useful comments\nand suggestions to further improve our manuscript.\nReferences\nlithium metal batteries. Nature Energy 5, 526, doi:10.1038/s41560-020-0634-5 (2020).\nlowtemperature non-aqueous lithium metal batteries. Nature Communications 14, 1081,\ndoi:10.1038/s41467-023-36793-6 (2023).\nlithium metal batteries. Journal of the American Chemical Society 146, 18281,\ndoi:10.1021/jacs.4c01735 (2024).\nfor high-voltage lithium metal batteries. Journal of the American Chemical Society 146, 5964,\ndoi:10.1021/jacs.3c11798 (2024).\nlithium metal batteries. Nature Chemistry 16, 922, doi:10.1038/s41557-024-01497-x (2024).\nAdvanced Materials 36, e2406359, doi:10.1002/adma.202406359 (2024).\nlithium metal batteries. Nature Energy 8, 934, doi:10.1038/s41560-023-01282-z (2023).\nbattery. Angewandte Chemie International Edition 63, e202316717,\ndoi:10.1002/anie.202316717 (2024).\ndoi:10.1016/j.chempr.2023.06.002 (2023).\ncontaining ionic liquid in the presence of lithium cations. Electrochemistry Communications 77,\n128, doi:10.1016/j.elecom.2017.02.014 (2017).\n14, 50, doi:10.1038/s41565-018-0284-y (2019).\n10, 365, doi:10.1038/s41560-025-01716-w (2025).\nelectrolytes. Nature Energy 7, 94, doi:10.1038/s41560-021-00962-y (2022).\nProceedings of the National Academy of Sciences of the United States of America 122,\ne2418623122, doi:10.1073/pnas.2418623122 (2025).\nmetal battery. Journal of the American Chemical Society 143, 18703, doi:10.1021/jacs.1c09006\n(2021).\nbatteries. ACS Applied Materials & Interfaces 14, 44470, doi:10.1021/acsami.2c13517 (2022)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "accept revised manuscript", "reviewer_3": "The authors have appropriately addressed the my concerns and requests by providing additional data and clarification. I recommend accepting the revised manuscript for publication.", "reviewer_4": "The revised manuscript clearly explains the impact of moderate dipole moments on interphase stability through a self-cleaning mechanism. It is thorough, well organized, and I recommend its publication in Nature Communications."}, "author_response": ""}]

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