Enhanced sodium storage in hard carbon via solvent co-intercalation electrolyte enabling Ah-level pouch cells at low temperatures

Domain: neuroscience
Published: 2026-02-06
Editorial outcome: minor_revision

Abstract
Sodium ion batteries are attracting extensive interest due to their low cost and abundant sodium resources. However, sodium ion batteries still suffer from severe performance degradation at low temperatures due to the conflict between ion desolvation and diffusion. Herein, we design a co-intercalation ether electrolyte to achieve solvent co-intercalation in the hard carbon negative electrode, thereby bypassing the slow desolvation process while ensuring rapid ion diffusion in electrolyte and hard carbon. The optimized solvation structure also promotes the formation of a thin, inorganic-rich solid electrolyte interface, facilitating interfacial ion transport. As a result, the co-intercalation electrolyte enables hard carbon to deliver good low-temperature performance, with an initial Coulombic efficiency of 80.5% at −50°C (20 mA g-1) and a capacity retention of 93% after 200 cycles (100 mA g-1). Moreover, an Ah-level full cell retains cell stack-level (excluding packaging) specific energy of 163 Wh kg-1 at 25°C and 107 Wh kg-1 at −50°C (100 mA g-1), demonstrating the practical feasibility of this strategy for wide-temperature sodium ion batteries. This work has the potential to overcome the long-standing trade-off between low-temperature ion desolvation and diffusion, offering an approach for electrolyte design toward wide-temperature sodium ion batteries.

Introduction
The global energy landscape is shifting toward low-carbon solutions and electrochemical energy storage (EES) technology has become essential for supporting the expanding use of clean energy. Sodium ion batteries (SIBs) show significant potential in EES due to abundant sodium resources and cost-effectiveness1–3. Currently, SIBs are approaching commercialization in terms of energy density and cycle life at room temperature. However, the increasing demand for using batteries in cold environments calls for SIBs with reliable performance at low temperatures. And the most promising hard carbon (HC) negative electrode in SIBs shows unsatisfactory low-temperature performance, especially below −30°C, due to slow ion diffusion in the electrolyte, across the solid electrolyte interface (SEI), and within HC itself4–6. Even worse, sluggish desolvation and thicker SEI formation under low temperatures further hinder the ion transport at the negative electrode/electrolyte interface7,8. These challenges of HC under low temperatures are intrinsically connected to the electrolyte, which demands manipulation on electrolyte to ensure efficient Na+ storage under low-temperature conditions.

The weakly solvated electrolytes (WSEs) are proposed as one attractive strategy in improving the low temperature performance of HC by weakening the Na+-solvent interaction. Specifically, the use of low-polarity cyclic ether solvents can weaken their coordination with Na⁺ and thus accelerate the desolvation process9–11. While the WSEs bring the conflict of the decrease ionic conductivity in electrolyte, because the weak Na+-solvent interaction lowers the solvent’s ability to dissociate sodium salts. For example, Li et al.12 experimentally compared the ionic conductivity of various cyclic ether-based electrolytes and found that solvents with weaker coordination ability, while facilitating faster desolvation, resulted in electrolytes with lower ionic conductivity. And the WSE developed by Wang et al.13 showed improved electrochemical performance at low temperatures, while its rate performance at room temperature was far inferior to that of conventional electrolytes with high ionic conductivity. Besides, these WSEs primarily focus on desolvation process with little-to-no attention to the slow ion diffusion inside HC negative electrode, and they lack validation in Ah-level cell packs under low temperatures. Hence, the design of tailor-made electrolyte to enhance the low-temperature adaptability of HC negative electrode remains challenging and is of great importance in the practical application for wide-temperature SIBs.

Herein, we design a co-intercalation electrolyte (CIE) toward HC negative electrode, successfully overcoming the trade-off between Na⁺ desolvation and ionic conductivity in electrolyte. Our CIE allows ions to bypass the slow desolvation process at interface and achieves sufficient diffusion in electrolyte and HC simultaneously. The free solvent regulation significantly widens the operating temperature of CIE to −50°C while ensuring sufficient solvent co-intercalation for ion storage. Meanwhile, the precisely regulated solvation structure enables appropriate participation of anions in the solvation sheath, resulting in the formation of a thinner, inorganic-rich SEI. This interphase facilitates efficient Na⁺ transport across the SEI. As a result, our CIE greatly improves the low temperature performance of HC negative electrode and is further demonstrated in Ah-level SIBs cell pack. Typically, the Na||HC cell using CIE has an initial Coulombic efficiency (ICE) of 93.7% at 25°C, and an ICE of 80.5% even at the low temperature of −50°C, achieving a performance improvement compared to the existing electrolytes for HC. The CIE empowers the HC to deliver a capacity retention of 93.5% after 200 cycles at −50°C. Further assembled Ah-level HC||NaNi1/3Fe1/3Mn1/3O2 (NFM) pouch cell can achieve specific energy of 163 Wh kg−1 at 25 °C and 107 Wh kg−1 at −50°C. The CIE concept offers insights in designing electrolyte for broadening the operating temperature range of SIBs, especially under low-temperature conditions.

Results
Electrolyte design and solvation structure
The co-embedded behavior of the solvent can significantly accelerate ion transport by directly skipping the slow desolvation process14–16. This mechanism was initially discovered in graphite negative electrodes, which enabled their application in SIBs17,18. However, the cycling performance was severely affected by the excessive volume expansion of graphite during charge/discharge19. In contrast, HC materials have a larger layer spacing and exhibit highly uniform volume expansion and contraction during charge/discharge, providing enough space for solvent embedding/de-embedding20. However, the behavior of solvent co-intercalation in HC has not been thoroughly investigated, especially its enhancement mechanism of ion transport rate at low temperatures has not been thoroughly explored. It has been shown that linear ether solvents such as diethylene glycol dimethyl ether (G2) can form a stable chelating coordination structure with Na+, which not only achieves solvent co-embedding and avoids the slow desolvation process, but also exhibits an good ability to dissociate sodium salts, thus endowing the electrolyte with high ionic conductivity21,22. However, when the temperature is lower than −30°C, such linear ether-based electrolyte systems experience a sharp drop in ionic conductivity, salt precipitation and even solidification, which severely impede ion transport23. To solve this problem, a CIE was designed in this work. The design principle is based on the following findings: there are always some solvents in the electrolyte that do not participate in the coordination and only act as ion transport media, which are called free solvents. For this reason, we replaced this part of free solvent with 2-methyloxolane (MO) which has a lower melting point (−136°C, Table S1) and is less polar. Through this strategy, not only the co-embeddable solvation structure is retained and the ion transport kinetics are accelerated, but also the working temperature range of electrolyte in low-temperature environment is significantly widened by increasing the solvation entropy.

To highlight the progressiveness of our designed electrolyte, we also used common co-intercalation ether electrolyte (CCE) and WSE as controls, and the specific electrolyte formulation is shown in Table S2. According to the optical photographs in Figure S1, CCE starts to have salt precipitation at −30°C, and it is even completely solidified at −50°C. On the other hand, WSE and our CIE remain liquid at −50°C, which effectively guarantees the possibility of electrolyte application at low temperatures. The ion transport mechanisms of various electrolytes at low temperatures are shown in Fig. 1, among which the CIE has the fastest ion transport rate due to the presence of the co-intercalation behavior.

To investigate the solvation structure of the different electrolytes, theoretical calculations and experimental characterization were carried out. Firstly, the electrostatic potential maps and binding energies with Na+ of the two solvents were calculated using density functional theory (DFT) (Fig. S2), which suggests that the oxygen atoms in G2 show a stronger negative electronegativity and form a chelating coordination structure with a much higher binding energy than that of MO (−0.29 eV vs. −0.13 eV). This indicates that Na+ will be more preferentially coordinated with G2, while MO mainly acts as a co-solvent. Fourier transform infrared spectroscopy (FTIR) was used to investigate the coordination environments in different electrolytes, as shown in Fig. 2a. The characteristic peaks at 1103 and 1021 cm−1 are attributed to C-O-C stretching vibrations in pure G2 and MO solvents, respectively24,25. After dissolving sodium salt, a new characteristic peak appears at 1081 cm−1 in CCE and CIE, which was attributed to coordination of Na+ with G2 solvent, while the characteristic peak of 1016 cm−1 in WSE corresponded to the coordination of Na+ with MO solvent. Interestingly, in CIE, the MO solvent characteristic peak remains at 1021 cm−1, which indicates that MO is hardly coordinated with Na+. In addition, we obtain similar results by Raman (Fig. 2b), where both MO in CIE and pure MO show characteristic peak located at 920 cm−1. These results suggest that in CIE, MO hardly participates in the solvation structure, and Na+ is mainly coordinated to G2. The existence of interaction between the solvents is further investigated by 1H nuclear magnetic resonance (NMR), as shown in Fig. 2c. It can be found that the chemical shifts of G2 are shifted to the downfield after mixing the two solvents, which implies that there is a dipole interaction between MO and G2. This makes the density of the electron cloud around the hydrogen nucleus of G2 decrease26. The calculated binding energy between the MO and G2 molecules is −0.11 eV (Fig. S3), with this negative value indicating the presence of a spontaneous dipole-dipole interaction between the two. Such dipole interactions would weaken the coordination of G2 to some extent, thus providing the possibility for anions to enter the solvation structure. As shown in Figs. 2d and S4, variable-temperature conductivity tests were conducted for three electrolytes. When the temperature is lower than -20 °C, the ionic conductivity of CCE decreases rapidly, which can be attributed to the precipitation of NaPF6 in the electrolyte. While CIE maintains a high ionic conductivity at low temperatures and is always higher than that of WSE. Therefore, compared to the WSE, the CIE designed here can effectively solve the decrease in ionic conductivity and promote ion transport at low temperatures. In addition, we also tested the physical parameters of different electrolytes, such as the electrochemical window and transference number (Figs. S5, S6).

Molecular dynamics (MD) simulations were carried out to further investigate the solvation structure of the electrolyte under different temperature gradients. The radial distribution functions (RDF) of different electrolytes are shown in Fig. 2e, f and S7-8. In CCE, the average coordination number of G2 is 1.60 and that of PF6− is 0.87. While in CIE, the coordination number of G2 decreases to 1.41, the coordination number of PF6− increases to 1.10, and that of MO is 0.11. This is attributed to the introduction of MO which has a dipole interaction with G2, slightly weakening the coordination of G2 and Na+. In addition, MO hardly coordinates with Na+, which is consistent with the results of Raman and FTIR. In WSE, the average coordination number of MO is 0.93 and PF6− is 3.05, constituting a typical aggregate-type (AGG) solvation structure rich in anions. The solvation structure at low temperature (−20, −40°C) was further simulated, and the changes in the coordination number of each component are compared (Figs. 2g, S9). With the decrease in temperature, the coordination of the solvent decreases, and more anions enter the solvation structure. To investigate the effect of this change on the solvation sheath, we perform a statistical analysis of the various solvent-anion coordination (Fig. S10) to accurately describe the typical solvation environments of different electrolytes at different temperatures. As shown in Fig. S11, for CCE at 25°C and −20°C, solvent-separated ion pairs (SSIPs) dominate. While when the temperature further decreases, the solvation structure transforms to be dominated by contact ion pairs (CIPs), suggesting that more anions will enter the solvation structure as the temperature decreases. For CIE, it always maintains the solvation sheath of the CIPs; a co-embeddable G2-Na+ chelating structure is included, and a moderate amount of anions is also introduced, which facilitates the formation of an anion-derived, more inorganic, and thinner SEI27,28. In addition, the temperature-stable solvated structure in CIE guarantees the same co-embedded behavior at low temperatures as at 25°C. In contrast, WSE always maintains the solvation sheath of AGGs, in which more anions are introduced into the solvation structure. Too many anions can decompose rapidly at negative electrode/electrolyte interface, leading to the accumulation of inorganic substances on the surface and the formation of thicker and inhomogeneous SEI29. Therefore, it is necessary to introduce a moderate amount of anions like CIE to form thin SEI for effective ion transport. Figs. 2h and S12 are snapshots of MD simulations of the solvation environments of the three electrolytes. And Na+ is surrounded by the G2 solvent in the CIE, ensuring the solvent co-intercalation mechanism. The solvation structures of the CIPs are further clarified by DFT, and as shown in Fig. 2i. Na+ coordinated with two G2 molecules (average distance of 2.38 Å) forms the first solvation sheath, while PF6− is mainly distributed in the second solvation sheath. Overall, we precisely regulate the solvation structure to retain the co-embeddable chelating coordination for facilitating ion transport, while also enabling an appropriate number of anions to enter the solvation sheath for promoting the reconstruction of a more favorable SEI structure.

Electrochemical performance
To evaluate the electrochemical performance of the electrolyte, Na||HC cells were assembled. The structural characteristics of HC are shown in Fig. S13, and for HC, the reversible capacity during the first cycle is critical for less sodium loss. As shown in Fig. 3a, CIE provides a specific capacity of 300.5 mAh g−1 and an initial Coulombic efficiency (ICE) of 93.7% during the first cycle at 25°C, while CCE and WSE provide specific capacities of 299.6 and 300.1 mAh g−1 and ICE of 87.7% and 91.4%, respectively. The three electrolytes deliver similar reversible capacities, while CIE has the highest ICE. The severe electrolyte decomposition creates a thick SEI and affects ICE30,31, which may explain the lower ICE of CCE. For CIE and WSE, the introduction of anions enables the construction of inorganic-rich SEI, and the appropriate number of anions in CIE consumes less Na+ during film formation, thus guaranteeing a high ICE for HC in CIE. The rate performance of HC is shown in Figure S14, where CIE still provides a specific capacity of 195.0 mAh g−1 at 2 A g−1, while CCE and WSE maintain a specific capacity of 204.6 and 131.4 mAh g-1, respectively. This phenomenon suggests that the ionic conductivity plays a crucial role in the rate performance at 25°C. The CIE effectively improves the ionic conductivity while retaining the co-intercalation behavior, so the rate performance is close to that of CCE. While WSE has poor rate performance at 25°C due to lower ionic conductivity. Figure 3b further shows the rate performance of HC at −20°C. Considering that all three electrolytes deliver similar ionic conductivity at this temperature, the variability in rate performance primarily originates from the mode of ion diffusion and interface composition. Interestingly, the rate performance of WSE is better than that of CCE at low temperature, which is attributed to the formation of a thin inorganic-rich SEI. The thicker SEI of CCE restricts the rate of Na+ transport and even hinders solvent co-embedding, thus resulting in poor rate performance at low temperatures. In contrast, CIE with both solvent co-embedding and thin SEI show good rate performance at low temperatures. The sodium storage performance of HC at low temperature was further tested (Fig. 3c). The HC using CIE can provide a specific capacity of 269.6 mAh g−1 at −20°C with an ICE of 91.4%, and maintains more than 80% of the ICE even at a low temperature of −50°C. Such performance is comparable to existing high-performance reports (Fig. 3d). In contrast, CCE and WSE show lower capacity and ICE at low temperatures (Fig. S15). In particular, CCE can hardly work normally at temperatures below −30°C. CIE also exhibits good cycling stability (Fig. 3e), with a capacity retention of 91.0% after 2000 cycles (capacity decay 0.0045% per cycle) at 500 mA g−1 (25°C). The capacity retention of CCE and WSE is 83.7% and 90.8%, respectively. Moreover, the cycling performance of the CIE at different specific currents was also tested (as shown in Fig. S16), all of which show high stability. Furthermore, the cycle stability of HC with electrolytes under low-temperature environment was tested. The capacity retention rate using CIE is as high as 96.1% after 200 cycles at −40°C (100 mA g−1). And the capacity retention rate is still 93.5% after 200 cycles at −50°C, and the average Coulombic efficiency is 99.8% (Fig. 3f). Such low-temperature Na+ storage performance is attributed to the enhanced overall ion transport process and anion-derived thin SEI, whereas CCE and WSE exhibit poor cycle performance at low temperatures (Fig. S17). To sum up, CIE has achieved high reversible and high-rate sodium storage at 25°C and low temperatures with good cycling stability, which has a high potential for low-temperature SIB applications.

Interfacial chemistry
The electrode/electrolyte interface is crucial for ion transport and cycling stability, and different electrolytes determine the structure and composition of the SEI. Firstly, the composition of SEI was characterized by X-ray photoelectron spectroscopy (XPS) (Figs. 4a-b, S18-21). Three peaks located at 284.8, 286.3, and 289.2 eV are observed in the C 1s spectra, corresponding to C-C, C-O, and O-C=O, respectively32. This mainly corresponds to the organic components in the SEI. The higher intensity C-C peaks in the CCE indicate that the SEI contains more organic compounds. In addition, at low temperatures, the intensity of the C-C peaks of all three electrolytes is higher than that at 25°C, which indicates that the electrolyte decomposition is more severe and more decomposition products are found in the low-temperature environment. The same results are verified in both O 1s and F 1s spectra, with an increase in both Na2CO3 and NaF at low temperatures. Combined with the research on solvation structure mentioned above, more anions enter the solvation sheath as the temperature decreases, and anions catalyze the formation of an SEI enriched in inorganic components33. WSE derived SEI contains the most inorganic compounds, especially NaF, which originates from the decomposition of PF6− in the solvation structure. Although NaF has high mechanical strength and can effectively protect the electrode34, excessive NaF will increase the interfacial resistance and hinder the migration of Na+35,36. In contrast, CIE derived SEI contains an appropriate amount of NaF, which not only protects the electrode but also does not affect the rapid transport of Na+. In addition, we characterized the HC electrode with different etching depths after low-temperature cycling. As etching progresses, it is found that the peak intensity of some inorganic components (such as Na2CO3 and NaF) in SEI decreases. This indicates that the inorganic components within the SEI are predominantly distributed in the outer layer. By comparing the variation in F element content at different etching depths (Figs. 4a-4b, and S22), CIE shows a more significant decrease in F element content after the first etching compared to WSE, and the same trend was observed for Na2CO3. This indicates that inorganic components of SEI in CIE are confined to a thinner surface layer. The structure of SEI was further tested by high-resolution transmission electron microscopy (HRTEM). As shown in Figs. S23-24, the HC negative electrode using CCE shows the thickest (~32 nm) and inhomogeneous SEI at 25°C. While the SEI of WSE and CIE is thinner, especially the SEI of CIE, which is only about 16 nm. This is attributed to the participation of anions in the decomposition of CIE and WSE to form thinner SEI. At low temperatures, severe electrolyte decomposition leads to thickening of the SEI. However, even at −40°C, the SEI of the HC negative electrode using CIE is only 30 nm (Fig. 4c), while the SEI of WSE increases to 44 nm thickness at low temperatures (Fig. 4d). Thicker SEI hinder the transport of Na+, resulting in poor performance.

Time of flight secondary ion mass spectrometry (TOF-SIMS) is a characterization method capable of obtaining the 3D structure of SEI37. Figures S25a and 4e show the variation in the content of the main substances on the HC electrode using the CIE electrolyte with sputtering time after cycling at 25°C and −40°C, which is mainly composed of organic components (C2HO-, CH2O-, PO2-) and inorganic components (NaF2-, NaCO3-, NaO-). The peak intensity of each component in the SEI formed at 25°C is slightly lower than that at −40°C, indicating that low temperature will cause more decomposition of the electrolyte, which is consistent with the XPS results. Moreover, the structure of SEI formed by WSE at −40°C was also tested (Fig. 4f). It can be clearly observed that the peak intensity in CIE shows a sharp decrease after ~10 s of sputtering, which indicates that the electrolyte decomposition components in CIE form a thinner layer concentrated at the electrode surface. While the peak intensity of WSE decreases more gently. In addition, 3D reconstructed images visualize the depth structure of the SEI (Figs. 4g, S25b and S26), where it is evident that the WSE-formed SEI has a higher content of inorganic compounds and is evenly distributed in deeper layers of the SEI. In contrast, in CIE, both organic and inorganic compounds in the SEI are highly concentrated at the surface (especially NaCO3- and NaO-), forming a thinner and denser SEI. In conclusion, although WSE forms an inorganic-rich SEI, the excess anions in the solvation sheath cause the SEI to be too thick, which increases the interfacial resistance. Whereas, CIE is able to form an inorganic-rich, thin, and dense SEI, which effectively promotes the transport of ions across SEI (Fig. 4h)38. Moreover, the thinner SEI will reduce the consumption of Na+, which is the reason why CIE can enable HC negative electrode to have high ICE at both room temperature and low temperatures.

Co-intercalation behavior and ion diffusion
As confirmed in the above characterizations, the solvation structure of CIE is invariant at 25°C and low temperatures, retaining the chelating coordination structure of G2 with Na+. To confirm the co-intercalation behavior, the sodium storage process was further investigated by in situ Raman (Fig. 5a, b). There are two characteristic peaks at 1350 cm−1 and 1580 cm−1 corresponding to D and G peaks, respectively. As the discharge proceeds, the G peak redshifts, which is due to the insertion of Na+ increasing the layer spacing and electron density, thus weakening the C-C bond39. When the discharge depth was further increased, the characteristic peak of the intercalation compound appeared at 1475 cm−1 for the HC electrode in both electrolyte systems. Interestingly, when CIE is used, a new characteristic peak appeared near 1083 cm−1, which corresponds to the Na+-G2 chelate co-embedded in the carbon layer40,41, while this peak did not appear when WSE is used. This indicates that the solvent G2 can be co-embedded in the carbon layer with Na+ in CIE, whereas the solvent is completely desolvated in WSE and only Na+ enters the carbon layer. To investigate the behavior of the co-intercalation in a low-temperature environment, we discharged the Na||HC cells using CIE and WSE to different potentials at −40°C and then took out the HC electrodes for testing. To ensure credibility of the results, the electrolyte was repeatedly cleaned using solvents to eliminate residual electrolyte. As shown in Figs. 5c and S27, the G peaks are shifted compared to the raw electrodes, and the characteristic peaks of the Na-C intercalation compounds appear, which is the same as those observed at 25 °C. Moreover, the characteristic peak of G2 co-embedded in the carbon layer appears only when CIE is used. Figure 5d shows the XRD pattern of the HC electrode discharged to 0 V under different conditions. Compared with the raw electrode, the electrode discharged to 0 V has some small sharp peaks at 30–35° corresponding to the characteristic peaks of the quasi-metallic state Na42, which indicates that the electrode is in a fully discharged state. More importantly, when discharged to 0 V, the (002) peak of HC is shifted to a lower angle. When WSE is used, only Na+ is embedded in the carbon layer, and the peak shift is smaller. When CIE is used, the (002) peak shifts to a larger extent, which indicates that the solvent is co-embedded in the carbon layer with Na+. In addition, when charged to 2.5 V, the (002) peak returns to the same angle as the raw electrode (due to the similarity of the XRD pattern of each group of samples after complete desodiation, only one group of samples is provided here for comparison), which indicates that the co-intercalation has high reversibility. Furthermore, we demonstrated the absence of a desolvation process by comparing liquid NMR with magic-angle spinning solid-state NMR (MAS NMR), as shown in Fig. S28. The peak of Na+ in HC after using CIE is 0.9 ppm, exhibiting only a very slight shift compared to the Na⁺ peak in the electrolyte (1.3 ppm). The slight downfield shift in the signal is attributed to the deshielding effect caused by the ring current of delocalized π electrons in the hard carbon and the paramagnetism of the interaction43. This indicates that during the process of Na⁺ embedding into the carbon layer, its solvation environment remains intact without undergoing desolvation. However, in WSE, the MAS NMR and liquid NMR spectra exhibit significant differences, with a broad peak appearing at 8.6 ppm in the MAS NMR. This corresponds to the bare Na+ into the carbon layer44,45, indicating that WSE has undergone a complete desolvation process. We also calculated the stepwise desolvation energy via DFT for different solvation structures (Table S3)46. The chelation coordination structure of CIE has a desolvation energy of 25.5 kJ/mol for the first G2 and 28.3 kJ/mol for the second G2. These two values are very close and both significantly higher than the desolvation energy for removing the MO (12.3 kJ/mol). This also suggests that partial desolvation within the CIE is theoretically unlikely, enabling two G2 molecules and one Na⁺ to embed as a whole into the carbon layer without desolvation at the interface.

The fundamental difference between desolvation and co-intercalation behavior lies in the disparity of solvation structures. For WSE, due to the weak interaction between MO solvents, the energy required to overcome the solvation energy barrier is relatively low. This enables the MO to desolvate easily and completely from the solvation sheath, with only Na+ entering the interlayer, which is a highly desolvation process. For CIE, G2 exhibits strong interactions with Na⁺, resulting in a high desolvation energy barrier that makes the desolvation process difficult. Thus, the fundamental difference between the two systems arises from the distinct interactions between the solvent and Na+. In CIE, the strong solvent-ion interactions result in a high desolvation barrier, which drives the system to adopt co-intercalation. Under electric field drive, the stable chelating solvation structure formed by G2 and Na⁺ undergoes directed diffusion to SEI. Within the SEI, organic and inorganic components cannot achieve atomic-level compaction, inevitably leaving nanoscale voids or grain boundaries. These minute, interconnected voids provide pathways for solvated Na⁺ clusters. Meanwhile, the CIE-formed SEI exhibits thinner layers, facilitating rapid penetration of solvated clusters through the SEI without desolvation, ultimately embedding them into the carbon layers.

In order to deeply analyze the diffusion kinetics of different electrolytes in the HC bulk phase, a series of experiments and theoretical calculations were carried out. Firstly, the ion diffusion process in HC materials at low temperature was investigated by GITT (Fig. S29). CIE has higher diffusion coefficients in both the discharge and charge processes (Figs. 5e and S31). Especially after being discharged to 0.6 V, the diffusion coefficients of CIE show an obvious gap compared with WSE, and at this time, the intercalation process starts. Higher diffusion coefficients indicate that co-intercalation favors ion transport in the bulk phase. Then, the diffusion behavior of Na+ is analyzed by in situ impedance (Fig. S32). We fitted the Rct (charge transfer impedance) values to analyze the specific interface process (Fig. S33). The two electrolytes show a similar trend: during the initial discharge (OCV-1 V), Rct gradually increases, which corresponds to the polarization process. Subsequently, the increased Na+ adsorbed on the active sites result in a decrease in Rct (1 V–0.5 V)47. The additional semicircle in WSE between 0.2 V–0.4 V is due to the difference in interface and ion transport between the two electrolytes. The thickness, uniformity, and rate of ion transport of the interface may all contribute to additional impedance characteristics48. The 0.2–0.4 V range corresponds to the process of ion embedding between layers. For CIE, its unique co-intercalation mechanism enables high ion transport rates, with Na+ rapidly intercalating between layers and exhibiting small concentration polarization. Such rapid ion migration can cause the interfacial impedance to overlap with the charge transfer process, making it impossible to distinguish as an independent semicircle in the Nyquist plot. However, in WSE, slow ion transport and thicker SEI contribute to additional distinguishable impedance feature. As the discharge proceeds further, Na+ diffuses in the bulk phase, the electrochemical behavior gradually stabilizes, and the change of Rct tends to be gentle. The trend of Rct values during charging process is mirror image of the discharging process, which indicates that the sodium storage process in HC is reversible49. For CIE, this also proves the reversibility of the co-intercalation behavior. The value of Rct at the end of the charging process is smaller than the value at the beginning of the discharge, which is due to the formation of a stable SEI during the first charging and discharging process, thereby accelerating the ion diffusion kinetics50. Moreover, due to the presence of unavoidable defects in the HC material, some Na+ is irreversibly adsorbed on the HC surface, increasing the conductivity and reducing the Rct value. Regardless of this, CIE always has smaller Rct values, indicating a faster charge transfer rate. Especially during the discharge process, the Rct values are significantly smaller than those of WSE, which can be attributed to the higher ionic conductivity across the interface. We further investigated the diffusion kinetic mechanism of the co-intercalation behavior by DFT calculations. As shown in Fig. 5f, a chelate of Na+-G2 is introduced into the two carbon layers, and the diffusion barrier is calculated by optimizing different migration paths (Fig. S35). Due to the electron-donating effect of oxygen in the solvent G2, the interaction of Na+ with the carbon layers is weakened (Figs. 5g, S33)51. Meanwhile, the co-embedding of the solvent also widens the layer spacing. The synergistic effect of these two key factors leads to a small diffusion energy barrier (0.163 eV, Fig. 5h), which is lower than that of Na+ in the interlayer (0.224 eV, Fig. S36). Such a lower diffusion energy barrier explains the merit of solvent co-intercalation mechanism.

Electrochemical performance of pouch cell
To verify the practical applicability of CIE, we assembled a 1.2 Ah (163 Wh kg−1) pouch cell with O3-type NFM positive electrode and HC negative electrode (Fig. 6a). Figure 6b shows the cycling performance of the pouch cell at different temperatures. At 25°C, the pouch cell provides a capacity of 1.18 Ah. As the temperature decreases, we verify the possibility of its operation at low temperatures, at −50 °C, the pouch cell still provides a capacity of 0.79 Ah (107 Wh kg−1), demonstrating a significantly better low-temperature adaptability than that of conventional electrolytes. Moreover, even at −50°C, the charge/discharge curve maintains a stable voltage plateau (Fig. 6c). Figure 6d shows the cycling performance of the pouch cell at −20°C with 94.1% capacity retention after 300 cycles. In addition, in order to visually verify the practical application potential, the LED lights are continuously powered at a low temperature of −50°C. The fully-charged state pouch cells keep the LED lights working for more than 10 hours (Fig. 6e). This demonstration confirms that the CIE has good prospects for practical applications, especially at low temperatures. We further compare our results with some previously reported low-temperature sodium-ion pouch cells (Fig. 6f, Table S6)26,52–58, and our work is comparable to most of the advanced reported works, which fully proves that the CIE developed by us has important application value for SIBs in cold environments. More detailed pouch cell parameters are shown in Table S7.

In summary, we have developed a co-intercalation electrolyte based on the solvent co-embedding mechanism for efficient Na+ ion storage in HC at low temperatures. The MO solvent is added into linear ether G2 to precisely regulate the solvation structure and expand the electrolyte’s low-temperature range via an entropy-driven effect. Experiments and simulations reveal that the Na+-G2 chelating coordination structure can be well co-embedded in HC negative electrode, avoiding the sluggish traditional desolvation process and significantly improving ion diffusion at low temperatures. Meanwhile, the high sodium salt dissociation ability of G2 ensures good ionic conductivity in electrolyte. And the dipole interaction between solvents drives an appropriate number of anions into the solvation sheath, inducing the formation of a thin layer of inorganic-rich SEI. This multi-scale synergistic design enables highly reversible and high-rate sodium storage at low temperatures. Consequently, the CIE enables HC to deliver an initial Coulombic efficiency of 80.5% at −50°C and a capacity retention of 93% after 200 cycles. Moreover, an Ah-level full cell retains 163 Wh kg−1 at 25 °C and 107 Wh kg−1 at −50 °C. This study demonstrates the significance of the solvent co-embedding mechanism in improving low-temperature ion storage and provides a strategy for designing electrolytes toward cold environments.

Methods
Materials
The sodium hexafluorophosphate (NaPF6, 99.8% min), diethylene glycol dimethyl ether (G2, 99.8% min) and 2-methyloxolane (MO, 99.8% min) were purchased from DoDoChem. Common ether electrolyte (CCE) was prepared by dissolving 0.8 M NaPF6 in G2 solvent. Weakly solvated electrolyte (WSE) was prepared by dissolving 0.8 M NaPF6 in MO solvent. Co-intercalated electrolyte (CIE) was prepared by dissolving 0.8 M NaPF6 in G2:MO=7:3 vol%. All electrolyte preparation processes were carried out in a glove box filled with Ar (H2O < 0.1 ppm and O2 < 0.1 ppm). The electrolyte salts and solvents we purchased are of anhydrous grade. After purchase, they are placed in the glove box for direct use.

Hard carbon (HC) materials were purchased from Baisige New Energy Co., Ltd. The HC negative electrode was prepared by mixing the active material, CMC (CMC2200, purchased from Daicel Corporation), SBR (BM-451B, purchased from Zeon Corporation) and conductive carbon black (weight ratio 92:1.5:3.5:3) in deionized water to form a homogeneous slurry. The slurry was spread on a copper foil (thickness 18 μm) and dried at 80°C for 8–12 h. Then, it was perforated into a circular electrode sheet with a diameter of 12 mm and a loading of 2.5–3.0 mg cm-2. Glass fiber (GF/D, Whatman) was used as separator and metallic sodium (purchased from Canrd Technology Co. Ltd.) was used as counter electrode. These components were assembled with HC negative electrode to assemble Na||HC coin cells. The diameter of the glass fiber is 16 mm and the thickness is 0.68 mm. The diameter of the sodium metal electrode is 15.6 mm and the thickness is 0.4 mm.

The Ah-level pouch batteries were composed of O3-NaNi1/3Fe1/3Mn1/3O2 (NFM) positive electrode and HC negative electrode coated on Al foil. The specific details and parameters are shown in Table S7.

Electrochemical tests
The Na||HC half cells with various electrolytes were assembled in Ar-filled glove boxes using 2032-type coin cells with Na metal and HC. The amount of electrolyte was controlled at 100 μL for each Na||HC cell. The voltage ranges of Na||HC cells and HC||NFM pouch batteries were 0 to 2.5 V and 1.8 to 3.8 V, respectively. For pouch cells, cycle at 50 mA for 2 h, followed by 100 mA for 1 h, then 200 mA for 2.5 h. After a 36 h rest period, charge at 200 mA until full capacity is reached, then discharge to 1.8 V to complete the activation process. The entire procedure is conducted at 45°C. The Na+ storage performance was examined in a galvanostatic charge-discharge test on NEWARE Battery Test System (BTS-610, Shenzhen, China). The ion conductivity of the electrolyte was measured at different temperatures using the CHI660D electrochemical workstation through electrochemical impedance spectroscopy (EIS) technology. Galvanostatic intermittent titration technique (GITT) testing was performed by NEWARE battery system. The test was performed in the third charge-discharge cycle at 20 mA g−1 with a pulse time for 30 min and the intervals for 2 h. Acquire 1 data point every 5 seconds. LSV (scan rate: 10 mV s−1, voltage range: 2.5–5.5 V) tests were implemented by CHI660D electrochemical workstation. In situ EIS test: discharge from open circuit voltage to 0 V, then charge to 2.5 V, with a specific current of 20 mA g−1. The testing frequency range is 105–10−2 Hz, with an amplitude of 5 mV.

Characterization
Fourier transform infrared (FTIR) spectra were performed to analyze the solvation structure on an FTIR Spectrometer (Nicolet iS10). Collect background spectra before testing and perform automatic subtraction. Raman spectra of the electrolytes were collected with a spectrometer (Thermo Fischer DXR, laser wavelength 532 nm, scanning range 700–1000 cm−1). The SEI on HC surface was characterized by X-ray Photoelectron Spectroscopy (XPS, ThermoFisher ESCALAB 250Xi with Al Kα radiation (1486.6 eV)). The cycled HC electrode was disassembled from the tested cell in an argon-filled glove box and rinsed with solvent, and subsequently the electrode was transferred to the instrument chamber in an inert atmosphere pending XPS testing. All XPS spectra were corrected with reference to the C 1s peak of the C-C bond located at 284.8 eV. High-resolution transmission electron microscope (HRTEM) images were obtained by the JEM-F200 transmission electron microscope at 200 kV. The 1H NMR was conducted on Bruker AVANCE spectrometer (Bruker, Germany). The 23Na solid-state NMR characterization was performed on a Bruker AVANCE III 400 MHz spectrometer at a magic angle rotation (MAS) rate of 15 kHZ. The time-of-flight secondary ion mass spectrometry (TOF-SIMS) data was collected on ION-TOF-GmbH-TOF.SIMS 5-100, Germany, equipped with a 30 keV Bi3+ gun for analysis beam, Cs+ gun for sputtering beam. We normalized TOF-SIMS data using SurfaceLab software accompanying the ION-TOF instrument. The crystal structures of the HC were characterized by a powder X-ray diffractometer (X’Pert PRO, PANalytical, 5–90° within 5 min) with Cu Kα radiation (λ=0.15406 nm).

Theoretical calculations
The binding energy between solvent molecules and Na+ was calculated on density functional theory (DFT) by the Gaussian 16 package. The geometry optimization was conducted at B3LYP/6-31+g(d) level and a higher level of B3LYP/6-311++g(d,p) was applied for the single point calculation. All calculations consider the empirical revision of DFT-D3. The solvation effect was considered with the universal solvation model of polarized continuum model (PCM). The electrostatic potential involved in the analysis was evaluated by Multiwfn.

The molecular dynamics simulations were performed using the Gromacs 2019 with all-atom optimized potential liquid simulation (OPLS-AA) force field. Force field parameters for PF6− were obtained directly from the literature, while force field parameters for other electrolyte components were generated using the LigParGen web server. Advanced restrained electrostatic potential (RESP2) atomic charges were employed, which were calculated using the Multiwfn program. The PACKMOL program was used to construct the initial simulation boxes with dimensions of 70×70×70 Å3 and filled with electrolyte components. First, the temperature was allowed to slowly ramp up from 0 K to 298 K in 1 ns, then, 10 cycles of quench-anneal kinetics were performed from 298 K to 500 K within 4 ns to eliminate the persistence of the metastable state. The particle-mesh Ewald (PME) method with a cut-off distance of 12 Å was applied to treat the electrostatic interactions and the van der Waals forces. During the pre-equilibrium simulation, the temperature is controlled by the V-rescale algorithm and the pressure is controlled by the Berendsen. Finally, MD simulations were performed in the NPT system for 20 ns at a temperature of 298 K and a pressure of 1 bar. During the final simulation, the temperature is controlled by the Nose-Hover algorithm and the pressure is controlled by the Parrinello-Rahman. The final 10-ns NPT simulation was output every 10000 steps and used to analyze the radial distribution function (g(r)) and the solvation structure. Visualization of simulated structures used the Visual Molecular Dynamics program (VMD). We used TCL script and analyzed the solvation structure based on the Python package MDTraj.

The diffusion energy barriers of Na+/Na+-G2 chelates in carbon layers were calculated using Vienna Ab-initio Simulation Package (VASP) by a method of the density-functional theory (DFT). The generalized gradient approximation (GGA), as implemented in the Perdew-Burke-Ernzerhof (PBE) functional, was employed for exchange-correlation. Dispersion effects were accounted for using the DFT-D3 method. A cutoff energy of 400 eV was applied throughout the calculations. To achieve accurate results, partial occupancy of the Kohn-Sham orbitals was allowed, utilizing the Gaussian smearing method with a width of 0.05 eV. These settings ensured energy convergence to within 10−5 eV. Atomic positions were relaxed until the forces on each atom were smaller than 0.03 eV Å−1. Visualization was carried out using VESTA software. The atomic coordinates from calculations have been provided in the “Supplementary Data” file.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Zhang et al. developed an ether-based electrolyte (CIE) for low-temperature sodium-ion batteries based on the solvent co-embedding mechanism. By introducing MO solvent to regulate the solvation structure of linear ether G2, the entropy effect is utilized to broaden the low-temperature adaptation window of the electrolyte. This collaborative design achieved reversible sodium storage at low temperature and high rate: The hard carbon anode achieved an initial efficiency of 80.5% at -50 ℃, and the capacity retention rate was 93% after 200 cycles. The Ah-class full battery maintains an energy density of 107 Wh/kg at -50 ℃. This work reveals the breakthrough significance of the solvent co-embedding mechanism for low-temperature ion storage. The following comments may help the author to improve the manuscript.\n1. In the introduction, the author writes: “Herein, we design a co-intercalation electrolyte (CIE) toward HC anode, successfully overcoming the trade-off between Na⁺ desolvation and ionic conductivity in electrolyte. Our CIE allows ions to bypass the slow desolvation process at interface and achieve sufficient diffusion in electrolyte and HC simultaneously.” Moreover, the main text also repeatedly emphasizes that the designed electrolyte only undergoes intercalation at the negative electrode and there is no desolvation behavior. However, the data in the article cannot fully support the author's core viewpoint. The fact that the original Raman and other characterizations can prove the existence of a solvent-sodium ion coordination structure in hard carbon does not indicate that the negative electrode is entirely a coordination structure and there is no desolvation process at the interface. Please supplement relevant characterization data to further support your viewpoint.\n2. “As shown in Figure 3a, CIE provides a specific capacity of 300.5 mAh g-1 and an initial Coulombic efficiency (ICE) of 93.7% during the first cycling at 25 ℃, while CEE and WSE provided specific capacities of 299.6 and 300.1 mAh g-1 and ICE of 87.7% and 91.4%, respectively.” Why is the initial efficiency of the HC anode in this article so high, especially for the comparison sample CCE? In the research on the pre-sodization of HC anodes, the comparison samples mostly used 1M NaPF6 in diethylene glycol dimethyl ether (G2), but the ICE was only 65%-70%. Similarly, 0.8 M NaPF6 in diethylene glycol dimethyl ether (G2) was attempted, and the ICE was only 68%. Why is the ICE level so high in this article?\n3. When comparing CIE and WSE electrolytes, authors should focus on discussing the underlying reasons for the differences in desolvation behavior and co-intercalation behavior of their solvation structures, rather than analyzing changes in interfacial composition and impedance. Please make relevant discussions and analyses in the text. Additionally, it is necessary to explain that at the anode interface, WSE undergoes a complete ion desolvation process, whereas CIE maintains the integrity of its solvation structure and reacts through a co-intercalation mechanism. However, why do both WSE and CIE exhibit excellent rate performance at low temperatures?\n4. the author describes the formation of the interface between CIE electrolyte and HC anode as follows:“For CIE, it always maintains the solvation sheath of the CIPs, a co-embeddable G2-Na+ chelating structure is included, and a moderate amount of anions is also introduced, which facilitates the formation of anion-derived, more inorganic, thinner SEI. In addition, the temperature-stable solvated structure in CIE guarantees the same co-embedded behavior at low temperatures as at room temperature.”The author points out that the electrolyte always maintains co-embedding behavior regardless of whether it is at room temperature or low temperature. The core function of the traditional SEI is to \"allow ion conduction but prevent solvent molecules from directly contacting the electrode active material\", in order to avoid continuous solvent decomposition and electrode structure damage. The CIE strategy explicitly relies on the \"overall embedding of sodium solvate ions (Na⁺ -solvent clusters) into hard carbon\", which means that solvent molecules will inevitably enter the electrode interior along with the ions. This is fundamentally in conflict with the \"solvent barrier\" function of the SEI on the surface. How to explain?\n5. Recent related publications on low temperature electrolyte design are suggested to referred, e.g., Interdiscip. Mater. 2023 ;2 (4): 569-588；Interdiscip. Mater. 2023; 2(6): 833-854.", "reviewer_2": "The topic of this manuscript is for developing low-temperature electrolyte for hard carbon anode material, which is highly relevant and timely in the field of energy storage devices for usage at low-temperature. The authors have made a significant effort to develop a new concept namely co-intercalation ether electrolyte (CIE), which helps to improve the Na+ desolvation kinetics. The manuscript is relatively complete so after solving these issues the reviewer believes this manuscript is suitable for nature communications for publication.\n1. What is the most critical parameter for formulating a CIE electrolyte? the selection of solvent or salt, or pore of hard carbon?\n2. Figure 1 concluded the difference among CCE, WSE and CIE. However, I am a little confused that why WSE will not freeze and forming a dual anion solvation structure? The number of anions in electrolyte should be the same with cation.\n3. Although the main idea is for the co-intercalation of electrolyte into hard carbon anode, this reviewer still would like to know the basic performance of the electrolyte such as the ionic conductivity, electrochemical stability window and sodium transference number. These physical parameters are also crucial, especially as a reference for the peer researchers.\n4. In figure 5f the in-situ EIS of CIE and WSE seems different at 0.2 to 0.4 V. There is an additional semicircle in Nyquist plot of WSE. Why is this happening?\n5. XPS etching is more likely to describe SEI layer on hard carbon. The authors may need to supplement two XPS etching results together with TOF-SIMs to evaluate the difference in thickness and component of SEI layer with different electrolyte.", "reviewer_3": "This study introduces a novel co-intercalation ether electrolyte (CIE) to overcome the long-standing trade-off between Na⁺ desolvation and diffusion in hard carbon anodes for sodium-ion batteries at ultra-low temperatures. By enabling solvent co-intercalation, it bypasses sluggish desolvation and ensures rapid ion transport, leading to excellent low-temperature performance including an initial Coulombic efficiency of 80.5% at -50°C and a capacity retention of 93% after 200 cycles, even demonstrating 107 Wh kg⁻¹ in Ah-level full cells at -50°C. The authors should address the following issues:\n1. There are two abbreviations for one full name: CCE and CEE\n2. Explain in detail why the peak of Na-P (PF6-) is quite large at 25 degrees Celsius in Figure 2-e, but suddenly decreases at -40 degrees Celsius. Figure 2g also has a similar problem.\n3. You wrote \"It can be found that the chemical shifts of G2 are shifted to the downfield after mixing of the two solvents, which implies that there is a dipole interaction between MO and G2.\". I suggest that the authors calculate the interaction between MO and G2 directly via DFT.\n4. It would be better to calculate the desolvation energy of the solvent more rigorously. See Table 2 of Journal of Molecular Liquids 395 (2024) 123817."}, "author_response": "Dear Editor and Reviewers,\nWe sincerely thank you for processing our manuscript (Title “Solvent Co-Intercalation Electrolyte\nUnlocks Ah-Level Sodium Storage in Hard Carbon at Ultra-Low Temperatures”, Manuscript. ID:\nNCOMMS-25-46390). We appreciate the opportunity to address and clarify the issues raised in the\nreviewers’ report. We have carefully considered and responded to all comments to the fullest extent\npossible. For the ease of reference, the reviewers’ comments and suggestions are reproduced in black,\nour response is in blue. All changes in the revised manuscript and supporting information have been\nhighlighted in yellow. We hope that the revised manuscript will be suitable for publication in this\nrenowned journal.\nResponse to reviewers’ comments:\nReviewers #1 ................................................................................................................................ 2\nReviewers #2 ................................................................................................................................ 7\nReviewers #3 .............................................................................................................................. 14\nReviewer #1:\nZhang et al. developed an ether-based electrolyte (CIE) for low-temperature sodium-ion batteries\nbased on the solvent co-embedding mechanism. By introducing MO solvent to regulate the solvation\nstructure of linear ether G2, the entropy effect is utilized to broaden the low-temperature adaptation\nwindow of the electrolyte. This collaborative design achieved reversible sodium storage at low\ntemperature and high rate: The hard carbon anode achieved an initial efficiency of 80.5% at -50 ℃,\nand the capacity retention rate was 93% after 200 cycles. The Ah-class full battery maintains an energy\ndensity of 107 Wh/kg at -50 ℃. This work reveals the breakthrough significance of the solvent co-\nembedding mechanism for low-temperature ion storage. The following comments may help the author\nto improve the manuscript.\nResponse: We sincerely thank Reviewers for taking the time to carefully review our manuscript. We\nhave made further revisions to our manuscript based on your questions and suggestions. Please see\nbelow our point-by-point responses.\n1. In the introduction, the author writes: “Herein, we design a co-intercalation electrolyte (CIE) toward\nHC anode, successfully overcoming the trade-off between Na⁺ desolvation and ionic conductivity in\nelectrolyte. Our CIE allows ions to bypass the slow desolvation process at interface and achieve\nsufficient diffusion in electrolyte and HC simultaneously.” Moreover, the main text also repeatedly\nemphasizes that the designed electrolyte only undergoes intercalation at the negative electrode and\nthere is no desolvation behavior. However, the data in the article cannot fully support the author's\ncore viewpoint. The fact that the original Raman and other characterizations can prove the existence of a\nsolvent-sodium ion coordination structure in hard carbon does not indicate that the negative electrode\nis entirely a coordination structure and there is no desolvation process at the interface. Please\nsupplement relevant characterization data to further support your viewpoint.\nResponse: Thanks for the reviewer’s valuable and insightful comment. We have carefully considered\nthe characterization methods to demonstrate the absence of a desolvation process and referred to\nrelevant literature. We ultimately obtained compelling evidence through a comparison of liquid and\nmagic-angle spinning solid-state nuclear magnetic resonance (MAS NMR) data:\nWe sodiated HC using different electrolytes at -40 ℃, then removed it and repeatedly cleaned it with\nsolvents to eliminate the influence of residual sodium salts on the 23Na NMR peak. The results are\nshown in Figure S28. The peak of Na+ in HC after using CIE is 0.9 ppm, exhibiting only a very slight\nshift compared to the Na⁺ peak in the electrolyte (-1.3 ppm). The slight downfield shift in the signal is\nattributed to the deshielding effect caused by the ring current of delocalized π electrons in the hard\ncarbon and the paramagnetism of the interaction.1 This indicates that during the process of Na⁺\nembedding into the carbon layer, its solvation environment remains intact without undergoing\ndesolvation. However, in WSE, the MAS NMR and liquid NMR spectra exhibit significant differences,\nwith a broad peak appearing at 8.6 ppm in the MAS NMR. This corresponds to the bare Na+ into the\ncarbon layer,2,3 this indicates that WSE has undergone a complete desolvation process.\n40 ℃.\nIn addition, we calculated the stepwise desolvation energies for different solvation structures, with\nresults shown in the Table S3. The chelation coordination structure of CIE has a desolvation energy\nof 25.5 kJ/mol for the first G2 and 28.3 kJ/mol for the second G2. These two values are very close and\nboth significantly higher than the desolvation energy for removing the MO (12.3 kJ/mol). This\neliminates the possibility of partial desolvation within the CIE, enabling two G2 molecules and one\nNa⁺ to embed as a whole into the carbon layer without desolvation at the interface.\nTable S3. The desolvation energy of different solvents.\nReaction ΔG (kJ/mol)\nNa+-(G2)\nn\nNa+-(G2)\n2\n→Na+-(G2)\n1\n+ G2 25.5\n(n = 1, 2) Na+-(G2) →Na+ + G2 28.3\nIn summary, in situ Raman spectroscopy confirms the presence of solvent in the carbon layer, while\nMAS NMR and theoretical calculations further confirm that no desolvation occurred at the interface:\nthe signal characteristics of Na⁺ in the solid NMR (in HC) and liquid NMR (in the electrolyte) were\nhighly similar, indicating that the solvation structure was co-embedded as a whole in the carbon layer.\nTherefore, based on the above data, we believe that CIE does not undergo desolvation at the interface.\nThanks again for your insightful question, which helped us improve the rigor of our paper. We have\nadded the above content to the revised manuscript in Page 11.\n2. “As shown in Figure 3a, CIE provides a specific capacity of 300.5 mAh g-1 and an initial Coulombic\nefficiency (ICE) of 93.7% during the first cycling at 25 ℃, while CEE and WSE provided specific\ncapacities of 299.6 and 300.1 mAh g-1 and ICE of 87.7% and 91.4%, respectively.” Why is the initial\nefficiency of the HC anode in this article so high, especially for the comparison sample CCE? In the\nresearch on the pre-sodization of HC anodes, the comparison samples mostly used 1M NaPF in\n6\ndiethylene glycol dimethyl ether (G2), but the ICE was only 65%-70%. Similarly, 0.8 M NaPF in\n6\ndiethylene glycol dimethyl ether (G2) was attempted, and the ICE was only 68%. Why is the ICE level\nso high in this article?\nResponse: We appreciate the reviewers’ valuable comments. The 87.7% ICE of the comparative\n3\nsample CEE can be explained by the following factors:\nFirstly, HC materials from different sources or synthesis processes exhibit significant variations in\nspecific surface area, pore structure, and defect concentration, which directly influence the ICE.\nFor the study of presodiation of HC anodes, in order to highlight the modification effect, HC materials\nwith high defect content may be used, which have high capacity in the slope region but low ICE.\nSome works improved ICE by reducing the specific surface area and defect concentration of HC,\nthereby giving it more platform capacity. For examples, when using 1M NaPF in G2 as the electrolyte,\nthe\nof 88.5%, Zhang et al. (Adv. Mater. 2024, 2412989) achieved an ICE of 90%, while that of Chou et\nal. (Angew. Chem. Int. Ed. 2024, 63, e202406889) reached 91.45%. These results sufficiently\ndemonstrate that the structure of HC directly influences the ICE.\nTo meet practical application requirements, the HC material in our work has lower specific surface\narea and fewer surface defects (I /I =1.54), resulting in an ICE of 87.7%. More importantly, the CIE\nD G\ndeveloped in this work further improves the ICE of HC to 93.7%, highlighting its advancement.\nOnce again, thank you for your thoughtful comments. We hope that the above explanation helps to\nclarify and resolve those concerns.\n3. When comparing CIE and WSE electrolytes, authors should focus on discussing the underlying\nreasons for the differences in desolvation behavior and co-intercalation behavior of their solvation\nstructures, rather than analyzing changes in interfacial composition and impedance. Please make\nrelevant discussions and analyses in the text. Additionally, it is necessary to explain that at the anode\ninterface, WSE undergoes a complete ion desolvation process, whereas CIE maintains the integrity of\nits solvation structure and reacts through a co-intercalation mechanism. However, why do both\nWSE and CIE exhibit excellent rate performance at low temperatures?\nResponse: Thanks for the reviewer’s valuable comment.\nFirstly, we delved into the fundamental causes underlying the differences in desolvation and co-\nintercalation behavior. The essence lies in the difference in solvation structure, which is mainly\ndetermined by the strength of the solvent-Na+ interaction. Based on our previous calculations of the\nbinding energies between different solvents and Na+, the binding energy between the G2 molecule and\nNa⁺ is significantly stronger than that of the MO molecule (-0.29 eV vs. -0.13 eV). Moreover, we\ncalculated the ΔG for complete desolvation of the solvent in the solvation shell for different\nelectrolytes (Table S3). For WSE, due to the weak interaction between MO solvents, the energy\nrequired to overcome the solvation energy barrier is relatively low. This enables the MO to desolvate\neasily and completely from the solvated sheath, with only Na+ entering the interlayer, which is a highly\ndesolvation process.\nFor CIE, G2 exhibits strong interactions with Na⁺. The ΔG for removing one G2 is 25.5 kJ/mol, while\nthat for removing the second G2 is 28.3 kJ/mol. Such high desolvation energy barrier makes the\ndesolvation process extremely difficult. Consequently, CIE adopts a unique solvent co-intercalation\nmechanism, in which two G2 molecules and one Na+ enter the carbon interlayer as an intact complex,\nthereby avoiding the high energy penalty of desolvation. The essence of this behavior is that the strong\nsolvation structure tends to maintain its stability and integrity to minimize the energy barrier.\nThus, the fundamental difference between the two systems arises from the distinct interactions\n4\nbetween the solvent and Na+. In CIE, the strong solvent-ion interactions result in a high desolvation\nbarrier, which drives the system to adopt co-intercalation.\nThe good rate performance of WSE at low temperatures can be attributed to following factors:\nFirstly, in WSE, MO is a weakly coordinated solvent with the core advantage of extremely low\ndesolvation energy. At low temperatures, the desolvation process is a rate controlled step of ion\ntransport. Although WSE cannot avoid the desolvation process like CIE, the energy barrier required\nfor weakly solvated electrolyte desolvation is relatively small, making it a common approach to\nimprove the low-temperature performance of electrolytes (e.g., Nat Commun. 2023, 14, 8326; Angew.\nChem. Int. Ed. 2024, e202400539).\nIn addition, WSE can form an anion-derived SEI. Although its performance is inferior to CIE,\ncompared to other electrolytes, WSE’s SEI contains more inorganic components, which effectively\naccelerates Na+ transport. Consequently, WSE exhibits favorable rate performance even at low\ntemperatures. More significantly, at low temperatures, CIE bypasses the desolvation process due to\nthe co-intercalation mechanism, and an appropriate number of anions participate to form a thinner SEI,\nmaking its rate performance superior to WSE.\nWe have included the above relevant content in the revised manuscript and highlighted it in yellow.\n4. the author describes the formation of the interface between CIE electrolyte and HC anode as follows:\n“For CIE, it always maintains the solvation sheath of the CIPs, a co-embeddable G2-Na+ chelating\nstructure is included, and a moderate amount of anions is also introduced, which facilitates the\nformation of anion-derived, more inorganic, thinner SEI. In addition, the temperature-stable solvated\nstructure in CIE guarantees the same co-embedded behavior at low temperatures as at room\ntemperature.” The author points out that the electrolyte always maintains co-embedding behavior\nregardless of whether it is at room temperature or low temperature. The core function of the traditional\nSEI is to \"allow ion conduction but prevent solvent molecules from directly contacting the electrode\nactive material\", in order to avoid continuous solvent decomposition and electrode structure damage.\nThe CIE strategy explicitly relies on the \"overall embedding of sodium solvate ions (Na⁺ -solvent\nclusters) into hard carbon\", which means that solvent molecules will inevitably enter the electrode\ninterior along with the ions. This is fundamentally in conflict with the \"solvent barrier\" function of the\nSEI on the surface. How to explain?\nResponse: Thanks for the reviewer’s valuable comment and question. You pointed out a contradiction\nbetween the core function of SEI (isolating solvents) and the solvent co-embedding behavior in CIE\nstrategy. We would like to take this opportunity to clarify in detail the unique ion transport mechanism\nin CIE and the specific role of the SEI.\n(1) Unique solvation structure\nIn CIE, Na+ is tightly surrounded by G2 molecules with chelating coordination. In this structure, the\nsolvent has a strong interaction with Na+, making it difficult to desolvation. Therefore, during the\nelectric field driven transport of Na+ to the HC anode, G2 molecules and Na+ form stable clusters,\nwhich can be regarded as a whole and embedded together in the carbon layer along the diffusion path\nof Na+ in the SEI.\n(2) The objects affected by SEI’s barrier function\nsolvation structure, which preferentially decompose over solvent molecules to form a thinner SEI layer\nwith high ionic conductivity, thereby facilitating solvent co-intercalation.4,5 In CIE, the barrier effect\nof SEI still exists, but the main target is that free solvent molecules do not participate in coordination\nand are not driven by electric field orientation. The SEI effectively prevents these solvent molecules\nfrom directly contacting the electrode material, thereby inhibiting subsequent decomposition and\nfurther enhancing stability.\n(3) Diffusion mechanism of solvent-Na+ chelates in SEI\nThe stable chelating solvation structure formed by G2 and Na+ diffuses directionally to SEI under\nelectric field driving. Within the SEI, organic and inorganic components cannot achieve atomic-level\ncompaction, inevitably leaving nanoscale voids or grain boundaries. These minute, interconnected\nvoids provide pathways for solvated Na⁺ clusters. Meanwhile, the CIE-formed SEI exhibits thinner\nlayers, facilitating rapid penetration of solvated clusters through the SEI without desolvation,\nultimately embedding them into the carbon layers. This diffusion phenomenon is also occurred in Xie\net al.6 work for discussing the SEI formed on Nb O anode, which lithium ions can pass through the\n2 5\nnanoscale gaps between organic and inorganic substances in the SEI layer in a solvated form.\nIn summary, the CIE strategy does not deny the barrier function of the SEI, but rather focuses the\nbarrier object of SEI from all solvents to free solvent molecules that are not driven by electric fields\nthrough a unique ion transport mechanism and SEI composition. The coordinating solvents form a\nstable cluster with Na+ and pass through SEI as a whole, achieving co embedding.\nThanks again to the reviewer for raising this key question, which has prompted us to clarify the ion\ntransport and SEI mechanism of CIE strategy more clearly. Relevant discussion has been updated in\nrevised manuscript in Page 11.\n5. Recent related publications on low temperature electrolyte design are suggested to referred, e.g.,\nInterdiscip. Mater. 2023 ;2 (4): 569-588；Interdiscip. Mater. 2023; 2(6): 833-854.\nResponse: Thanks for your valuable advice. We have carefully read the references you provided and\nbased on their insights, we have improved the content of the manuscript and cited them in the revised\nmanuscript, ref 33 and 11.\nFor the first reference, a detailed review was conducted on the solvation structure of electrolytes in\ngraphite-based lithium-ion batteries, and it was pointed out that “more CIP or AGG in the first\nsolvation sheath can significantly improve the electrochemical performance of the system by\npromoting the formation of a robust anion‐derived SEI.” This viewpoint provides important theoretical\nsupport for us to explain why the SEI film structure formed by CIE at low temperatures is more stable.\nTherefore, we cited this literature in the relevant section of the manuscript discussing the stability of\nlow-temperature SEI (ref. 33). The second reference emphasizes the core role of solvation structures\nin regulating electrode/electrolyte interface reactions, and delves into the interactions between cations,\nanions, and solvent molecules. In addition, the review also points out that solvation structures and their\ninduced interface characteristics have a decisive impact on rechargeable batteries. This viewpoint is\nhighly consistent with the focus of our work, so we have cited this literature (ref. 11) in the introduction\nto better explain the research background and motivation.\nReviewer #2:\nThe topic of this manuscript is for developing low-temperature electrolyte for hard carbon anode\nmaterial, which is highly relevant and timely in the field of energy storage devices for usage at low-\ntemperature. The authors have made a significant effort to develop a new concept namely co-\nintercalation ether electrolyte (CIE), which helps to improve the Na+ desolvation kinetics. The\nmanuscript is relatively complete so after solving these issues the reviewer believes this manuscript is\nsuitable for nature communications for publication.\nResponse: We sincerely thank Reviewer’s valuable and positive comments on our manuscript. We\nhave made further revisions to our manuscript based on Reviewer’s valuable suggestions. We believe\nthat the additional supplements and the corresponding revisions to address the comments substantially\nstrengthen our manuscript. Please see below our point-by point responses.\n1. What is the most critical parameter for formulating a CIE electrolyte? the selection of solvent or\nsalt, or pore of hard carbon?\nResponse: Thanks for the reviewer’s valuable question. The selection of solvent to be the most critical\nparameter in preparing CIE. This is because the solvent directly determines the formation of the\nsolvation structure, thereby governing whether co-intercalation behavior occurs and its reversibility.\nFirstly, strong coordinating solvents are indispensable. For example, G2 can form strong interactions\nwith Na+ through chelating coordination, resulting in a high desolvation energy barrier. This ensures\nthat solvent molecules can co-embed with Na+ into HC. Secondly, to achieve co-intercalation behavior\nat low temperatures, it is also necessary to introduce another low-melting-point solvent to replace the\nnon-coordinated free solvent within the solvation structure. This solvent requires weak interactions\nwith Na+ to prevent participation in the first solvation shell, while its low melting point broadens the\nliquid phase of the electrolyte at low temperatures. Moreover, such weakly coordinated solvents can\nintroduce an appropriate quantity of anions into the outer solvation shell through dipole-dipole\ninteractions with strongly coordinated solvents. This promotes the preferential decomposition of\nanions to form a thin, stable SEI. Therefore, the properties of the solvent fundamentally determine the\nsolvation structure and co embedding behavior, which are the most critical influencing factors.\nFor sodium salts, we should select those with high ionic conductivity and easy dissociation. Altering\nthe salt type or concentration mainly influences the kinetic transport and interfacial composition of the\nelectrolyte, thereby affecting the electrochemical performance. However, these factors play an\noptimization role more on the basis of the occurrence of the co-intercalation mechanism, rather than\ndetermining whether co-intercalation itself can occur. Therefore, although the selection of sodium salt\nis very important, it is still a secondary parameter.\nAs for the pore structure of hard carbon, it may exert a certain influence on the initial coulombic\nefficiency or electrolyte wettability, but these are secondary effects that occur under the premise of co-\n7\nintercalation mechanism. The pore structure itself does not directly participate in or determine the\nstability of solvation structure and co-embedded clusters, so its influence is relatively limited.\n2. Figure 1 concluded the difference among CCE, WSE and CIE. However, I am a little confused that\nwhy WSE will not freeze and forming a dual anion solvation structure? The number of anions in\nelectrolyte should be the same with cation.\nResponse: Thanks a lot for the reviewer’s careful reading and insightful comment.\nWSE remains liquid primarily because MO has a low melting point (-136 °C), enabling WSE to\nmaintain a wide liquid range at low temperatures. Regarding the apparent formation of a multi-anion\nsolvation structure in WSE, which seems to contradict the principle of electrical neutrality, arises from\na schematic representation. Figure 1 depicts the most typical solvation structure in WSE as derived\nfrom molecular dynamics (MD) simulations.\nThe MD simulation quantifies the number of neighboring particles around Na+ over a period of motion.\nThe coordination number represents the statistical average of anions and solvent molecules at varying\ndistances from Na+ (Nat Energy. 2024, 9, 57-69.; Nat Commun. 2025, 16, 7917; Adv. Mater. 2023, 35,\n2208340). For WSE, we calculated a PF - anion coordination number of 3.05 and a MO solvent of\n6\n0.93. In addition, we have calculated the frequency of various coordination situations in WSE, and\n[Na+-anion -solvent ] has the highest frequency, representing the most typical solvation structure.\n3 1\nTherefore, we have plotted the most typical first solvation structure of WSE in Figure 1, which may\nhave caused the confusion.\nYour belief that the electrolyte is generally electrically neutral is absolutely correct. Although the most\ntypical solvation structure of WSE is a coordination of one Na+ and three anions, this does not mean\nthat all Na+ strictly maintain this coordination structure at all times. And, in our statistics, we focus on\na single Na+ as the center, but for a Na+ coordinated by three anions will also have these three anions\nshared and coordinated by other cations. In other words, an anion does not only belong to a Na+, it\nmay be present in the solvent sheath of two or more cations simultaneously. To provide you with a\nclear visual understanding, we have drawn a schematic diagram (Figure R2.1). This sharing\nmechanism ensures the presence of rich anions in the solvation structure of a single Na+, but the\nnumber of anions and cations in the entire electrolyte system is strictly equal. This explains why\nweakly solvation electrolytes form anion-rich coordination (Nat Commun. 2025, 16, 3344; Angew.\nChem. Int. Ed. 2025, 64, e202508152).\nFigure R2.1. Schematic diagram of coordination in the anion-rich solvation structure.\nIn summary, Figure 1 is intended to illustrate the typical solvation structure of a single sodium ion,\n8\nwhich may have caused some confusion. We sincerely appreciate your insightful question, which has\nallowed us to clarify this point more clearly. We hope the above explanation satisfactorily addresses\nyour concerns.\n3. Although the main idea is for the co-intercalation of electrolyte into hard carbon anode, this reviewer\nstill would like to know the basic performance of the electrolyte such as the ionic conductivity,\nelectrochemical stability window and sodium transference number. These physical parameters are also\ncrucial, especially as a reference for the peer researchers.\nResponse: Thanks for the reviewer’s valuable comment and suggestion. Following the suggestion, we\nhave supplemented the corresponding experiments and conducted systematic testing on the\nfundamental physicochemical parameters of the electrolyte including ionic conductivity,\nelectrochemical stability, and transference number).\nThe ionic conductivity of the electrolyte was tested using the AC impedance method (Figure S4 and\n2d). When the temperature is below -20 ℃, the ionic conductivity of CCE declines rapidly, whereas\nCIE maintains a high ionic conductivity at low temperatures, consistently higher than WSE. Therefore,\ncompared to WSE and CCE, our designed CIE effectively addresses the issue of low ionic conductivity\nat low temperatures, thereby promoting ion transport.\nFigure S4 and 2d. Nyquist plots of (a) CCE, (b) CIE and (c) WSE at different temperatures. (d) Ionic\nconductivity of various electrolytes at temperatures ranges from 25℃ to -50℃.\nWe assembled Na||stainless steel cells and evaluated the electrochemical stability window of the\nelectrolyte using linear sweep voltammetry (LSV). The results are shown in Figure S5, where the\noxidation decomposition potential for WSE is approximately 3.7 V, CIE is 4.2 V, and CCE is 4.3 V.\nFor SIBs, the typical operating voltage for the cathode is around 4 V, and the electrochemical stability\nwindow at 4.2 V for CIE ensures that the electrolyte works well within the normal operating voltage\n9\nrange.\nFigure S5. LSV curves of different electrolytes.\nWe measured the transference numbers of different electrolytes by combining chronoamperometry\ncurves with AC impedance (Figure S6). Tests were conducted in Na||Na symmetric cells, with\ntransference numbers calculated using the following formula:\n𝐼 (∆𝑉−𝐼 𝑅 )\ns 0 0\nt = \n+ 𝐼 (∆𝑉−𝐼 𝑅 )\n0 s s\nWhere 𝐼 is the initial current, 𝐼 is the steady-state current, 𝑅 is the pre-polarization impedance,\n0 s 0\n𝑅 is the post-polarization impedance, and ∆𝑉 is the DC bias voltage (10 mV). The calculated t for\ns +\nCCE, CIE and WSE were 0.52, 0.61 and 0.66 respectively. The anion-dominated solvation structure\ncontributes to enhanced ionic transference number, hence WSE has the highest t . Due to the\n+\nintroduction of MO, anions are introduced into the outer layer of the solvation sheath, resulting in a\nhigher ion transfer number of CIE than CCE, which is beneficial for reducing polarization and\nimproving rate performance.\nFigure S6. t and corresponding chronoamperometry profiles of Na||Na symmetric cells with (a) CCE,\n+\n(b) WSE and (c) CIE.\n4. In figure 5f the in-situ EIS of CIE and WSE seems different at 0.2 to 0.4 V. There is an additional\n10\nsemicircle in Nyquist plot of WSE. Why is this happening?\nResponse: Thanks for the reviewer’s valuable question. The additional semicircle in the Nyquist plot\nof WSE is due to the differences in the interface and ion transport between the two electrolytes. The\nthickness, uniformity, and rate of ion transport of the interface may all contribute to additional\nimpedance characteristics.7 The 0.2-0.4 V range corresponds to the process of ion embedding between\nlayers. For CIE, its unique co-intercalation mechanism enables high ion transport rates, with Na+\nrapidly intercalating between layers and exhibiting small concentration polarization. Such rapid ion\nmigration can cause the interfacial impedance to overlap with the charge transfer process, making it\ndifficult to distinguish as an independent semicircle in the Nyquist plot. However, Na+ insertion in\nWSE requires a complete desolvation process. Due to the presence of more anions in the solvation\nstructure of WSE, it will decompose in a short period of time and rapidly accumulate on the negative\nelectrode to form SEI, which is thicker.8 Therefore, in WSE, ion transport is a relatively slow step,\nthereby contributing an additional, distinguishable impedance feature (an additional semicircle). As\nthe sodiation process progresses (<0.2 V), the interlayer Na+ concentration gradually saturates,\nconcentration polarization decreases, and the additional semicircle no longer prominent. The relevant\ndiscussion has been supplemented in Page 12 in revised manuscript.\n5. XPS etching is more likely to describe SEI layer on hard carbon. The authors may need to\nsupplement two XPS etching results together with TOF-SIMs to evaluate the difference in thickness\nand component of SEI layer with different electrolyte.\nResponse: Thanks a lot for the reviewer’s careful reading and insightful comment. Following the\nsuggestion, we have supplemented the XPS depth etching data for HC electrodes after cycling at low\ntemperatures for different electrolytes (Figures S19-21).\nAs etching progresses, it was found that the peak intensity of some inorganic components (such as\nNa CO and NaF) in SEI decreased. This indicates that the inorganic components within the SEI are\n2 3\npredominantly distributed in the outer layer. By comparing the variation in F element content at\ndifferent etching depths (Figure S22), we observe that CCE consistently exhibits lower F content. In\ncontrast, WSE forms a F-rich SEI due to the presence of abundant anions in the solvation structure,\nbut this may increase interfacial resistance and hinder Na+ migration. Another noteworthy point is that\nCIE shows a more significant decrease in F element content after the first etching compared to WSE,\nand the same trend was observed for Na CO . This further indicates that SEI components in CIE are\n2 3\nmore concentrated at the surface, which is conducive to the formation of thinner SEI. This finding is\nconsistent with TOF-SIMS results.\nFigure S20 and 4a. XPS spectra of cycled HC anode after different etching times using CIE at -40 ℃.\n40 ℃.\nFigure S22. The variation of F element content in SEI layer under different etching times.\nWe believe that these supplementary experiments have helped us to analyze the differences in\nthickness and composition of SEI more deeply. We would like to reiterate our gratitude again for taking\nthe time and effort to review our manuscript. The above content has been added to the revised\nmanuscript.\nReviewer #3:\nThis study introduces a novel co-intercalation ether electrolyte (CIE) to overcome the long-standing\ntrade-off between Na⁺ desolvation and diffusion in hard carbon anodes for sodium-ion batteries for\nultra-low temperatures. By enabling solvent co-intercalation, it bypasses sluggish desolvation and\nensures rapid ion transport, leading to excellent low-temperature performance including an initial\nCoulombic efficiency of 80.5% at -50°C and a capacity retention of 93% after 200 cycles, even\ndemonstrating 107 Wh kg-1 in Ah-level full cells at -50°C. The authors should address the following\nissues:\n1. There are two abbreviations for one full name: CCE and CEE\nResponse: Thanks to the reviewer for astutely pointing out the issue of inconsistent electrolyte\nabbreviations. We have thoroughly reviewed the entire manuscript and uniformly corrected all\nreferences to the electrolyte abbreviation to CCE.\n2. Explain in detail why the peak of Na-P (PF6-) is quite large at 25 degrees Celsius in Figure 2-e, but\nsuddenly decreases at -40 degrees Celsius. Figure 2g also has a similar problem.\nResponse: Thanks for the reviewer’s valuable comment. We have carefully examined the issue you\nraised and found that it is due to color labeling errors in the data of Na-P (PF -) and Na-O (MO) in\napologies for this oversight in our manuscript, which has now been rectified.\nIn fact, Na-P (PF -) corresponds to the yellow data line in Figure 2f. When the temperature decreases\n6\nfrom 25°C to -40°C, the peak intensity of Na-P (PF -) actually increases slightly. This indicates that as\n6\nthe temperature decreases, the anion’s coordination tendency increases, leading to an increase in the\ncoordination number. This result is consistent with that shown in Figure 2g.\nFigure 2e, f. The RDF of CIE at (e) 25℃ and (f) -40℃.\nThanks for raising this issue, which has enabled us to identify this important drawing error. We have\nredrawn Figures 2e-f in the manuscript to ensure that the color labeling of all data lines is accurate and\ncorrect.\n14\n3. You wrote \"It can be found that the chemical shifts of G2 are shifted to the downfield after mixing\nof the two solvents, which implies that there is a dipole interaction between MO and G2.\". I suggest\nthat the authors calculate the interaction between MO and G2 directly via DFT.\nResponse: Thanks for the reviewer’s valuable comment and suggestion. We have calculated the\ninteraction between MO and G2 via DFT, using the same basis set for all calculations (see supporting\ninformation). The calculated binding energy between the MO and G2 molecules is -0.11 eV (Figure\nS3), this negative value indicating the presence of a spontaneous dipole-dipole interaction between the\ntwo.\nFigure S3. The binding energy between the MO and G2.\n4. It would be better to calculate the desolvation energy of the solvent more rigorously. See Table 2 of\nJournal of Molecular Liquids 395 (2024) 123817.\nResponse: Thanks for the reviewer’s valuable comment and suggestion. We have carefully read the\nliterature you recommended and calculated the desolvation energies of different solvents with\nreference to Table 2.\nThe results are shown in Table S3. The chelation coordination structure formed by G2 and Na+ has a\ndesolvation energy of 25.5 kJ/mol when the first G2 molecule is removed, and the energy required to\nremove the second G2 molecule is 28.3 kJ/mol, both of which are much higher than the 12.3 kJ/mol\nrequired to remove the MO solvent. This indicates that desolvation is difficult to occur in CIE, thereby\nenabling the possibility of solvent co-embedding. We have cited this reference in the corresponding\nsection of the revised manuscript (ref. 46).\nTable S3. The desolvation energy of different solvents.\nReaction ΔG (kJ/mol)\nNa+-(G2)\nn\nNa+-(G2)\n2\n→Na+-(G2)\n1\n+ G2 25.5\n(n = 1, 2) Na+-(G2) →Na+ + G2 28.3\nThe literature provided by the reviewers has offered valuable guidance, enabling us to enhance the\nrigor of our desolvation energy calculations. We have added the calculation results and discussions to\nthe revised manuscript, hoping that the above modifications can meet your requirements.\n15"}, {"round": 2, "reviewer_comments": {"reviewer_1": "In response to the various deficiencies and uncertainties identified in this paper, the author has conducted in-depth and clear experimental characterizations for each item and supplemented them with theoretical calculations for verification. Therefore, it is proved that the CIE does not undergo desolvation at the interface and the unique ion transport mechanism in the CIE and the specific role of the SEI are elaborated in detail. This series of improvements has greatly enhanced the quality and depth of the proposed research. Thanks to these improvements, the paper now meets the criteria for publication in the journal.", "reviewer_2": "I have carefully checked the revised manuscript and now it meets the standard of NC.", "reviewer_3": "I recommend the acceptance of this version of the manuscript."}, "author_response": ""}]

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