Introduction

Rechargeable batteries operating under extreme conditions are often required to have exceptional durability across a wide range of temperatures1,2. Yet, the temperature range of current battery technology is rather limited due to the rapid performance decay at elevated /subzero temperatures3,4,5. Electrolyte, acting as the “blood” to connect all the components in the battery, plays a vital role in determining the electrochemistry performance, especially at extreme temperatures. The viscosity and desolvation energy of the electrolyte dramatically increase at low temperatures, slowing down ion migration and subsequently leading to decreases in battery output voltage and capacity6,7. While, at elevated temperatures, electrolytes vigorously decompose at the interphase of electrodes despite of the improved ion migration kinetics, posing a serious threat to the long-term cycling stability and safety of batteries8,9,10.

Broadening the operating temperatures of batteries faces intrinsic trade-offs and limitations. Taking ethers or carboxylate esters as an example, they could serve as co-solvents in low-temperature electrolytes due to their low viscosity and freezing point, these solvents, however, significantly deteriorates the chemical stability at high temperatures11,12. Vice versa, to ensure high thermal and chemical stability at elevated temperatures, solvents such as phosphates and fluorides are introduced to the electrolytes13,14, while these solvents have poor capability to dissolve salts and compatibility with other solvents, leading to significant voltage polarization at low temperatures. Clearly, conventional electrolytes with cocktail recipe struggle to meet the demands at elevated and subzero temperatures concurrently15,16. Exploring of new electrolyte that has “temperature-adaptive” feature to simultaneously address the issues arising from both high and low temperatures is, therefore, expected to boost the electrochemical performance under extreme conditions.

Solvation structure of electrolytes plays a vital role of impacting the battery performance. There are numerous studies demonstrating that the understanding of solvation structures, although mostly conducted at ambient temperature, have achieved tremendous progress in the past years. In general, the formation of solvation structure is balanced among the intricate interplay among ion-ion, ion-solvent (ion-dipole), and solvent-solvent (dipole-dipole) interactions17,18. Through optimizing the ion-dipole interaction, the most commonly presented one in the solvation structure, electrolytes with weak solvation19,20 or highly concentrated salt21,22 have been explored with success to some extent. As temperature being changed, however, the degree of salt dissociation and the solvating capability of solvents both vary accordingly, which inevitably affects the solvation structure and, subsequently, the electrolyte properties as a whole23. Unfortunately, such a substantial temperature dependence of solvation structures has long been neglected, especially the effect of dipole-dipole interaction on the solvation structure at high and/or low temperatures, if any, remains elusive24. Considering its much higher temperature sensitivity than ion-solvent interactions25, we argue that understanding the underlying dipole-dipole interaction mechanism could help us to identify suitable solvents for different purpose, but more importantly, we may be able to design temperature responsive electrolyte through further manipulating the dipole-dipole interaction rather than the common ion-dipole interaction, which potentially opens another window for wide-temperature electrolyte design.

Through regulating the dipole-dipole interactions at various temperatures, we indeed explore an electrolyte by dissolving NaPF6 in 2-methyltetrahydrofuran (MeTHF), tetrahydrofuran (THF) and anisole (AN) dilute (denoted as SMTA), which demonstrates great temperature-adaptive feature in this work. Specifically, by adjusting the interactions between anti-solvents (AN) and co-solvents (MeTHF and THF), SMTA electrolyte achieves high thermal stability under elevated temperature and synchronously, fast kinetics at subzero temperature. We find that, at high temperature, AN shows strong interaction with MeTHF and thus suppresses parasitic reactions, while at low temperatures, AN exhibits strong interaction with THF and thereby inhibits salt precipitation. Such temperature-adaptive feature enables hard carbon anodes working stably with high capacity in SMTA electrolyte within a wide temperature range of ‒60–55 °C. Furthermore, the enhancement of the performance on a series of electrolytes validates the universality and effectiveness of this concept. Our finding offers a approach to the development of wide-temperature electrolytes.

Results

Electrolyte solvation structure

The solvation structure of Na+ in SMTA electrolyte was firstly analyzed by molecular dynamics (MD) simulation. In the equilibrated SMTA system at various temperatures, the radial distribution functions (RDFs) of Na+ with O atoms and Na+ with F atoms (Fig. 1a, b and Supplementary Fig. 1) reveal that THF, MeTHF, and PF6- enter the primary solvation sheath of Na+. Notably, the coordination number (CN) of Na+-AN remains 0 among all temperature range, indicating that AN does not participate in the primary solvation structure. With temperature dropping from 55 °C to −40 °C, the average CN of Na+ with O atoms in THF decreases from 1.22 to 1.19 while that of Na+ with MeTHF increases from 0.94 to 1.0 (Fig. 1c). The RDFs and CN results implies that as temperature drops, the solvation structure experiences a decrease in the proportion of THF and a corresponding increase in the proportion of MeTHF. In addition, the steady-state distance (r) between Na+ and solvent molecules exhibited significant variations (Fig. 1d, e and Supplementary Fig. 2). The r between Na+ and O atom in MeTHF (denoted as r1) and that between Na+ and O atom in THF (denoted as r2) were calculated at various temperatures (Fig. 1d, e and Supplementary Fig. 2), where shorter distance indicates stronger interaction26,27. In the solvation structure of SMTA, the values of r1 (2.17 Å at 25 °C and 2.56 Å at 55 °C) are significantly higher than r2 (2.09 Å at 25 °C and 2.30 Å at 55 °C), while those of r1 are smaller than r1 at −40 °C (Fig. 1e). These results indicate a stronger interaction between Na+ and THF at 55 °C and a stronger interaction between Na+ and MeTHF at −40 °C.

Fig. 1: Solvation structures of various electrolytes.
Fig. 1: Solvation structures of various electrolytes.The alternative text for this image may have been generated using AI.
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Na+ RDF in SMTA system at 55 °C (a) and −40 °C (b) obtained by MD simulations. c CN of the SMTA electrolyte at 55 °C and −40 °C. The representative primary solvation structures for the optimized SMTA systems at 55 °C (d) and −40 °C (e) obtained by MD simulations. f IR spectra obtained from selected pure solvents and electrolytes. SM: 1.0 M NaPF6 in MeTHF; ST: 1.0 M NaPF6 in THF. g The fitted Raman spectra of free MeTHF/THF molecules collected from the SMTA electrolyte solutions at 55 °C and −40 °C. h 19F NMR in the SMTA electrolyte at 55 °C and −40 °C.

Infrared spectroscopy (IR) analysis was carried out to further obtain a comprehensive understanding of the Na+-solvent complexes in the SMTA system (Fig. 1f). Taking SMTA electrolyte at 25 °C as an example, direct observation of coordination peaks corresponding to Na+-THF (1052 cm–1) and Na+-MeTHF (993 cm–1) show that both THF and MeTHF are involved in the primary solvation sheath. Meanwhile, no change in the characteristic peak of the C-O-C stretching vibration at 1040 cm–1 in the AN solvent is observed. IR observations demonstrate that the absence of coordination between Na+ and AN, which is in good accordance with MD calculation results (Fig. 1a, b and Supplementary Fig. 1). Raman spectra reveal that with temperature decreasing from 55 °C to −40 °C (Fig. 1g and Supplementary Fig. 3), the proportion of free MeTHF and THF molecules changes from 79:21 to 54:46. The variation of anions in solvation structure as function of temperature was further analyzed by 19F nuclear magnetic resonance (NMR). With temperature dropping from 55 °C to −40 °C, the F peaks in PF6- undergoes an upfield shift resulting from an increase in electron cloud density around F- (Fig. 1h). Such change could be attributed to a reduction in the interaction force between Na+ and the anions, facilitating the dissociation of salts, inhibiting the formation of unstable clusters, and ultimately enhancing conductivity of electrolyte at low temperatures. Both simulation and experimental results confirm that the solvation structure transforms from a THF-dominated configuration at high temperatures to a MeTHF-dominated solvated structure at low temperatures.

Dipole-dipole interaction in solvent-antisolvent

The molecular structures of three solvents are illustrated in Fig. 2a. The letters (from A to L) in Fig. 2a represent the hydrogen atoms at their respective positions in each molecular structure, and their 1H NMR chemical shifts are listed in Supplementary Table 1. The variations in the solvation structure of the SMTA system with temperatures were investigated through temperature-dependent one dimensional (1D) 1H NMR tests (Fig. 2b). The continuous shifts of all 1H nuclei in THF and MeTHF as the temperature decreases indicate the continuous change of the solvation structure in response to temperature. Specifically, with temperature dropping (Fig. 2b and Supplementary Fig. 4), all characteristic peaks of THF shifts upfield, and the extent of this shift gradually decreases compared to the solvation at room temperature, indicating a weakening of the interaction between Na+ and THF. On the contrary, the shift in all characteristic peaks of MeTHF continues to increase compared to that at room temperature, suggesting a strengthening of the interaction between Na+ and MeTHF. We further use two-dimensional (2D) 1H-1H correlation spectroscopy (COSY) to reveal proton coupling between various molecules (Fig. 2c, d and Supplementary Fig. 5). Specifically, the hydrogen atoms at A and B sites in THF primarily interact with the hydrogen atoms at C and D sites in MeTHF. Only the hydrogen atom at J site in the methoxy group of AN interacts with both MeTHF and THF molecules, resulting in a strong and directional coupling effect. At 55 °C and 25 °C (Fig. 3c and Supplementary Fig. 5), no coupling signals of AN with other MeTHF protons is observed except for α-H (hydrogen on carbon directly connected to the functional group) proton coupling (J, D), indicating a highly directional and stable intermolecular interaction. In sharp contrast, at −40 °C, new coupling peaks emerge at positions (J, C), (J, F), and (J, G), suggesting a reduction in directional interaction (Fig. 2d).

Fig. 2: Experimental analysis of dipole-dipole interactions between solvents.
Fig. 2: Experimental analysis of dipole-dipole interactions between solvents.The alternative text for this image may have been generated using AI.
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a Schematic illustration showing the molecular structure of three solvents. Letters from A to L represent the H locations in the molecules. b 1H NMR spectra of SMTA electrolyte at 25 °C, 10 °C, 0  °C, −10 °C, −20 °C, −30 °C and −40 °C. 1H-1H COSY NMR spectra of SMTA electrolyte at 55 °C (c) and −40 °C (d). For example, (J, A) indicates the coupling between proton at J position in AN molecule and proton at A position in THF molecule. Notably, the coupling within the green box identifies the coupling peak between protons on AN and protons on MeTHF. 1H DOSY-NMR spectra of SMTA electrolyte at 55 °C (e) and –40 °C (f). g Schematic illustration of solvent interactions in SMTA electrolyte at 55 °C and −40 °C.

Fig. 3: Systematic Investigation of Physicochemical and Electrochemical Property Alterations in SMTA Electrolyte.
Fig. 3: Systematic Investigation of Physicochemical and Electrochemical Property Alterations in SMTA Electrolyte.The alternative text for this image may have been generated using AI.
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a Optical photographs of SMT electrolyte after storage at 55 °C and –60 °C. 1H NMR of long-stored SMT (b) and SMTA (c) electrolytes at 25 °C (two weeks) and 55 °C (two days). d Differential scanning calorimetry (DSC) heating curves of SMTA electrolyte. e The viscosity and ionic conductivity of SMTA electrolyte were measured at different temperatures from –60 °C to 25 °C. f The GCD curves of HC | |Na cells equipped with the SMTA electrolyte at 55 °C, 25 °C, 0 °C, −20 °C, −40 °C, −50 °C, and −60 °C. g The GCD curve of the HC | |Na cell with SMT and SMTA electrolytes at –50 °C. h Cycling performance of HC | |Na cells with SMT and SMTA electrolytes at 55 °C. i Cycling performance of HC | |Na cells with the SMTA electrolyte at 25 °C and −40 °C at a specific current of 100 mA g-1. j The desolvation energy of THF, MeTHF in SMTA electrolyte at 55 °C and –40 °C (above). ΔE(-THF) represents the energy required to remove one THF molecule from each solvation structure, while ΔE(-MeTHF) indicates the energy for removing one MeTHF molecule. Arrhenius behaviour of the resistance corresponding to Na+ transport through SEI and charge-transfer processes (below). The semicircles in the mid-frequency and high-frequency regions represent charge transfer and ion transport properties in the SEI, respectively.

We collected diffusion-ordered spectroscopy (DOSY) to elucidate the strength of interactions among distinct solvates. At 25 °C, compared to the SMT electrolyte, the 1H DOSY NMR spectrum of the SMTA electrolyte demonstrates a decreased overlap in the diffusion dimension (D) of the 1H DOSY NMR spectra for THF and MeTHF. This observation indicates that, the dipole-dipole interactions between THF and MeTHF molecules are significantly weakened (Supplementary Figs. 6 and 7) in the presence of AN. The peak integrals are fitted to the Stejskal-Tanner equation to calculate the D (Supplementary Figs. 8 and 9). Noted that “Dbefore” represents the D of molecules in the SMT electrolyte without AN, whereas “Dafter” denotes the D of molecules in the SMTA electrolyte after the introduction of AN. A higher ratio of Dbefore /Dafter indicates a stronger interaction between AN and the solvent. The calculated Dbefore /Dafter values of MeTHF are 1.08, 1.10, and 1.14 at 55, 25, and -40 °C (Fig. 2e, f), respectively, while those of THF are 1.01, 1.05, and 1.34, respectively. 1H DOSY NMR results illustrate that AN possesses stronger binding affinity towards MeTHF at high temperatures (Supplementary Figs. 8 and 9) and stronger affinity towards THF at low temperatures. Overall, both 1D and 2D NMR results show that the solvation change is enabled by the varieties of dipole-dipole interactions between AN-MeTHF and AN-THF. As the temperature rises, the AN solvent exhibits stronger dipole-dipole interactions with MeTHF, leading to the formation of a solvent structure dominated by THF. At low temperatures, however, the interaction between AN and THF becomes stronger, resulting in the formation of a solvent structure primarily consisted of MeTHF (Fig. 2g and Supplementary Fig. 10).

Physicochemical and electrochemical properties of SMTA electrolyte

Changes in the intermolecular interactions, accompanied by alterations in solvation structure, lead to significant variations in both physical and electrochemical properties of the electrolyte across a wide temperature range. For electrolytes, the most significant challenges are severe parasitic reactions at high temperatures and salt precipitation at low temperatures. We chose 1.0 M NaPF6 dissolved in MeTHF and THF (1:1 in volume, denoted as SMT) as a control sample to illustrate the effect of AN anti-solvent. We can clearly see that the color of SMT electrolyte darkens after storage at 55 °C. Such a poor chemical stability is mainly attributed to the decomposition of cyclic ether solvents in SMT at high temperature to form alkyl species, as evidenced by 1H NMR results (Fig. 3b and Supplementary Figs. 11-12). Additionally, the 1H DOSY-NMR spectrum of the SMT electrolyte at 55 °C reveals the appearance of a methoxy peak at a chemical shift of 3.28 ppm (Supplementary Fig. 6), further confirming the decomposition and ring-opening of cyclic solvents. On the contrary, such decomposition process is evidently inhibited in SMTA system (Fig. 3a, c, and Supplementary Figs. 11-12). With temperature dropping, both SMT and SMTA shows good chemical stability after long-term storage, nevertheless, salt readily precipitates at −60 °C in the SMT electrolyte (Fig. 3a), which inevitably impairs the ion diffusion kinetics at low temperatures. Fortunately, the addition of AN to electrolyte significantly inhibits the salt precipitation at low temperatures. The SMTA electrolyte has a low freezing point of −77.5 °C, allowing to maintain a high conductivity even at low temperatures (Fig. 3d). The conductivity and viscosity of the SMTA system at different temperatures were measured (Fig. 4e). The SMTA electrolyte exhibited an impressive conductivity of 2.4 mS cm–1 and a low viscosity of 3.88 cP at –40 °C. Even at –60 °C, the SMTA electrolyte retains a conductivity of 1.57 mS cm–¹, which is sufficient to support battery operating at such an extremely low temperature. Evidently, the addition of AN not only enhances the high-temperature stability of the electrolyte but also lowers its freezing point at low temperatures, thereby enabling SMTA as an wide-temperature electrolyte.

Fig. 4: Mechanism study of SMTA electrolyte across a wide temperature range.
Fig. 4: Mechanism study of SMTA electrolyte across a wide temperature range.The alternative text for this image may have been generated using AI.
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a ESP distribution in the molecular structures of MeTHF, THF and AN. b The ring-open decomposition process of MeTHF molecules in the presence of NaPF6 salt and the mechanism of improved chemical stability by the interactions between AN and MeTHF. The symbols δ+ and δ− in the structure represent the partially positive and negative charge states, respectively. c Schematic illustration showing the mechanism of inhibiting salt precipitation through dipole-dipole interactions.

The electrochemical performance of SMTA electrolyte was investigated in HC | |Na half cells. The high ionic conductivity of the SMTA electrolyte at low temperatures favors fast Na+ transport kinetics. As revealed in cyclic voltammetry (CV) results, CV curves exhibit excellent reversibility and no significant change in redox potentials over a wide temperature range (Supplementary Fig. 13). The galvanostatic charge/discharge (GCD) curves of HC | |Na cells equipped with SMTA electrolyte at a specific current of 100 mA g-1 across a wide temperature range were shown in Fig. 3f. HC anodes exhibit high specific capacities over a wide temperature range. At 55 °C, 25 °C, 0 °C, −20 °C, −40 °C, −50 °C, and −60 °C, the discharge capacities are 301.4 mAh g–1, 293.1 mAh g–1, 265.4 mAh g–1, 246.8 mAh g–1, 225.3 mAh g–1, 180.8 mAh g–1, and 112.9 mAh g–1, respectively. The Na+ insertion/extraction process in HC anodes, which is composed of adsorption (slope region in GCD curve) and insertion/deposition (plateau region in GCD curve) process, is highly reversible even at −50 °C with SMTA electrolyte. After 200 cycles, the capacity retention is as high as 99.3% (Supplementary Fig. 14). As for SMT electrolyte, on the contrary, HC only delivers a capacity of 57.5 mAh g-1, with complete vanishing of insertion/deposition process28. The low ionic conductivity as a result of salt precipitation leads to sluggish kinetics (Fig. 3g and Supplementary Fig. 15) in SMT, preventing the effective insertion of Na+29,30. In addition to the improved kinetics at low temperatures, the cycling stability of HC operated in SMTA electrolyte is also significantly improved compared with those in SMT electrolyte especially at high temperatures. At 55 °C, the capacity retention of HC cycled in SMTA electrolyte is 73.5% after 200 cycles (Fig. 3h) while that of HC in SMT only remains 33.1% after 150 cycles. HC anode also shows high stability after extended cycling in SMTA electrolyte. The capacity retentions of HC after 700 cycles are 80.1% and 87.0%, respectively, at 25 °C and −40 °C (Fig. 3i and Supplementary Fig. 16). The role of AN ensures a rapid kinetic process and excellent cycle stability in SMTA electrolyte across wide temperatures.

Given that the desolvation process is crucial for the kinetics of Na+ storage in HC anodes31, we calculated the desolvation energies of solvation structures in SMTA electrolyte at 55 °C and –40 °C by density functional theory (DFT). As evidenced in Fig. 3j, ΔE (-THF) is significantly higher than ΔE (-MeTHF) at 55 °C, but becomes smaller with temperature dropping to –40 °C. Such an observation indicates that the binding force between Na+ and THF solvents is stronger than that between Na+ and MeTHF at high temperature but weaker at low temperature. The leapfrog improvement in interfacial dynamics is well demonstrated by temperature-dependent electrochemical impedance spectroscopy (EIS) measurements and the fitted results according to the classical Arrhenius law (Fig. 3j and Supplementary Fig. 17). The activation energy of Na+ in SMTA electrolyte through SEI transport (Ea, SEI) and charge transfer process (Ea, ct) is 14.1 kJ mol–1 and 17.7 kJ mol–1, respectively. These Ea values are relatively low, and temperature has minimal influence on them, emphasizing the significance of intermolecular interactions between AN and solvents in the SMTA electrolyte in promoting charge transfer processes at the electrode interface.

Mechanism of dipole-dipole interactions

To understand the underline mechanism for high-temperature instability, we investigated a series of solvents and electrolytes after being stored at various temperatures (Supplementary Fig. 18). No obvious changes is observed in all the solvents after storage at high temperatures, however, the addition of NaPF6 salt leads to significant discoloration in the SM electrolyte (1.0 M NaPF6 in MeTHF), even stored at an ambient environment of 25 °C. 1H-NMR spectra (Supplementary Figs. 19-20) show that new alkyls peaks (-CH, -CH2, -OCH3, etc) emerges in SM electrolyte after two weeks storage at 25 °C, confirming that the structural decomposition of MeTHF is the root cause of the degradation in high-temperature performance of the electrolyte. The electrostatic potential (ESP) of these three molecular structures was calculated by DFT to analyze their surface charge distributions (Fig. 4a). Unlike THF, which exhibits high structural symmetry and low ring strain, the presence of a methyl group in MeTHF leads to a highly asymmetrical charge distribution. The electron-withdrawing effect of oxygen (O) enhances the positive charge on the α-carbon adjacent to the neighboring O atom, leading to an increase in its chemical reactivity and the enhancement of the acidity of α-H. In SMT electrolyte, the reaction between NaPF6 and trace amounts of water produces the [PF₅OH]⁻ anion. Subsequently, the nucleophilic [PF₅OH]⁻ anion selectively attacks the positively charged α-carbon atom within the O-C bond of MeTHF (Fig. 4b), leading to the formation of an active intermediate with an anionic chain end. This process weakens the bond energy of the C-O bond, ultimately resulting in the ring-opening decomposition of MeTHF. Continuous addition of anions to the monomers via the reaction facilitates the growth of the polymer chain. The addition of AN significantly improves the chemical stability of MeTHF molecules. As evidenced in 2D 1H-1H COSY results, the methoxyl group of AN shows a strong and directional interaction with the α-H in MeTHF at high temperatures, reducing the acidity of the α-H and diminishing the positive charge on the α-C atom. The interactions between AN and MeTHF, therefore, significantly enhances the stability of the O-C bond and stabilizes the MeTHF structure. At low temperatures, all electrolytes display high chemical stability (Supplementary Fig. 21), thus salt precipitation and freezing primarily influence the low-temperature performance. As revealed by 2D NMR (Fig. 2c, d), this transition hinders the formation of ordered clusters, thus promoting an increase in entropy. From thermodynamic point of view, rising entropy potentially favors the formation of a homogeneous solution, thus prevents salt precipitation and stabilizes the liquid-phase system (Fig. 4c)32.

Overall, the temperature adaptive feature of SMTA electrolyte is realized by manipulating dipole-dipole interactions among solvents (Fig. 5). At elevated temperatures, thermal motion among molecules significantly intensifies. Due to the presence of methoxy and methyl side chains, AN and MeTHF possess considerable steric hindrance, which slows down molecules movements and increases the frequency of molecular collisions between them, thus leading to an enhancement of their interactions. The strong interactions between AN and MeTHF inhibits the parasitic decomposition of MeTHF and promotes the formation of stable complexes. While at sub-zero temperatures, on the contrary, the thermal motion of all molecules is dramatically hindered, thus instead of the steric hindrance, polarization effect among molecules become the primary factor influencing interactions. The greater difference in polarizability is, the stronger the intermolecular interactions become. The molecular polarizability of AN, MeTHF, and THF is 13.05, 9.81 and 7.94, respectively33. The symmetrical structure of THF renders it less sensitive to polarization changes at low temperatures. The large difference in polarizability between AN and THF leads to an enhanced dispersive force between them, leading to less directional interactions between AN and MeTHF. Consequently, the solvation structure is less ordered, the system entropy increases, and thus the solubility of salt is enhanced.

Fig. 5
Fig. 5The alternative text for this image may have been generated using AI.
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Schematic illustration showing the challenges encountered by wide-temperature electrolytes and the temperature-adaptive transformation of solvation structures in SMTA electrolyte.

Discussion

To validate the effectiveness of this concept, we selected dimethoxybenzene (DMB), 1,3,5-trimethoxybenzene (TMB) and cyclopentyl methyl ether (CME) as antisolvents due to their identical structure to AN, particularly in their possession of methoxy groups as active functionalities. The specific molecular structures are depicted in Supplementary Fig. 22. Simultaneously, THF and MeTHF remained as solvents. Three electrolytes, SMT-DMB (1 M NaPF6 in THF: MeTHF: Dimethoxybenzene with a volume ratio of 17:17:6), SMT-TMB (1 M NaPF6 in THF: MeTHF: Trimethoxybenzene with a volume ratio of 17:17:6) and SMT-CME (1 M NaPF6 in THF: MeTHF: Cyclopentyl methyl ether with a volume ratio of 17:17:6), were prepared and subsequently tested in HC | |Na half cells. They demonstrated significantly improved performance compared to SMT, especially under room and low temperature conditions (Supplementary Figs. 23-24). The obtained results not only validate the significant feasibility of this strategy but also offer a practical approach to enhancing the temperature adaptability of electrolytes by regulating the strength of dipole-dipole interactions.

Although the composition and properties of the solid electrolyte interphase (SEI) play a significant role in the electrochemical performance of batteries, our findings in this work confirm that the physicochemical properties of the electrolyte itself have a more profound impact on battery performance under extreme temperatures. We utilized X-ray photoelectron spectroscopy (XPS) to investigate the interfacial chemistry at different temperatures (25 °C and –40 °C) and etching depths of HC electrode after cycling. The components of the SEI in SMT and SMTA electrolytes exhibited good uniformity at various depths, resulting in the formation of a predominantly inorganic-rich SEI composed of NaF/Na2CO3. These results suggest that there are no significant differences in the interfacial components formed at different temperatures. (Supplementary Figs. 25–28). The superior electrochemical performance of the SMTA electrolyte at both high and low temperatures is not primarily attributed to the SEI, but rather results from improvements in the physicochemical properties of the electrolyte itself.

In summary, we found that regulating dipole-dipole interactions could endow solvation structure with temperature adaptability, enabling high-performance sodium-ion batteries to operate stably within a wide temperature range. Such a temperature-adaptive feature could simultaneously meet the demands both at high and low temperatures. In the SMTA electrolyte, AN shows strong interaction with MeTHF and stabilizes the α-H in molecular at high temperatures, thereby suppressing parasitic reactions and enhancing thermal stability. At low temperatures, on the other hand, AN exhibits strong interaction with THF, thus inhibiting salt precipitation and improve kinetics. Leveraging this advantage, we have ensured that hard carbon anodes maintain high capacity and competitive cycling stability even across an extensive temperature range (− 60 °C–55 °C). Our work elucidates the significance of dipole-dipole interactions in regulating the solvation structure and points out a direction for the development of wide-temperature rechargeable batteries.

Methods

Materials

NaPF6, THF and MeTHF were purchased from Dodo chem and used as received. Anisole (AN) was purchased from Aladdin Reagent Co., Ltd. and used as received. 1,3-Dimethoxybenzene was purchased from Maclin Inc. and was used after being dried over a molecular sieve. 1,3,5-Trimethoxybenzene and Cyclopentyl methyl ether were purchased from Maclin Inc. and used as received. All the solvents mentioned above were stored in aluminum-plastic bottles. A pipette with a polypropylene tip was used to transfer the solvents into 5 ml glass bottles, which were then placed on a magnetic stirrer to dissolve the salt at room temperature. MTA solution was prepared by THF: MeTHF: AN with a volume ratio of 17:17:6. MT solution was prepared by MeTHF: THF with a volume ratio of 1:1. MA solutions were prepared by MeTHF: AN with a volume ratio of 37:13. TA solutions were prepared by THF: AN with a volume ratio of 37:13. MA solution was prepared by MeTHF: AN with a volume ratio of 37:13. SM or ST electrolytes were prepared by dissolving 1 M NaPF6 in MeTHF or THF, respectively. SMA electrolyte was prepared by dissolving 1 M NaPF6 in MeTHF: AN with a volume ratio of 9:1. STA electrolyte was prepared by dissolving 1 M NaPF6 to THF: AN with a volume ratio of 37:13. SMTA electrolyte was prepared by dissolving 1 M NaPF6 to THF: MeTHF: AN with a volume ratio of 17:17:6. SMT-DMB electrolyte was prepared by dissolving 1 M NaPF6 to THF: MeTHF: Dimethoxybenzene with a volume ratio of 17:17:6. SMT-TMB electrolyte was prepared by dissolving 1 M NaPF6 to THF: MeTHF: 1,3,5-Trimethoxybenzene with a volume ratio of 17:17:6. SMT-CME electrolyte was prepared by dissolving 1 M NaPF6 to THF: MeTHF: Cyclopentyl methyl ether with a volume ratio of 17:17:6. All the preparation procedure were carried out in an argon-filled glove box with H2O and O2 levels <0.01 ppm. Hard carbon materials were purchased from Kuraray Company. Sodium metal purchased from Sinopharm Group Chemical Reagent Co., LTD. (99.5%).

Electrochemical tests

HC electrodes were prepared by blending 80 wt.% HC, 10 wt.% acetylene black and 10 wt.% polyvinylidene fluoride binders to form a homogeneous slurry. The as-prepared slurry was pasted uniformly onto a carbon-coated Al foil and then dried at 120 °C overnight in a vacuum oven. All electrodes were cut into circular pieces with a diameter of 12 mm. The average loading mass is ∼5 mg cm−2. Remove the sodium block from kerosene and trim off the oxidized layer to expose the fresh, metallic-sheen sodium. Roll it into a uniform, 1-2 mm thick sheet using an iron rod. Cut the rolled sheet into 14 mm diameter disks with a hole punch in an Ar-filled glovebox. CR2025 coin-cells were used to prepare HC | |Na cells. A 19 mm diameter porous glass fibre (GF/D) was used as separators. All the cells were assembled in an Ar-filled glovebox with oxygen and moisture levels less than 0.01 ppm.

The electrochemical performances were carried out on CT-4008Tn-5V50mA-HWX battery tester. The room-temperature electrochemical tests were carried out at a constant temperature of 25 °C in an environmental chamber. The high temperature electrochemical tests were performed at a constant temperature of 55 °C in the temperature and humidity chamber. All low-temperature data were collected from these cells inside SCICOOLING freezers (BTC-SG7503-02F) for −60 ∼55 °C tests. Before working at LT, the cells were cycled at RT for 5 cycles with a specific current of 100 mA/g and then stored at the targeted temperature for 1 h before testing. The operation voltage range of HC | |Na cell was 0.001 ∼ 2.0 V. The HC | |Na cell was operated at a constant specific current of 100 mA g−1. CV experiments were applied to investigate the electrode kinetics at a scan rate of 0.5 mV s−1 within the same voltage range on a Princeton PMC CHS08A electrochemical workstation.

Characterization

The ionic conductivities of electrolytes were tested by a conductivity measuring meter (INESA DDS-307, Leici) within the temperature range from −60 to 25 °C. The DSC measurements were carried out in a DSC 214 (NETZSCH) differential scanning calorimeter from 25 to −120 °C with a cooling rate of 5 °C min−1. An investigation of solid electrolyte interphase on hard carbon cycled in SMTA electrolyte was characterized via a high sensitivity Kratos AXIS Supra X-ray photoelectron spectrometer (XPS) with Al Kα radiation (1486.6 eV). The sputtering with a power of 4 kV × 140 μA on a 3 mm × 3 mm surface was conducted via Ar ions, with a sputtering rate on Ta2O5 calibrated to be 10 nm min−1. All values of binding energy were referenced to the C 1 s peak of carbon located at 284.8 eV. Before the XPS characterizations, the cycled HC electrodes were washed three times to remove residual salts by the corresponding solvents, and dried at 50 °C in glovebox for 3 h to totally remove the solvents. The glovebox is connected to the vacuum transfer chamber of the XPS system to avoid the sample exposure to air. NMR (Bruker AVANCE NEO, 600 MHz) techniques were deployed to reveal the electrolyte solvation structures. samples were measured at 233.1 ± 0.1 K, 298.1 ± 0.1 K, and 328.1 ± 0.1 K without rotation and with 4 dummy scans prior to 16 scans. Acquisition parameters were set as follows: FID size = 64 K, spectral width = 24.5044 ppm, receiver gain = 4, acquisition time = 2.22 s, relaxation delay = 1 s, and FID resolution = 0.3 Hz. Deuterated acetone was placed in an external coaxial insert and then in the NMR tube with the samples. The 1H chemical shifts were referenced to acetone-d6 at 2.05 ppm (at high temperatures, Dimethyl-d6 sulfoxide at 2.50 ppm) and TMS was used as an internal reference.

The Raman spectra of the electrolytes were recorded using a spectrometer (XploRA INV, Horiba), excited by a 532 nm laser. The electrolyte samples were cooled down to the target temperature with a nitrogen coolant and controller.

Computational details

MD simulations were carried out by a Materials Studio software, reversion 201834. The MD simulation package Forcite was used for all the simulations with COMPASS II force field35. The model for SMTA electrolyte contains *NaPF6, *THF molecules, *MeTHF molecules and *AN molecules. The obtained box is a 39.33 × 39.33 × 39.33 Å3 cube. The corrected charge simulations for molecules were performed using DMol3 packge36, based on Density Functional Theory (DFT). In these simulations, the pure density functional method M06-L was employed, which is a widely-used formulation within the meta-generalized gradient approximation(m-GGA) method37. The electronic energy was considered self-consistent when the energy change was smaller than 10−5 eV, while the tolerance convergence in ionic was 10−5 eV, too. Furthermore, the van der Waals correction of Grimme’s DFT-D3 model was adopted38. The simulation temperature was set at 298, 233 and 328 K, respectively. The Ewald summation method was used for the electrostatic interactions between the permanent charges with either permanent charges or induced dipole moments with K = 63 vectors39. Multiple timestep integration was employed with an inner timestep of 0.5 fs (bonded interactions), a central time step of 1.5 fs for all non-bonded interactions within a truncation distance of 8.0 Å and an outer timestep of 3.0 fs for all non-bonded interactions between 7.0 Å and the non-bonded truncation distance of 14–16 Å. The reciprocal part of Ewald was calculated every 3.0 fs. A Nose‒Hoover thermostat was used to control the temperature with the associated frequencies of 10‒2 and 0.1 × 10‒4 fs. A Berendsen barostat was used to control the pressure with a decay constant of 0.1 ps40. The atomic coordinates were saved every 2 ps for post-analysis. Each system will be subjected to an Annealing process from 300 K to 500 K to help the mixture disperse more evenly. Subsequently, initial equilibration runs of ~5 ns were performed in an NPT ensemble to obtain the equilibrium box size that is used in the follow-up equilibration and production runs of ~1 ns performed in the NVT ensemble.