Regulation of anion–Na⁺ coordination chemistry in electrolyte solvates for low-temperature sodium-ion batteries

Significance

Sodium-ion batteries have attracted extensive attention because of the abundant sodium resources and low cost. However, SIBs still suffer severe capacity degradation related to the sluggish desolvation and the incurred low kinetics at electrode surface, especially at low temperatures. Herein, anions with high electron donicity, trifluoroacetate (TFA⁻), are applied to regulate the solvation chemistry with enhanced Na⁺ desolvation in a wide temperature range from 40 to −60 °C. The strongly coordinating TFA⁻ enables more PF₆⁻ anions and decreased diglyme molecules as the anion-reinforced solvate to boost the low-temperature electrochemical performance. Temperature-dependent solvates were also disclosed to highlight the role of anions on solvation chemistry and will favor the deployment of SIBs at extreme temperatures.

Abstract

High-performance sodium storage at low temperature is urgent with the increasingly stringent demand for energy storage systems. However, the aggravated capacity loss is induced by the sluggish interfacial kinetics, which originates from the interfacial Na⁺ desolvation. Herein, all-fluorinated anions with ultrahigh electron donicity, trifluoroacetate (TFA⁻), are introduced into the diglyme (G2)-based electrolyte for the anion-reinforced solvates in a wide temperature range. The unique solvation structure with TFA⁻ anions and decreased G2 molecules occupying the inner sheath accelerates desolvation of Na⁺ to exhibit decreased desolvation energy from 4.16 to 3.49 kJ mol⁻¹ and 24.74 to 16.55 kJ mol⁻¹ beyond and below −20 °C, respectively, compared with that in 1.0 M NaPF₆-G2. These enable the cell of Na||Na₃V₂(PO₄)₃ to deliver 60.2% of its room-temperature capacity and high capacity retention of 99.2% after 100 cycles at −40 °C. This work highlights regulation of solvation chemistry for highly stable sodium-ion batteries at low temperature.

Rechargeable sodium-ion batteries (SIBs) have been considered as a promising candidate for large-scale energy storage systems because of abundant sodium resources (1–5). Nevertheless, the electrochemical performance of SIBs at low temperature, especially below −20 °C, remains challenging for the urgent demand for deploying in extreme environments (6–11). This is mainly derived from the kinetic barriers of ion transport in electrolytes, cation desolvation at the electrode-electrolyte interface, ion transport inside the interfacial layer, and electron and ion transport within electrode materials (12–15). Noticeably, the interfacial Na⁺ desolvation is more sensitive to low temperature and induces dominating charge transfer impedance (16, 17). The sluggish process favors undesired Na plating within the incompatible solid electrolyte interphase (SEI) (18–20). It was revealed to depend on the competitive coordination of cations with anions and solvent molecules (21, 22), and thereby, manipulation of the solvation chemistry in electrolytes is responsible for the interfacial kinetics of Na⁺ desolvation in SIBs at low temperature.

Solvate structures are crucial for the interfacial kinetics of Na⁺ and dictate the electrochemical performance of SIBs. Exploring weakly solvating solvents is a facile approach to reduce the desolvation energy with minor solvent molecules occupying the inner solvation sheath (23, 24). The weakly solvating tetrahydrofuran was reported to enable low Na⁺ desolvation energy to achieve 74.0% of its room-temperature capacity of commercial hard carbon at −20 °C (25). Lowering the interaction between cations and solvent molecules by adjusting the concentrations is also feasible, that is, an electrolyte of ultralow concentration is proposed to boost the Na⁺ desolvation (26–28). However, these electrolytes suffer from low salt dissociation and hence inferior ionic conductivity, which limits their practical applications. Na⁺-solvent co-intercalation without desolvation was attempted in a few layered materials in couple with specific solvents (29). Although regulation of salt concentrations and solvents is effective for the configuration of inner solvation sheath, the role of anions on solvation behavior is innegligible. Understanding the comprehensive solvation chemistry regarding Na⁺, anions, and solvent molecules is of paramount importance for low-temperature SIBs.

In this work, anions with high electron donicity, trifluoroacetate (TFA⁻), are applied to regulate the solvation chemistry of 1.0 M NaPF₆ in the diglyme (G2) electrolyte and accelerate the desolvation of Na⁺ in a wide temperature range. The strongly coordinating TFA⁻ enables more PF₆⁻ anions and decreased G2 molecules as the anion-reinforced solvate of Na(G2)_4.78(PF₆)_1.33(TFA)_0.22 and Na(G2)_3.46(PF₆)_4.34(TFA)_0.19 at 25 and −40 °C, respectively. This induces the anion-derived robust SEI layer with enhanced mechanical strength and low desolvation energy from 40 to −60 °C. It favors uniform Na deposition with ultrahigh Coulombic efficiency of 99.9% over 500 cycles at 25 °C and enables the cell of Na||Na₃V₂(PO₄)₃ to deliver 60.2% of its room-temperature capacity and high capacity retention of 99.2% after 100 cycles at −40 °C.

Results

Solvation Chemistry of Na⁺.

Desolvation energy is greatly related to the interactions among Na⁺-solvents and Na⁺-anions, and Gutmann donor number (DN) acts as a descriptor for the solvating capability (30–32). High-DN solvents endow the enhanced capability to dissolve sodium salt and preferentially occupy the inner solvation sheath, however, the electrolyte suffers from sluggish interfacial kinetics due to its high desolvation energy (33–35). Of equal importance, high-DN anions can decrease the coordination number of solvents to exhibit potential on boosting the Na⁺ desolvation (36, 37). With ultrahigh DN of 34.0 kcal mol⁻¹, TFA⁻ is introduced into G2-based electrolytes to regulate the Na⁺ solvates. The higher DN of anion, its stronger interaction with Na⁺, indicating the strongly coordinating capability of TFA⁻ to occupy the inner solvation sheath (Fig. 1 A and B and SI Appendix, Table S1). This leads to poor dissociation of NaTFA in G2-based electrolytes and low ionic conductivity, for example, 0.20 mS cm^-2 in 1.0 M NaTFA-G2 vs. 7.59 mS cm⁻² in 1.0 M NaPF₆-G2 at 20 °C (SI Appendix, Fig. S1 and Table S2). As a result, 0.1 M NaTFA is introduced as an electrolyte additive in 1.0 M NaPF₆-G2 electrolyte to manipulate the Na⁺ solvates (SI Appendix, Figs. S1 and S2). It exhibits moderate ionic conductivity (6.87 mS cm⁻²) and ultrahigh Coulombic efficiency (CE, 99.9%) in asymmetrical cells of Na||Cu.

Fig. 1.

Solvation chemistry of Na⁺. (A) Donor number of commonly used anions. (B) Binding energy of Na⁺ with anions. (C) Raman spectra and (D and E) proportion of free G2 and solvated G2 in different electrolytes. (F) ²³Na NMR spectra of different electrolytes. (G) Coordination number from MD simulation of electrolytes without and with NaTFA.

Raman spectra were employed to unveil the solvation chemistry of Na⁺ (Fig. 1C and SI Appendix, Fig. S3). Free G2 molecules show three characteristic vibration peaks at 804.7, 824.2, and 850.9 cm⁻¹, marked as F1, F2, and F3, respectively, which correspond to the coupled CH₂ rocking mode and COC stretching mode (38, 39). Extra vibration peaks assigned to Na⁺-solvated G2 at 839.4, 861.1, and 870.3 cm⁻¹ are marked as S1, S2, and S3, respectively. The proportion of Na⁺-solvated G2 is 47.7% in 1.0 M NaPF₆-G2 (electrolyte w/o NaTFA) and increases to 54.4% in 1.1 M NaPF₆-G2 (Fig. 1D and SI Appendix, Table S3). It largely decreases to 41.9% with addition of 0.1 M NaTFA into 1.0 M NaPF₆-G2 (electrolyte with NaTFA). This is further verified by the proportion of free G2 (F1, F2, and F3) and Na⁺-solvated G2 (S1, S2, and S3) in Fig. 1E, highlighting the role of strongly coordinated TFA^- in the solvation structure. As shown in ²³Na NMR in Fig. 1F, the chemical shift increases from −6.86 to −6.45 ppm after addition of NaTFA, and the difference becomes more obvious in 1.0 M NaTFA-G2, indicating the deshielding Na⁺ by the surrounding electron cloud with more anions (40, 41). The broadened signal of the electrolyte with NaTFA implies the complicated local environment of Na⁺, composing of contact-ion pairs and aggregates. The weakened interaction between Na⁺ and G2 molecules in the presence of TFA^- is confirmed by the ¹⁷O NMR spectra (SI Appendix, Fig. S4).

The Na⁺ solvation is further studied by classical molecular dynamics (MD, Fig. 1G and SI Appendix, Fig. S5). The theoretical coordination number that all TFA⁻ anions occupy the inner Na⁺ sheath as contact-ion pairs (CIPs, one O atom in TFA^- coordinates with one Na⁺) is 0.18, and a higher value of 0.22 is realized in the electrolyte with NaTFA, validating formation of aggregates (AGGs) with one O atom in TFA⁻ coordinated with two or more Na⁺. Accordingly, the coordination number of G2 decreases from 5.09 to 4.78, and coordinated PF₆⁻ increases from 1.30 to 1.33 in the presence of TFA⁻ (Fig. 1G and SI Appendix, Fig. S6). The enhanced Na⁺-PF₆⁻ interaction is also verified by Raman spectra with higher intensity of PF₆⁻ (SI Appendix, Fig. S7). The highly coordinating TFA⁻ enables more PF₆⁻ anions and less G2 molecules to occupy the inner solvation sheath of Na⁺, favoring the construction of reinforced Na⁺-anion coordination, namely, Na(G2)_4.78(PF₆)_1.33(TFA)_0.22.

Na Plating/Stripping.

The scanning electron microscopy (SEM) images show micron-sized granular Na deposits with numerous cracks in the electrolyte w/o NaTFA, while dense Na deposits with smooth surface is realized in the electrolyte with NaTFA (Fig. 2 A and B and SI Appendix, Fig. S8). The much thinner thickness of 84.8 vs. 107.8 μm, suggests the smaller surface area exposed to the electrolyte with well-suppressed side reactions (SI Appendix, Fig. S9). As the height and modulus images of atomic force microscopy (AFM) shown (Fig. 2 C and D and SI Appendix, Fig. S10), a flat and smooth surface with roughness (Rq) of 32.5 nm is observed in the electrolyte with NaTFA, compared with an obvious protuberance with Rq of 118.1 nm in the electrolyte w/o NaTFA. The average mechanical strength of the SEI greatly enhances from 0.86 to 5.20 GPa in the presence of TFA^-, indicating its robustness. These highlight that the anion-reinforced solvates favor the uniform Na deposition with a robust SEI layer.

Fig. 2.

Na plating/stripping. (A and B) SEM images and (C and D) histogram of Young’s modulus of Na deposits in electrolytes without and with NaTFA. Insets are 3D AFM height images with a data scale of 1,800 nm. (E) Coulombic efficiency and (F) charge–discharge curves of Na||Cu cells in the electrolyte with NaTFA, with a fixed capacity of 0.5 mAh cm⁻² at 0.5 mA cm⁻². (G) Radar map of tip overpotential (η_t), plateau overpotential (η_p), nucleation overpotential (η_n), voltage polarization (ΔE), and initial Na loss. (H) LSV curves and (I) Tafel plots in the electrolyte without and with NaTFA.

To verify the superiority of anion-reinforced solvates and the as-formed robust SEI, Na utilization during platting/stripping is tested in Na||Cu cells. Relatively high average CE of 99.8% with random values, especially after 300 cycles, are displayed in the electrolyte w/o NaTFA, indicating the continuous decomposition of the electrolyte (Fig. 2 E and F and SI Appendix, Fig. S11). Ultrahigh and ultrastable CE of 99.9% over 500 cycles is realized in the electrolyte with NaTFA. As summarized in the radar map, the average voltage polarization reduces from 46.2 to 24.1 mV in the presence of TFA⁻, indicating enhanced interfacial kinetics. Besides, smaller tip, plateau, and nucleation overpotentials, as well as lower initial Na loss are realized, which implies the enhanced stability and reversibility of Na plating/stripping with the help of NaTFA (Fig. 2G and SI Appendix, Table S4). A larger fixed capacity of 1.0 mAh cm⁻² of deposited Na at 0.5 mA cm⁻² is also demonstrated (SI Appendix, Fig. S12), emphasizing the superior reversibility of Na platting/stripping in the electrolyte with NaTFA.

The decomposition behavior of sodium salt/solvent was revealed by linear sweep voltammetry (LSV) in Fig. 2H. A sharp reduction peak around 1.1 V is attributed to the decomposition of PF₆⁻, which produces the harmful components such as HF and results in the unstable SEI. Noticeably, an additional reduction peak at ca. 1.4 V, attributed to TFA⁻ reduction, appears in the electrolyte with NaTFA and significantly decreases the peak intensity of PF₆⁻ decomposition, indicating the enhanced interfacial property. The larger exchange current density for Na stripping/plating is realized in the electrolyte with NaTFA in Fig. 2I. This is attributed to the anion-reinforced solvates, induced by TFA⁻ anions, promoting formation of the robust SEI with fast interfacial kinetics and highly reversible and uniform Na plating/stripping.

Interfacial Kinetics.

The superiority of NaTFA on electrochemical performance is investigated in half cells Na||Na₃V₂(PO₄)₃ (NVP). A high reversible capacity of 103.3 mAh g⁻¹ and capacity retention of 90.6% after 1,000 cycles is realized in the electrolyte with NaTFA at 1.0 A g⁻¹ (Fig. 3A and SI Appendix, Fig. S13). It delivers a much higher capacity, especially at high current density, with 94.5 and 84.6 mAh g⁻¹ at 2.0 and 3.0 A g⁻¹ in the electrolyte with NaTFA, compared to 81.9 and 66.7 mAh g⁻¹ in the electrolyte w/o NaTFA (Fig. 3 B and C and SI Appendix, Fig. S14), suggesting the potential of the electrolyte with NaTFA for fast-charging batteries. The uniform and inorganic-rich cathode electrolyte interphase (CEI) and greatly decreased charge-transfer resistance during long-term cycling are presented (SI Appendix, Figs. S15 and S16). These are responsible for the excellent electrochemical performance.

Fig. 3.

Interfacial kinetics. (A) Cycling performance of Na||NVP cells in the electrolyte without and with NaTFA at 1.0 A g⁻¹. (B) Rate capability of Na||NVP cells from 0.05 to 3.0 A g⁻¹ and (C) charge–discharge curves in the electrolyte with NaTFA. (D) HOMO/LUMO energy, (E) electrostatic potential density distribution, and (F) desolvation energy of four representative solvation configurations. (G) Nyquist plots of symmetric Na_1.5V₂(PO₄)₃||Na_1.5V₂(PO₄)₃ cells from 313 to 213 K. (H) Desolvation energy of Na⁺ desolvation derived from Nyquist plots. (I) Schematic of desolvation energy in a wide temperature range.

Density functional theory (DFT) calculation is performed to reveal the multifunctional TFA⁻ on redox and (de)solvation. Four dominant solvation configurations from MD simulation (SI Appendix, Table S5), Na(G2)₃, Na(G2)₂(PF₆), Na(G2)(PF₆)₂, and Na(G2)(PF₆)(TFA), are selected to calculate the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels. The HOMO energy levels increase greatly with TFA⁻ occupying the inner solvation sheath, indicating its prior decomposition on the NVP surface as the fluoride-rich CEI (Fig. 3D and SI Appendix, Fig. S17). The electrostatic potential (ESP) mapping discloses that negative moieties are concentrated on anions, and especially, a more negative value is observed in TFA⁻ anions, leading to the highest LUMO energy level of Na(G2)(PF₆)(TFA) cluster in Fig. 3E. The interfacial desolvation energy obeys the following order: Na(G2)₃ > Na(G2)₂(PF₆) > Na(G2)(PF₆)₂ > Na(G2)(PF₆)(TFA), exhibiting the accelerated Na⁺ desolvation with the participation of TFA⁻ anions (Fig. 3F).

The desolvation energy is evaluated by electrochemical impedance spectroscopy (EIS) from 313 to 213 K according to the Arrhenius law (37, 42–46),

$$\mathrm{k}=\mathrm{A} \text{exp}\left(\frac{{-\mathrm{E}}_{\mathrm{a}}}{\text{RT}}\right),$$ [1]

where k, A, E_a, R, and T represent rate constant, preexponential constant, activation energy, standard gas constant, and absolute temperature, respectively. The influence of the electrode–electrolyte interface was eliminated in the symmetric NVP cells at half charge state [Na_1.5V₂(PO₄)₃] rather than the Na||Na cells. Among the commonly used NaPF₆-based electrolyte, G2 solvent displays the lowest desolvation energy than that with G1 and PC (4.16 vs. 5.18, 5.69 kJ mol⁻¹ from 40 to −10 °C, SI Appendix, Fig. S18 and Table S6). The introduction of NaTFA further decreases the desolvation energy to 3.49 kJ mol⁻¹, emphasizing superiority of the electrolyte with NaTFA. Besides, as operating in a wider temperature range from 40 to −60 °C (Fig. 3 G and H and SI Appendix, Tables S7 and S8), a turning point is observed at −20 °C in both electrolytes, which is attributed to the transformation of solvation structures. The electrolyte with NaTFA exhibits obviously decreased Na⁺ desolvation energy of 3.49 kJ mol⁻¹ (40 to −20 °C) and 16.55 kJ mol⁻¹ (−20 to −60 °C), compared with the 4.16 and 24.74 kJ mol⁻¹ in the electrolyte w/o NaTFA, respectively. As the desolvation energy of G2-based electrolyte shown (SI Appendix, Fig. S19 and Table S9), the higher DN of anions (PF₆⁻: 2.5, TFSI⁻: 5.4, CF₃SO₃⁻: 16.9 kcal mol⁻¹), the lower desolvation energy, verifying the strongly coordinating capability of high-DN anions. The ionic conductivity in the wide temperature range also exhibits the turning point at −20 °C (SI Appendix, Fig. S20 and Table S10). As illustrated in Fig. 3I, TFA⁻ anions accelerate the Na⁺ desolvation in the wide temperature range, and the transition of desolvation energy indicates the different solvation configuration beyond and below −20 °C.

Temperature-Dependent Solvation.

The temperature-dependent solvation configuration is explored through MD simulation and Raman spectra at 25 and −40 °C. There exhibit two different snapshots and radial distribution functions at room and low temperatures (Fig. 4 A and B and SI Appendix, Figs. S21–S23). Na⁺ is mainly solvated by G2 molecules with a sharp peak of Na-O_G2 pair at 25 °C, and PF₆⁻ anions extensively occupy the Na⁺ inner sheath with a relatively high intensity of Na-F_PF6 pair at −40 °C, indicating the strong Na⁺-PF₆⁻ interaction at low temperature. For the high-DN anion, TFA⁻ slightly decreases from 0.22 to 0.19 as the temperature decreases, however, it is still larger than the theoretical value of 0.18 that all TFA⁻ anions occupy the inner sheath as CIPs (SI Appendix, Fig. S5), indicating the strongly coordinating capability of TFA⁻ anions even at −40 °C. The decreased proportion of AGGs containing TFA⁻ might be attributed to the weakened activity of ions and sluggish thermodynamic rate at low operating temperatures. The coordinated G2 and PF₆⁻ display the opposite tendency during the cooling process, in which coordinated G2 decreases from 4.79 to 3.46 and coordinated PF₆⁻ increases from 1.33 to 4.34 in the electrolyte with NaTFA (Fig. 4C and SI Appendix, Table S11). Besides, the addition of NaTFA enhances the coordinated PF₆⁻ at both 25 and −40 °C with formation of more CIPs and AGGs containing PF₆⁻ in the wide temperature range. Both electrolytes exhibit the higher peak intensity at 740.2 cm⁻¹ assigned to PF₆⁻ at low temperature in Raman spectra (Fig. 4D and SI Appendix, Fig. S24). This exhibits the enhanced coordinated-PF₆⁻ proportion in the presence of TFA⁻.

Fig. 4.

Temperature-dependent solvation. Snapshots of solvation configuration and radial distribution functions of Na-O_G2, Na-F_PF6, and Na-O_TFA pairs in the electrolyte with NaTFA at (A) 25 °C and (B) −40 °C. (C) Coordination number of solvents and anions in the electrolyte with NaTFA. (D) Raman spectra of the electrolyte with NaTFA, (E) atomic ratio of F in the SEI with Ar⁺ sputtering for varied times, and (F) atomic ratios of F and P and their ratios at 25 and −40 °C.

The in-depth X-ray photoelectron spectroscopy (XPS) spectra were employed to examine the chemical components of the SEI layer through Ar⁺ sputtering. The addition of NaTFA endows the high contents of inorganic species, such as NaF (683.2 eV in F 1 s spectra) and Na₂O (529.5 eV in O 1 s spectra, Fig. 4E and SI Appendix, Figs. S25–S28) (21, 44, 47), indicating the strongly coordinating capability and preferential decomposition of TFA⁻. The higher intensity of NaF and Na₂O with Ar⁺ sputtering for 120 s than 0 s indicates the enrichment of inorganic species in the SEI inner layer, while organic species are mainly located in the SEI outer layer. Low temperature favors formation of phosphide in both electrolytes, implying that more PF₆^- anions occupy the Na⁺ inner solvation sheath at −40 °C (Fig. 4F and SI Appendix, Fig. S29). Higher atomic F is found in the electrolyte w/o NaTFA, which is attributed to the enhanced coordinated PF₆⁻ at −40 °C. In the electrolyte with NaTFA, F can be derived from PF₆⁻ and TFA⁻ anions, but it shows an obvious decrement, indicating the decomposition of TFA⁻ anions (dominant part of F resources) is slightly weakened at low temperature. The opposite tendency of P:F ratio indicates that more PF₆⁻ and less TFA⁻ anions occupy the Na⁺ inner solvation sheath at low temperature, which is in accordance with the Raman and MD simulations on solvation structure.

Electrochemical Performance.

Fig. 5 A and B show the temperature-dependent performance of Na||NVP cells from 25 to −40 °C, as well as in SI Appendix, Fig. S30. Similar capacities can be obtained in both electrolytes at 25 °C, but Na||NVP in the electrolyte with NaTFA exhibits obvious superiority on capacity retention at sub-zero temperature. Capacities of 53.4 and 105.6 mAh g⁻¹ at −40 and 25 °C are achieved in the electrolyte with NaTFA, higher than 40.7 and 90.3 mAh g⁻¹ in the electrolyte w/o NaTFA, respectively. Besides, lower polarization voltages are maintained in the electrolyte with NaTFA during the long-term cycling (SI Appendix, Fig. S31). The improved low-temperature rate capability indicates the accelerated desolvation of Na⁺ by TFA⁻ (Fig. 5C and SI Appendix, Fig. S32). The cell of Na||NVP in the electrolyte with TFA⁻ exhibits 60.2% of its room-temperature capacity and high capacity retention of 99.2% after 100 cycles at -40 °C, much higher than 50.8% and 91.3% in the electrolyte w/o NaTFA (Fig. 5D and SI Appendix, Fig. S33). This is also guaranteed by the broadened electrochemical window and uniform Na deposition at −40 °C (Fig. 5E and SI Appendix, Fig. S34). The superiority of NaTFA on Na anode and NVP cathode is studied in the Na||NVP cells after 50 cycles. The introduction of NaTFA greatly improves Na deposition, to show smooth anode surface in electrolyte with NaTFA, against numerous cracks in electrolyte w/o NaTFA (SI Appendix, Fig. S35). The contrast is more obvious at −40 °C. Of note, the introduction of NaTFA and operation temperature have negligible effect on the cycled NVP (SI Appendix, Figs. S36 and S37). Combining the EIS spectra of Na||Na and NVP||NVP symmetric cells at both 25 and −40 °C (SI Appendix, Fig. S38), the impedance increment is mainly derived from the anode side during the cooling process, which may be attributed to the uneven Na surface with numerous cracks. The interfacial behaviors of Na⁺ solvates in the presence of TFA^- are depicted in Fig. 5F, to decrease the desolvation energy with boosted kinetics in a wide temperature range and encourage fast-charging and low temperature performance of SIBs.

Fig. 5.

Electrochemical performance. (A) Temperature-dependent performance of Na||NVP from at 20 mA g⁻¹ and (B) charge–discharge curves in the electrolyte with NaTFA. (C) Rate performance of Na||NVP and (D) charge–discharge curves at 100 mA g⁻¹ at −40 °C. (E) Electrochemical window at −40 °C. (F) Schematic of interfacial behaviors.

Discussion

In summary, all-fluorinated TFA⁻ anions with ultrahigh electron donicity are introduced into an ether-based electrolyte to regulate anion-reinforced solvates. The inner Na⁺ solvation sheath is composed of TFA⁻, more FP₆⁻, and decreased G2 molecules, with Na(G2)_4.78(PF₆)_1.33(TFA)_0.22 at 25 °C and Na(G2)_3.46(PF₆)_4.34 (TFA)_0.19 at −40 °C, which effectively accelerates the desolvation of Na⁺ under the wide temperature range. This solvation configuration induces the anion-derived inorganic-rich and robust SEI layer to enable ultrahigh and ultrastable Na plating/stripping CE. These merit cells of Na||NVP with 60.2 % of its room-temperature capacity and high capacity retention of 99.2% after 100 cycles at −40 °C. This work emphasizes the anion-reinforced coordination in electrolyte solvates, providing insights into solvation chemistry regulation for durable, fast-charging and low-temperature SIBs.

Materials and Methods

Materials.

Sodium trifluoroacetate (NaTFA, ≥99.0%) is purchased from Xiya Reagent. NaPF₆, NaCF₃SO₃, NaTFSI, PC, G1, and G2 (Battery grade) were purchased from Dodochem. Prepared electrolytes are dried with 4 Å molecular sieves. Na metal (99.7%, Aladdin) was rolled and pressed as Na chips (10 mm diameter) before assembling cells. Commercial Na₃V₂(PO₄)₃ (NVP) was purchased from HF-Kejing.

Material Characterizations.

Raman spectra were recorded on a spectrophotometer (DXR, Thermo Fisher Scientific). ¹⁷O and ²³Na NMR spectra of liquid electrolytes were recorded on a 400 MHz Bruker AVANCE III. The signals of ¹⁷O and ²³Na chemical shifts were quoted to the internal standard of methanol-d⁴ and 0.1 M NaCl in deuteroxide, respectively. The morphology and thickness of deposited Na on copper foils were observed via scanning electron microscopy (SEM, JEOL JSM7500F) with an accelerating voltage of 5.0 kV. Roughness and Young’s modulus were obtained by atomic force microscopy (AFM, Bruker Multimode 8.0) in a glove box. The thickness of the interfacial layer on cathodes was characterized by transmission electron microscopy (TEM, JEOL ARM-200F) with a voltage of 80 KV. The cycled NVP is characterized by powder X-ray diffraction (XRD, Rigaku, MiniFLex600) with Cu Kα radiation. XPS analysis was performed on the X-ray Photoelectron Spectrometer (Thermo Fisher), in which a monochromatic Al k_α radiation (hv = 1,486.6 eV) was employed as an X-ray source. The monochromator is equipped with an Ar⁺ ion sputtering gun, and the sputter rate was normalized by SiO₂ (25 nm min⁻¹) with a sputter area of 1.0 mm × 1.0 mm. The Na anodes or NVP cathodes were taken out of the Na||NVP cells with a cutoff voltage of 2.8 V after 50 cycles at 100 mA g⁻¹. All the batteries were assembled and disassembled in a glove box, and the electrodes were washed several times with G1. All the samples were loaded into an airtight vessel without exposure to ambient air. This is also applicable for the SEM and XRD tests for electrode stability during the long-term cycling.

Electrochemical Measurements.

The cathode slurry was prepared by mixing NVP, Super P, and polyvinylidene fluoride in a weight ratio of 8:1:1 with a certain amount of N-methylpyrrolidinone. It was coated onto an aluminum foil with a doctor blade and dried at 60 °C for 12 h in vacuum. The active material loading is ca. 1.5 mg cm⁻² on each pellet. Glass fiber (Whatman) is used as the separator with 60 μL electrolyte.

CR2032-type coin cells were assembled in an Ar-filled glovebox and tested on a NEWARE battery cycler (CT-3008 W). EIS, Tafel plot, and LSV were tested by an electrochemical workstation (CHI760E). Na||Cu cells were assembled for Coulombic efficiency tests, where Cu foil was cleaned by deionized water and ethyl alcohol. Symmetric Na_1.5V₂(PO₄)₃||Na_1.5V₂(PO₄)₃ cells were prepared for calculating the desolvation energy. Na||NVP half cells were applied for the impedance at 0, 10, 50, 200, and 1,000 cycles. Na||Na cells were assembled for Tafel tests at a scan rate of 0.1 mV s⁻¹. Na||stainless cells were prepared for LSV tests at a scan rate of 1.0 mV s⁻¹.

Theoretical Calculations.

Quantum chemistry calculations were carried out with the Gaussian 16 software package. The structure optimization, molecular orbital energy as well as binding energy calculation were performed with the B3LYP density functional and the 6-311+G(d, p) basis set (48, 49). The electrostatic potential mapping (ESP) was acquired by the further calculation of Gaussian check files. The SMD implicit solvation model was used to describe the solvation effect. Acetone (ε = 20.49) was used as the solvent for calculation of Na⁺ complexes. Molecular dynamics (MD) simulations were conducted by using the GROMACS package with AMBER03 force field (50). The MD parameters for Na⁺ were in the built-in force field parameters, where FP₆⁻, G2, and TFA⁻ were generated by ACPYPE and the corresponding atomic charges were based on RESP charges (51). The atomic charges of solvent were corrected by a factor of 1.5. Upon quasi-equilibrium of the system, a total of 90 ns MD simulation was carried out for each electrolyte. First, NVT runs were performed at 398.15 K for 40 ns, and then, NPT runs of 50 ns were performed at 298.15 K to ensure the system equilibrium. The last 10 ns in NPT runs were used for calculating the radial distribution functions and coordination numbers in various electrolytes. The snapshot of MD simulation is produced by VMD software (52).

The desolvation energy (E_d) was theoretically calculated by the DFT method as follows:

$${\mathrm{E}}_{\mathrm{d}}={\mathrm{E}}_{\text{total}}-{\mathrm{E}}_{\text{Na}}-{\text{nE}}_{\mathrm{G}2}.$$

Here, E_total, E_Na, and E_G2 are the total energy of inner Na⁺ cluster, energy of Na⁺ alone, and energy of G2 molecule, respectively, and n is the coordination number of G2. According to this definition, a more negative value of E_d endows a faster desolvation process.

Supplementary Material

Appendix 01 (PDF)

Data, Materials, and Software Availability

All study data are included in the article and/or SI Appendix.