Asymmetric Benzene Sulfonamide Sodium Salt Enabling Stable Cycling in Solid‐State Sodium Metal Batteries

Abstract

With renewable energy and electric vehicles driving demand, safer and cost‐effective alternatives to lithium‐ion batteries are being sought. This study explores the development of a novel sodium salt, sodium (benzenesulfonyl)(trifluoromethanesulfonyl) imide (NaBTFSI), for all‐solid‐state sodium metal batteries (ASSSMBs). NaBTFSI offers a promising electrolyte option by improving sodium‐ion transference number ($T_{\text{Na}}^{+}$), conductivity, and stability of sodium metal (Na°) anode cycling. When combined with poly(ethylene oxide), NaBTFSI forms safe solid polymer electrolytes with high mechanical strength, effectively mitigating dendrite growth and polarization issues common in sodium anodes. Characterization shows NaBTFSI enhances the electrochemical performance through π–π stacking interactions, which stabilize the polymer matrix and increase ionic conductivity (≈4.0 × 10^{− 4} S cm⁻¹) at elevated temperatures (70 °C). NaBTFSI‐based electrolytes exhibit higher stability with sodium anodes than the conventional sodium bis(trifluoromethanesulfonyl)imide salt, supporting prolonged cycling in Na^o||Na^o symmetric cells and demonstrating potential for sustainable, high‐performance ASSSMBs.

A novel sodium salt, sodium (benzenesulfonyl)(trifluoromethanesulfonyl) imide, is developed for all‐solid‐state sodium metal batteries. It offers a high sodium‐ion transference number and stability with sodium metal . Combined with poly(ethylene oxide), it forms strong solid polymer electrolytes that suppress dendrite growth and polarization. π–π stacking interactions enhance ionic conductivity and stability, outperforming conventional bis(trifluoromethanesulfonyl)imide in cycling performance.

1. Introduction

The growing demand for energy storage solutions, driven by the widespread adoption of renewable energy sources and the rapid expansion of electric vehicles, has intensified the search for alternative battery technologies.^{[ 1} , ² , ^{3 ]} While lithium‐ion batteries (LIBs) currently dominate the market, they face limitations such as the scarcity and rising cost of lithium, as well as safety concerns linked to the use of flammable liquid electrolytes.^{[ 1} , ^{4 ]} To overcome these challenges, sodium batteries have emerged as a promising alternative due to the natural abundance and lower cost of sodium compared to lithium.^{[ 5 ]}

All‐solid‐state sodium metal batteries (ASSSMBs) have garnered considerable interest among the different sodium battery technologies. Solid polymer electrolytes (SPEs) are considered promising substitutes for traditional liquid electrolytes due to their remarkable flexibility, ease of processing, and strong adhesion to alkali metal anodes.^{[ 6} , ^{7 ]} Both academia and industry have shown significant interest in PEs, as they not only improve performance but also contribute to the overall safety and longevity of battery systems.^{[ 8} , ⁹ , ¹⁰ , ^{11 ]} By enabling the use of a sodium metal (Na°) anode—known for its high theoretical capacity—ASSSMBs have the potential to outperform conventional sodium‐ion (Na‐ion) batteries.^{[ 12} , ¹³ , ¹⁴ , ¹⁵ , ^{16 ]} The SPEs also eliminate the flammability risk associated with organic solvents, improving the overall safety of the system.^{[ 17} , ¹⁸ , ^{19 ]}

Despite these advantages, ASSSMBs face several technical hurdles. Key challenges include achieving high ionic conductivity in solid electrolytes, ensuring stable interfaces between the Na° anode and the solid electrolyte, and preventing the growth of sodium dendrites, which can lead to short circuits and battery failure.^{[ 13} , ^{16 ]} Addressing these issues is critical to unlocking the full potential of ASSSMBs.

A crucial element in advancing these types of batteries is the selection of appropriate sodium salts within the solid electrolyte. These salts facilitate Na‐ion movement and significantly influence the overall stability and performance of the battery. Integrating sodium salts into the solid electrolyte should address key challenges such as ionic conductivity, interface stability, and dendrite formation.^{[ 20 ]} Furthermore, salts play a crucial role in the mechanical properties of SPEs.^{[ 21} , ^{22 ]}

Benzene‐based salts, with their rigid aromatic structures, not only reinforce the polymer matrix to maintain the structural integrity of SPEs and minimize deformation under stress but also enhance intermolecular interactions between the salt and polymer chains. This results in a more cohesive and stable SPE with increased mechanical strength able to mitigate dendrite growth.^{[ 23} , ²⁴ , ^{25 ]} Moreover, these salts influence the sodium‐ion transference number ($T_{\text{Na}}^{+}$), which is vital for reducing cell polarization and enhancing overall performance and longevity.^{[ 16} , ^{26 ]} Recent efforts to enhance the efficiency and lifespan of Na° batteries have focused on suppressing anion movement while boosting Na‐ion transport.^{[ 27} , ^{28 ]} Strategies to achieve higher $T_{\text{Na}}^{+}$, reduce polarization, and prevent concentration gradients include: 1) covalent bonding, 2) Lewis acid–base interactions, and 3) hydrogen bonding. Covalent bonding chemically tethers anions to high molecular weight polymers or inorganic particles, effectively limiting their mobility and enhancing Na‐ion selectivity.^{[ 29} , ³⁰ , ³¹ , ^{32 ]} In contrast, Lewis acid–base interactions utilize additives with Lewis acidic functionalities to trap anions, thereby improving Na‐ion conductivity.^{[ 30 ]} Finally, hydrogen bonding interactions capture anions within the electrolyte matrix, using specific functional groups to restrict their movement and enhance transference without sacrificing ionic conductivity.^{[ 33} , ³⁴ , ^{35 ]}

In principle, weak attractive interactions between molecules can serve also as a valuable strategy for designing sodium salts with decreased anion mobility. Notably, π–π stacking interactions are crucial in aromatic systems and have gained significant interest across various disciplines, including semiconductor technology, biochemistry, and materials science.^{[ 36} , ^{37 ]} These interactions have been utilized to create organic electrode materials and binders for rechargeable batteries, including those based on lithium, sodium, magnesium, and zinc.^{[ 38} , ³⁹ , ⁴⁰ , ^{41 ]} In 2022, Qiao et al. introduced benzene‐based lithium salts, lithium (benzenesulfonyl)(trifluoromethanesulfonyl) imide (LiBTFSI) and lithium (2,4,6‐triisopropylbenzenesulfonyl)(trifluoromethanesulfonyl)imide (LiTPBTFSI), to enhance lithium‐ion transport selectivity and conductivity in polymer electrolytes.^{[ 42 ]} The LiBTFSI‐based electrolyte showed a threefold increase in lithium‐ion transference number as compared to LiTFSI, due to strong π–π stacking interactions between BTFSI⁻ anions and exhibited excellent compatibility with lithium anodes. Despite their potential, the role of π–π stacking interactions in modifying anionic structures and their effects on the essential physicochemical properties of sodium‐based polymer electrolytes—such as phase behavior, ionic conductivity, $T_{\text{Na}}^{+}$, and overall battery performance—has received limited attention. This oversight gives an opportunity for further exploration in developing high‐performance sodium battery systems.

This article presents the synthesis and characterization of a novel sodium (benzenesulfonyl)(trifluoromethanesulfonyl)imide (NaBTFSI) (Scheme  1a). The unique structure of NaBTFSI features significant delocalization of the negative charge of the imide moieties (Scheme 1c), which promotes a high degree of Na⁺ dissociation when blended with poly(ethylene oxide) (PEO) at varying concentrations to create SPEs and the benzene rings associate due to π–π stacking interactions. The miscibility of the salt within the PEO polymer matrix, the crystallization kinetics of the polymer, and its effect on the ionic conductivity were investigated in detail. Then, the electrochemical compatibility of the synthesized NaBTFSI electrolytes with Na° electrodes when compared to sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) was studied (Scheme 1b). The enhanced cyclability and notable resistance to dendritic sodium growth of the SPE in sodium symmetric cell were assessed, which can be related to the improved mechanical stability of the membranes.

Scheme 1.

Chemical structure of a) sodium (benzenesulfonyl)(trifluoromethanesulfonyl)imide (NaBTFSI) and b) sodium bis(trifluoromethanesulfonyl)imide (NaTFSI). c) π–π stacking interaction between two NaBTFSI molecules. Color code: O (red), S (yellow), F (green), N (blue), C (gray), and H (white).

2. Results and Discussion

2.1. Synthesis and Characterization of Sodium Salt (NaBTFSI)

One simple approach to synthesizing sulfonamide‐based salts involves coupling a nucleophilic component with an electrophilic counterpart in the presence of a base, as reported in previous studies by Zhang et al.^{[ 42} , ⁴³ , ⁴⁴ , ⁴⁵ , ^{46 ]} In this method, a preactivated nucleophile reacts with sulfonyl chloride, functioning as the electrophile, to produce an anion containing the two desired functional groups.

A three‐step synthesis was followed (Figure  1a) starting from commercially available trifluoromethanesulfonamide (CF₃SO₂NH₂), which is first converted to its potassium salt to enhance nucleophilicity. Potassium [(trifluoromethyl)sulfonyl]amide (CF₃SO₂NHK) is then reacted with benzenesulfonyl chloride (PhSO₂Cl) in a 1:1 molar ratio in the presence of an excess of triethylamine, with the reaction occurring in ACN. The resulting triethylammonium salt, incorporating both trifluoromethyl (–CF₃) and phenyl (–Ph) moieties, undergoes an acidic workup to yield the acidic form of the anion (CF₃SO₂–NH–SO₂Ph). Finally, N‐[(trifluoromethyl)sulfonyl] benzenesulfonamide is treated with sodium bicarbonate (NaHCO₃) in ethanol at RT for 16 h, yielding the corresponding sodium salt. Product confirmation was obtained via NMR, where the characteristic peak of the –CF₃ group was observed at −79.26 ppm, distinct from the starting material CF₃SO₂NHK (−80.41 ppm) (Figure S1, Supporting Information). Additional signals corresponding to the aromatic ring were identified in both ¹H NMR (7.44–7.93 ppm for H_arom) and ¹³C NMR (126.35–145.71 ppm for C_arom) (Figure 1b–d). ICP analysis confirmed a high sodium content of 90 ± 5%.

Figure 1.

a) Synthesis scheme for NaBTFSI and NMR spectra of the as‐prepared NaBTFSI: b) ¹H‐NMR, c) ¹³C‐NMR, and d) ¹⁹F‐NMR.

The thermal properties of NaBTFSI were investigated using thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis and compared to those of the reference salt, NaTFSI. As shown in Figure  2a, NaBTFSI exhibits a slightly lower decomposition temperature compared to NaTFSI, with T _d5 values of 418 and 430 °C, respectively. This difference may be attributed to the greater stability of the C—F bond in NaTFSI compared to the C—C bond in NaBTFSI. These findings suggest that replacing one trifluoromethyl group with an aromatic ring has minimal impact on the stability of the salt. Although the decomposition temperature of NaBTFSI is lower than that of NaTFSI, it remains well above the operating temperature of solid‐state batteries.

Figure 2.

Thermal properties of NaBTFSI and NaTFSI salts: a) TGA profiles and b) DSC traces.

Figure 2b presents the DSC traces for NaBTFSI and NaTFSI. The data reveal that while both salts exhibit similar T _m, with NaBTFSI melting at 255 °C and NaTFSI at 258 °C, there are certain differences in their thermal properties. The substitution of a –CF₃ group in NaTFSI with an aromatic ring in NaBTFSI significantly impacts the melting enthalpy (ΔH _m). NaTFSI exhibits a lower ΔH _m, attributed to the symmetric and flexible structure of the TFSI anion, which weakens ionic interactions and reduces lattice energy. In contrast, NaBTFSI shows a higher ΔH _m due to the presence of the benzenesulfonyl group, which increases the rigidity and polarity asymmetry of the anion. This structural modification enhances ionic interactions and lattice stability, contributing to the observed differences in thermal properties.

2.2. Preparation of SPE Membranes

Polymer electrolyte membranes with an average thickness of 180 μm were fabricated using a standard solvent casting method, followed by hot‐pressing. The NaTFSI/PEO and NaBTFSI/PEO electrolytes were prepared by dissolving the respective Na salts and PEO in ACN. Salt concentrations were adjusted to achieve specific EO/Na ratios of 32:1, 20:1, 12:1, and 8:1, and the resulting electrolytes were designated as NaBTFSI‐32, NaBTFSI‐20, NaBTFSI‐12, and NaBTFSI‐8. The corresponding nomenclature was applied for NaTFSI‐based electrolytes.

2.3. Thermal Characterization of the SPEs

Due to its high salt solvation capacity, PEO is a reference polymer for studying novel salts or additives in SPEs, making it suitable for battery applications (e.g., with LiTFSI). The impact of salt addition on PEO is particularly evident in its T _m, a phenomenon previously investigated across various aliphatic polyethers, predominantly NaTFSI‐12. In the case of NaTFSI‐8, PEO becomes amorphous, suggesting that although both salts are miscible with PEO, NaTFSI exhibits a stronger interaction with the polymer matrix.^{[ 47} , ^{48 ]} This same trend is observed in Figure  3c, which illustrates the behavior of the glass transition temperature (T _g). Increasing the concentration of amorphous PEO (i.e., adding more salt) raises the T _g, likely due to a stiffening of the EO helices when coordinating the cation, partially compensated by the flexibility of the anions through the S—N—S bonds, in addition to changes in the free volume of the material. Comparable trends have been observed in other PEO systems with various lithium salts.^{[ 49 ]}

Figure 3.

Second heating DSC scan for a) PEO/NaBTFSI and b) PEO/NaTFSI‐based electrolytes and PEO (M _w = 5 × 10⁶ g mol⁻¹) polymer matrix. c) Change in T _g as a function of PEO amorphous fraction and d) change in melting enthalpy (ΔH _m) as a function of the fraction of Na salt.

In the case of NaBTFSI, adding this salt results in a more pronounced increase in the T _g, suggesting that PEO chains exhibit increased rigidity in the presence of NaBTFSI. Furthermore, Figure 3d demonstrates the effect of salt concentration on the ΔH _m of electrolyte, with a faster reduction in enthalpy observed upon NaTFSI addition. These findings will impact on the ionic conductivity, as ionic transport in SPEs primarily takes place within the amorphous regions of the polymer.

2.4. Overall Crystallization Rate

DSC isothermal crystallization experiments, including both primary nucleation and growth, were conducted using DSC to assess the impact of the two sodium salts on the overall crystallization kinetics of PEO. Figure  4 presents the inverse of the half crystallization time (i.e., a quantity proportional to the overall crystallization rate) as a function of crystallization temperature (Equation (S1), Supporting Information). In both series of blends, a higher supercooling (or lower crystallization temperature) is needed to crystallize PEO as the amount of salt increases. A comparison of NaBTFSI‐12 and NaTFSI‐12 electrolytes clearly shows the effect of the salts on the crystallization of PEO. NaTFSI‐12 needs higher supercooling to crystallize, and the crystallization rate decreases considerably. On the contrary, NaBTFSI can crystallize at higher temperatures (between 37 and 44 °C) with a higher crystallization rate. This indicates that the NaTFSI salt has a higher affinity with PEO. These results are attributed to the dilution effect of the sodium salts on PEO. However, this effect is different for each of the two salts. For NaBTFSI, the crystallization rates were measurable for NaBTFSI‐8, whereas for NaTFSI, PEO becomes fully amorphous at this salt concentration. All the fitted data are presented in Table S2, Supporting Information.

Figure 4.

Overall crystallization rate (i.e., the inverse of the half crystallization time) as a function of temperature for a) PEO/NaBTFSI and b) PEO/NaTFSI SPEs at the indicated salt concentrations.

2.5. Diluent Effect of Sodium Salts (Flory–Huggins Theory for Polymer Diluent Mixtures)

The diluent effect of NaBTFSI and NaTFSI was studied using the Flory–Huggins theory for polymer–diluent mixtures (Equation (2)).

Figure  5a displays the plot of [(1/T _m − 1/$T_{\mathrm{m}}^0$)/${\upsilon }_1$] × 10³ versus υ ₁/T _m for all samples. According to Flory's theory, the data follow a straight line, indicating that both salts act as diluents for PEO. From the intercept of the straight line, the value of melting enthalpy per mole of repeating unit (ΔH _u) can be determined, and from the slope, the value of the interaction energy parameter (B) is calculated. According to Flory's theory, ΔH _u is an intrinsic property of the polymer and should be around 214 J g⁻¹ for PEO. However, as shown in Figure 5b, ΔH _u increases with higher sodium salt concentrations, indicating that complexes between PEO and the salts are formed. To further examine this phenomenon, X‐ray diffraction (XRD) spectra were obtained for all electrolyte samples and the PEO reference (Figure S2 and S3, Supporting Information). Characteristic diffraction peaks for PEO appear at 19.1° (corresponding to the (102) plane) and 23.3° (corresponding to the (112) plane). These peaks remain at the same positions across all electrolytes; however, their intensity decreases with increasing salt concentration, indicating enhanced amorphous character in the polymer matrix. Additionally, new peaks emerge in the SPEs at 11.3°, 14.6°, and 22.7°, associated with a crystalline complex formation between PEO and the sodium salts, most notably at higher salt concentrations (NaBTFSI‐8). This phenomenon, previously reported for PEO/LiTFSI systems,^{[ 50 ]} accounts for the observed increase in ΔH _u.

Figure 5.

a) Diluent effect for NaBTFSI and NaTFSI‐based SPEs. b) Variation of melting enthalpy per mole of repeating unit (ΔH _u) and c) interaction energy parameter (B) variation with salt fraction.

The different slopes (i.e., B values) reflect differences in the interaction energy parameter (Figure 5c) of the SPEs. The general trend is that this interaction parameter decreases with increasing salt concentration; conversely, conductivity increases with increasing salt concentration.

2.6. Ionic Conductivity

The assessment of ionic conductivity is essential for determining the suitability of SPEs in ASSSMBs. Figure  6 illustrates the temperature dependence of ionic conductivity for the synthesized SPEs, which display nonlinear behavior due to crystalline phase melting. At lower temperatures, NaBTFSI‐8 exhibits the highest ionic conductivity (≈2.7 × 10^{− 5} versus 5.4 × 10^{− 6} S cm⁻¹ for NaBTFSI‐20 at 40 °C), likely due to its lower melting transition at 48 °C (Figure 6a) and negligible crystallinity degree (Table S1, Supporting Information). At 70 °C, after all samples have undergone crystalline phase melting, the SPEs demonstrate similar conductivity values [(≈2.7 × 10^{− 4} S cm⁻¹ (NaBTFSI‐20) versus 1.3 × 10^{− 4} S cm⁻¹ (NaBTFSI‐32)]. Notably, NaBTFSI‐32 demonstrates the lowest ionic conductivity due to the very dilute salt content and its higher crystallinity (X _c = 58%) compared to NaBTFSI‐20 (X _c = 52%) at temperatures above the T _m (Figure 3a and Table S1, Supporting Information). Based on these results, an EO/Li ratio of 20 was chosen for further characterization, as SPEs with this ratio provided an optimal balance of mechanical properties and ionic conductivity.

Figure 6.

Comparison of the ionic conductivity a) for the NaBTFSI‐based SPEs and b) for reference electrolyte NaTFSI/PEO and NaBTFSI/PEO with EO/Li = 20.

Figure 6b compares the ionic conductivities of NaBTFSI‐20 and the reference electrolyte NaTFSI‐20. Below 60 °C, before the onset of crystalline phase melting, NaTFSI‐20 exhibits significantly higher ionic conductivity, ≈2.7 × 10^{− 5} S cm⁻¹, compared to 5.4 × 10^{− 6} S cm⁻¹ for NaBTFSI‐20 at 40 °C. This decreased ionic conductivity in NaBTFSI‐20 can be attributed to the higher T _g and crystallinity values compared to NaTFSI‐20 (Table S1, Supporting Information), as well as to the weaker plasticizing effect and lower mobility of the benzene‐based anions. Above 60 °C, the melting of the crystals occurs and the difference in ionic conductivity is minimized. These trends are consistent with results from DSC, supporting the relationship between crystallinity and ionic conductivity in these SPEs.

2.7. Transport Properties, Diffusion Coefficient, and Polarization Studies

Na‐ion conduction in PEO‐based electrolytes relies on segmental motion in the amorphous phase, where Na ions bond with oxygen atoms via Lewis acid–base interactions. However, anions interact less with the ether oxygen, leading to faster anion transport than Na‐ion transport. This imbalance results in a low $T_{\text{Na}}^{+}$, which causes salt concentration gradients when operating a battery. These gradients can lead to issues like voltage loss, high internal resistance, dendrite formation, and early battery failure. Therefore, polymer electrolytes with high $T_{\text{Na}}^{+}$ are preferred to reduce these problems and mitigate sodium dendrite growth. NaBTFSI‐based electrolytes exhibited higher $T_{\text{Na}}^{+}$ values than NaTFSI‐based ones (0.83 vs 0.70, respectively) (Figure S4, Supporting Information). This is likely due to the reduced anion mobility from the π–π stacking of benzene‐based anions with themselves. The σ _Na ⁺ of PEO‐based SPEs was calculated using Equation (4). Thanks to its higher $T_{\text{Na}}^{+}$, the NaBTFSI/PEO‐based SPE showed a greater fraction of ${\sigma }_{\text{Na}}^{+}$ than the NaTFSI/PEO‐based SPE (83% of the total ionic conductivity vs 70%), as it is noticeable in Figure  7a. The diffusion coefficient of the SPEs was calculated using Equation (5), and the results are shown in Figure 7b. As it can be observed, NaBTFSI‐20 and NaTFSI‐20 show similar values of diffusion coefficient, (≈1.9 × 10⁻¹² vs 1.7 × 10⁻¹² m² s⁻¹, respectively), in line with the results reported for other PEO‐based SPEs with conventional lithium sulfonimide salts.^{[ 51 ]}

Figure 7.

a) Total and Na‐ion conductivity and b) diffusion coefficient and difference in slope for NaBTFSI‐20 and NaTFSI‐20. Experimental and theoretical values of voltage versus current for c) NaBTFSI‐20 and d) NaTFSI‐20. All the measurements were carried out at 70 °C.

In concentration polarization experiments, the formation of ion concentration gradients in an electrolyte under applied current is studied. This phenomenon arises from unequal ion transport rates, particularly when anions migrate faster than cations. As a result, salt accumulates at one electrode and depletes at the other, leading to voltage drop and an increase in the internal resistance of the electrolyte over time. Ohm's law (Equation (6)) defines the relationship between voltage, current, and resistance in a circuit. This law assumes that resistance remains constant, a condition that holds true only under ideal circumstances. However, in practical electrochemical systems, especially those influenced by concentration polarization, deviations from this ideal behavior occur due to the dynamic nature of resistance. Concentration polarization arises when ion concentration gradients develop within the electrolyte during operation, typically as a result of uneven ion transport. These gradients increase the effective resistance of the system, causing additional voltage drops. This phenomenon is illustrated in Figure 7c,d for NaBTFSI‐20 and NaTFSI‐20, respectively. The slopes in these figures represent the change in voltage with respect to current density, and deviations from linearity highlight the impact of concentration polarization. Figure 7b further quantifies these deviations by comparing the difference in slopes between the two electrolytes. Notably, NaBTFSI‐20 demonstrates a smaller deviation from the ideal linear behavior predicted by Ohm's law, particularly at higher current densities (Figure 7c). This suggests that NaBTFSI‐20 exhibits better ion transport properties or reduced concentration polarization effects under similar operating conditions.

Conversely, NaTFSI‐20 experiences a more pronounced voltage deviation due to concentration polarization, as shown in Figure 7d. This increased deviation indicates higher internal resistance, which can adversely affect the performance of electrochemical cells. The additional voltage drop reduces the overall voltage output and increases energy inefficiency, as more energy is dissipated as heat rather than contributing to the desired electrochemical reaction. This behavior underscores the importance of selecting or designing electrolytes with minimized concentration polarization effects to enhance the efficiency and performance of energy storage and conversion devices.

2.8. Electrochemical Stability Window

The selection of cathode materials for ASSMBs is generally guided by the anodic stability of the SPEs. Figure S5, Supporting Information, illustrates the linear sweeping voltammetry (LSV) profile of the Na^o|| stainless steel (SS) cell using NaBTFSI‐20 and NaTFSI‐20 electrolytes at 70 °C. As seen, the electrolytes exhibit a minor oxidation at 3.5 V versus Na^o/Na⁺, which is attributed to the oxidative decomposition of EO‐based electrolytes at these potentials. This is due to the electrochemically unstable C—O bond in the polymer backbone, leading to the formation of terminal carboxyl groups or esters. Furthermore, at 5 V versus Na^o/Na⁺, a noticeable increase in current is observed, indicating the upper boundary of the electrochemical stability window of the electrolyte. This results in the full oxidation of the electrolyte, causing polymer chain excision and the generation of volatile products. This observation highlights the importance of selecting cathode materials that function within the anodic stability limits of the SPEs to ensure the safety and longevity of ASSMBs.

Cyclic voltammetry (CV) was conducted using copper (Cu) as the working electrode and Na⁰ as both the counter and reference electrodes to evaluate the cathodic stability of the electrolytes, as shown in Figure S6, Supporting Information. In the case of NaBTFSI‐20, the first cycle reveals two distinct and irreversible reduction events near 1.5 and 0.5 V versus Na⁺/Na⁰, which are attributed to initial solid electrolyte interphase (SEI) formation and the reductive decomposition of the electrolyte components. Notably, these processes are predominantly confined to the initial cycle, with subsequent scans exhibiting minimal current response, indicative of rapid passivation and the formation of a stable interphase on the Cu surface. In contrast, the NaTFSI‐20 system shows continued electrochemical activity over successive cycles, including oxidative features above 1.5 V, suggesting less effective SEI formation and ongoing interfacial reactions. These observations highlight the superior interfacial stability of NaBTFSI‐20, which forms a self‐limiting SEI that may contribute to enhanced safety and cycling reliability in ASSMBs.

2.9. Electrochemical Stability of Electrolyte/Na^o Electrode

The interface formed between the Na° anode and the electrolyte is a critical factor affecting the cycling performance of rechargeable ASSSMBs. Figure  8a,b presents the electrochemical impedance spectroscopy plots for NaBTFSI‐20 and NaTFSI‐20, respectively, tested at 70 °C. Both electrolytes displayed similar initial total resistance (R _T) values. Overtime, NaTFSI‐20 showed a more significant variation in R _T, attributed to the gradual formation of an interfacial passivation layer under static conditions. In contrast, NaBTFSI‐20 maintained a stable R _T overtime. These results suggest that the synthesized NaBTFSI is more stable against the Na° anode, requiring only a short period for complete interface stabilization. This stability may be attributed to the presence of less mobile anionic species, which restricts further reactions at the anode interface. The high interfacial resistance observed in both systems is primarily attributed to poor physical contact between the PEO electrolyte and the Na° anode, which hampers efficient Na⁺ transfer across the interface. In the case of NaTFSI‐20, this challenge is compounded by the formation of an unstable SEI, resulting in a steady increase in resistance over time. Conversely, NaBTFSI‐20 forms a more robust and stable SEI that could help suppress side reactions and dendrite formation.

Figure 8.

Normalized impedance spectra of Na^o||Na^o cells at 70 °C overtime for a) NaBTFSI‐20 and b) NaTFSI‐20. c) Galvanostatic cycling performance for the Na^o symmetric cells at 70 °C with a current density of 0.1 mA cm⁻² for NaBTFSI‐20 and NaTFSI‐20 and the zoom between d) 10 and 60 h, e) 200 and 250 h, and f) 350 and 400 h.

The successful operation of ASSSMBs necessitates highly reversible and dendrite‐free cycling of the Na° electrode. This can be evaluated through plating/stripping experiments performed under galvanostatic conditions. Such chronoamperometric testing provides insights into the behavior of the electrode/electrolyte interface during cycling, thus simulating real battery operating conditions. Figure 8c–f shows the voltage profiles of Na° symmetric cells subjected to a constant current density of 0.1 mA cm⁻² at 70 °C. The NaTFSI‐20 reference electrolyte was cycled for less than 50 h before an internal short‐circuit occurred. In contrast, the cell incorporating NaBTFSI‐20 demonstrated prolonged stability, sustaining continuous cycling for over 350 h without any apparent side reactions. This stability suggests the formation of a more robust SEI on the Na° anode in the NaBTFSI‐based cell. A stable SEI effectively suppresses continuous Na° dendrite growth, thus promoting a longer cell lifespan. Moreover, these results highlight the critical role of an electrolyte with high $T_{\text{Na}}^{+}$ in reducing cell polarization, promoting homogeneous Na° plating and reducing dendrite nucleation. Additionally, the structural rigidity conferred to the SPE by the aromatic ring in the synthesized NaBTFSI salt appears to contribute favorably to these effects.

3. Conclusion

This study presents the synthesis and characterization of the asymmetric benzene sulfonamide sodium salt NaBTFSI, demonstrating its potential as an effective electrolyte component for ASSSMBs. Polymer electrolyte membranes were prepared with NaBTFSI and characterized to evaluate their influence on polymer crystallinity, thermal properties, ionic conductivity, and electrochemical stability. NaBTFSI exhibited stable interaction with PEO as evidenced by the decrease in T _m and crystallinity, favoring the formation of amorphous regions crucial for appropriate ionic transport.

Key electrochemical measurements indicated that NaBTFSI‐based SPEs exhibit a higher $T_{\text{Na}}^{+}$ compared to NaTFSI‐based SPEs. This enhancement is attributed to the formation of strong π–π stacking interactions between pairs of BTFSI⁻ anions, which reduce concentration polarization and facilitate more uniform Na° plating. This feature significantly enhances cycle life, as confirmed in chronoamperometric tests, where NaBTFSI‐20 sustained continuous cycling for over 350 h under galvanostatic conditions without short‐circuiting. This study underscores the advantages of NaBTFSI as a novel sodium salt for polymer electrolytes with optimal transport properties and interfacial stability, which makes it a promising candidate for ASSSMB applications.

4. Experimental Section

4.1.

4.1.1.

Materials

Trifluoromethanesulfonamide (CF₃SO₂NH₂, 98%), magnesium sulfate (MgSO₄, anhydrous), and acetonitrile (ACN, 99.9% anhydrous) were purchased from Thermo Scientific. Potassium hydroxide (KOH), hydrochloric acid (HCl, 37%), ethanol (EtOH, 99%), and acetone (99%) were supplied by Scharlab. Benzenesulfonyl chloride (99%), triethylamine (99%, anhydrous), tert‐butyl methyl ether (MTBE, 99.8% anhydrous), sodium bicarbonate (NaHCO₃, 99.5%), and poly(ethylene oxide) [PEO (M _w = 5 × 10⁶ g mol⁻¹)] were received from Sigma–Aldrich. Deuterated acetone (acetone‐d ₆, 99.8% D) was purchased from Eurisotop. Battery grade Na° chips of 12 mm diameter were supplied by MSE. All reagents were used as purchased.

Synthesis of NaBTFSI

NaBTFSI was prepared according to the synthetic route depicted in Figure 1a. To a solution of trifluoromethanesulfonamide (15 g, 100 mmol, 1 eq) in acetone (30 mL), a solution of KOH (5.6 g, 100 mmol, 1 eq) in H₂O (8 mL) was added at 0 °C and the reaction mixture was stirred for 5 min at room temperature (RT). After evaporation of the solvent, the crude was used in the next step without further purification. A solution of potassium[(trifluoromethyl)sulfonyl]amide (18.7 g, 100 mmol, 1 eq) in ACN (100 mL) was added dropwise to a cooled solution (0 °C) of benzenesulfonyl chloride (17.6 mL, 100 mmol, 1 eq) in ACN (150 mL), followed by triethylamine (20.4 mL, 200 mmol, 2 eq) at the same temperature. The reaction mixture was stirred for 16 h at RT. Once the reaction was completed, the solvent was evaporated and the crude was acidified with concentrated HCl (80 mL). The acid form of the anion was extracted with MTBE. Organics were collected, dried over Mg₂SO₄, and evaporated, affording HBTFSI as a yellowish oil which was converted into the sodium salt by stirring for 16 h in EtOH (60 mL) with NaHCO₃ (15 g, 178 mmol, 1.2 eq). NaBTFSI was obtained (12.3 g, 40 mmol) as a white powder with 40% of total yield after three steps. ¹HNMR (300 MHz, acetone‐d ₆, ppm): δ = 7.93–7.90 (m, 2H, H_arom), 7.52–7.44 (m, 3H, H_arom). ¹³C NMR (75.5 ppm, acetone‐d ₆, ppm) δ = 145.7 (C_arom), 130.9 (C_arom), 128.1 (C_arom‐H), 126.3 (C_arom‐H). ¹⁹F NMR (283 MHz, acetone‐d ₆, ppm): δ = −79.26 (s, 3F). The NMR spectra are shown in Figure 1b–d. Inductively coupled plasma optical emission spectroscopy: sodiation degree = 90 ± 5%.

Chemical Characterization

Nuclear magnetic resonance spectroscopy [NMR, Bruker 300 Ultrashield (300 MHz for ¹H, 75.5 MHz for ¹³C, and 283 MHz for ¹⁹F)] was used to characterize the structure of the synthesized sodium salt. Chemical shifts (δ) are reported in ppm relative to residual solvent signals (acetone, 2.05 ppm for ¹H‐NMR and 29.84 ppm for ¹³C‐NMR). The following abbreviations indicate the multiplicity in ¹H NMR spectra: s, singlet; m, multiplet.

Sodium concentration was analyzed using inductively coupled plasma optical emission spectroscopy with a Horiba Ultima 2 System and AS500 autosampler. The instrument operated at 1.0 kW RF power with gas flows of 13 L min⁻¹ (plasma), 0.2 L min⁻¹ (sheath), and 0.25 L min⁻¹ (nebulizer), while samples were introduced at 0.87 mL min⁻¹. Calibration solutions were prepared by diluting 1000 mg L⁻¹ sodium standards with ultrapure water and nitric acid, and calibration curves were generated for sodium concentrations ranging from 0.1 to 100 mg L⁻¹. Samples of NaBTFSI were dissolved in ultrapure water at 0.5 mg mL⁻¹ for analysis.

Preparation of Polymer Electrolytes

Electrolyte membranes were fabricated by blending the as‐prepared NaBTFSI with PEO. A series of electrolyte compositions were formulated by varying the ratios of PEO/Na, (–CH₂CH₂O– ([EO])/([Na⁺] = 32, 20, 12, and 8). PEO was dissolved in ACN before the addition of NaBTFSI. The solutions were stirred for 24 h at RT to ensure homogeneity. The resulting solution was then cast onto a polytetrafluoroethylene (PTFE) dish and dried for 24 h at 35 °C in a P selecta Digitronic TFT drying oven with constant air flow. Hot‐pressing was applied to achieve uniform membrane. The samples were pressed over 1 min at 70 °C under pressure (≈2 tons cm⁻²) yielding membranes with an average thickness of 180 μm. Finally, all membranes were vacuum‐dried at 50 °C for 16 h to remove residual solvent.

Thermal Characterization

A simultaneous thermogravimetric analyzer (TG 209F1 Libra NETSCH) was used to determine the thermal stability of sodium salts. The samples were placed in an alumina crucible and heated from RT (≈25 °C) to 600 °C at a heating rate of 10 °C min⁻¹ under argon atmosphere. The decomposition temperature (T _d) was defined as the temperature corresponding to a 5% weight loss relative to the initial sample mass.

DSC (DSC Q2000, TA Instruments) was used to study the phase transition behavior of sodium salts and SPEs. Samples were hermetically sealed in aluminum pans in an argon‐filled glove box with an average ≈5−10 mg mass. The analysis of sodium salts was performed over two successive scans, using heating and cooling rates of 10 °C min⁻¹, within a temperature range of −80–300 °C, under argon atmosphere.

In the case of SPEs, DSC experiments were performed in a PerkinElmer 8000 DSC fitted with an Intracooler II, and ultrapure nitrogen atmosphere was used. For nonisothermal experiments, the samples were measured in a temperature range between −70 and 120 °C at 20 °C min⁻¹. The samples were first heated to 120 °C to erase thermal history, and then, cooling and second heating scans were performed.

The crystallinity degree was calculated from the second heating scans using the following equation:

$$\%X_{\mathrm{c}}=\frac{\Delta H_{\mathrm{m}}}{\Delta H_{\mathrm{m}}^0\hspace{0.17em}f}\times 100$$ (1)

where ΔH _m is the measured melting enthalpy, $\Delta H_{\mathrm{m}}^0$ is the equilibrium melting enthalpy (214 J g⁻¹ for PEO),^{[ 52 ]} and f is the PEO weight fraction in the sample.

For the isothermal overall crystallization rate, the experiments were performed as follows: first, the crystallization temperature (T _c ) range employed for each sample was determined by the methodology recommended by Lorenzo et al.^{[ 53 ]} After, the samples were evaluated as follows: 1) heating from 25 to 120 °C at 20 °C min⁻¹; 2) holding at 120 °C for 3 min; 3) cooling to T _c at 60 °C min⁻¹; 4) holding at T _c during 15–60 min to allow crystallization to saturate; and 5) heating from T _c to 120 °C at 20 °C min⁻¹ to measure the DSC heating scan after the isothermal crystallization.

XRD

X‐ray powder diffraction patterns were collected by using a Philips X'pert PRO automatic diffractometer operating at 40 kV and 40 mA, in theta–theta configuration, secondary monochromator with Cu Kα radiation (λ = 1.5418 Å) and a PIXcel solid‐state detector (active length in 2θ 3.347°). Data were collected from 5 to 70° 2θ (step size = 0.026 and time per step = 60 s) at RT. A 1° fixed soller slit and divergence slit giving a constant volume of sample illumination were used. The blends were heat treated before X‐ray measurement; all samples were heated to 120 °C at 20 °C min⁻¹ and cooled to RT at 20 °C min⁻¹ so that the conditions were the same as in the DSC.

Diluent Effect of Sodium Salts (Flory–Huggins Theory for Polymer Diluent Mixtures)

The Flory–Huggins theory for polymer diluent mixtures was used to demonstrate that NaBTFSI and NaTFSI act as solvent, the equation used was the following:

$$\frac{\frac{1}{T_{\mathrm{m}}}-\frac{1}{T_{\mathrm{m}}^0}}{{\upsilon }_1}=\frac{R}{\Delta H_{\mathrm{u}}}\frac{V_{\mathrm{u}}}{V_1}\left(1-\frac{B{V}_1}{R}\frac{{\upsilon }_1}{T_{\mathrm{m}}}\right)$$ (2)

where ΔH _u is the melting enthalpy per mole of repeating unit, V _u and V ₁ are the molar volumes of the polymer repeating unit and the diluent, respectively, ${\upsilon }_1$ is the volume fraction of the diluent, B is the interaction energy density character of the polymer–diluent pair, T _m is the melting temperature (taken from the DSC second heating run), and $T_{\mathrm{m}}^0$ is the equilibrium melting temperature (determined by the Hoffman–Weeks extrapolation method). All the temperatures are expressed in Kelvin degrees and R is the gas constant.

Ionic Conductivity

The ionic conductivity was evaluated using electrochemical impedance spectroscopy (EIS) on CR2032‐type cells, which were constructed by sandwiching a polymer electrolyte disc between two SS blocking electrodes (SS | PE | SS) within an argon‐filled glove box. The EIS measurements were performed with a VMP3 potentiostat (Biologic), employing a frequency range of 10⁻¹–10⁶ Hz and a signal amplitude of 10 mV. The cells were heated to 100 °C for 12 h before measurement to facilitate optimal contact between the electrolyte and electrodes. Conductivities were assessed over a 25–90 °C temperature range using a temperature chamber (BINDER), ensuring that thermal equilibrium was achieved for at least 1 h before each measurement. The ionic conductivity was determined using Equation (3):

$$\sigma =\frac{1}{R}·\frac{t}{A}$$ (3)

where R represents the bulk electrolyte resistance, t denotes the thickness of the electrolyte membrane, and A is the surface area. The conductivity was computed as the average of three cells for each sample.

Sodium Transference Number

The sodium transference number ($T_{\text{Na}}^{+}$) of the polymer electrolytes at 70 °C was determined through a combined approach involving alternating current (AC) impedance and direct current (DC) polarization measurements, utilizing a symmetric Na° | SPE | Na° cell. The $T_{\text{Na}}^{+}$ values were calculated using the method proposed by Hu et al.^{[ 54 ]} The measurements were carried out at 70 °C.

Na‐Ion Conductivity

The Na‐ion conductivity (${\sigma }_{\text{Na}}^{+}$) of PEO‐based SPEs was calculated by the measured total conductivity and $T_{\text{Na}}^{+}$, followed by Equation (4):

$$\sigma ={\sigma }_{\mathrm{T}}\times T_{\text{Na}}^{+}$$ (4)

Diffusion Coefficient

The same procedure was followed for the $T_{\text{Na}}^{+}$ test. The diffusion coefficient (D) of the SPEs was calculated from the relaxation profile of polarized cells by Equation (5) proposed by Newman et al.^{[ 55 ]}

$$a=-\frac{{\pi }^2}{L^2}D$$ (5)

where in a is the slope calculated from representing the Napierian logarithm of the voltage versus time, L is the thickness of the electrolyte, and D is the diffusion coefficient. The measurements were carried out at 70 °C.

Concentration Polarization Studies

This analysis was conducted at 70 °C using a VMP3 potentiostat (Biologic) using symmetric Na° | SPE | Na° cell. Different current densities were applied for the constant current cycling test, ranging from 0.005 to 0.1 mA cm⁻², with each half‐cycle lasting 2 h and with 2 h rest and EIS measurement between charge/discharge processes. Ohm's law (Equation (6)) was used to calculate the theoretical ΔV value at each studied current density for each sample.

$$I=\frac{\Delta V}{R_{\mathrm{T}}}$$ (6)

where I denotes the applied current density, R _T represents the total resistance of the electrolyte membrane, and ΔV is the voltage.

Electrochemical Stability Window

The electrochemical stability window of the polymer electrolytes was investigated using CV and LSV. Cathodic stability was examined via CV employing a copper disk electrode (15 mm diameter) as the working electrode, with Na^o serving as both the counter and reference electrodes. Measurements were conducted on a Biologic VMP3 potentiostat by scanning from the open‐circuit voltage (OCV) down to −0.5 V versus Na⁺/Na⁰ at a rate of 0.1 mV s⁻¹ at 70 °C. Anodic stability was evaluated through LSV using a stainless‐steel electrode (16 mm diameter) as the working electrode, again with Na⁰ (12 mm diameter) as both counter and reference electrodes, all assembled in a standard 2032 coin cell configuration. The potential was swept from the OCV up to 6.0 V versus Na⁺/Na⁰ under the same scan rate and temperature conditions.

Electrochemical Stability of Electrolyte/Na° Electrode

EIS measurements were conducted in Na° | SPE | Na° cells using a VMP3 potentiostat (Biologic), with frequencies ranging from 10⁻¹ to 10⁶ Hz and a signal amplitude of 10 mV to analyze the change in the interfacial resistance for 24 h. To investigate the electrochemical stability of the SPE/Na° interface, galvanostatic cycling was performed using a Maccor battery tester (series 4000). The cells were subjected to constant current cycling at a current density of 0.1 mA cm⁻², with each half‐cycle lasting 2 h. These measurements were carried out at 70 °C.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supplementary Material

Data Availability Statement

Research data are not shared.