Ethylene Carbonate-based Concentrated Electrolyte Composed of Asymmetric Amide Anion for Rechargeable Sodium-ion Batteries

Abstract

The phase behavior, Na⁺ coordination structure, Na⁺ conduction and transport properties, and battery performance of liquid electrolytes comprising sodium (fluorosulfonyl) (trifluoromethylsulfonyl) amide (NaFTA) and ethylene carbonate (EC) are investigated. A highly concentrated electrolyte with a molar ratio of EC/NaFTA = 1.5 is shown to exhibit a stable liquid state at room temperature. It is revealed that Na⁺ in the concentrated electrolyte coordinates—not only with EC but also with anions—to satisfy stable coordination numbers. From this specific coordination structure, dynamic ligand exchange of Na⁺ is shown to be enhanced, and [Na | NaFTA+1.5EC | Na_0.44MnO₂] cells are shown to achieve stable cycling performance by constant charge-discharge tests at 303 K.

1. Introduction

Renewable energy, which does not rely on fossil fuels, is crucial for achieving a sustainable society. However, renewable energy sources such as solar and wind power are high priority challenges because of output fluctuations, that depend on weather conditions, and the need for stabilizing power grids.¹ Therefore, the introduction of large-scale stationary battery systems is essential. Lithium-ion batteries (LIBs), with their high energy density, are one potential solution for effective stationary storage. However, the initial cost of these battery systems is expected to increase, owing to their scarcity and the geopolitical risks associated with lithium, thus prohibiting the progress of renewable energy.^2,3 Recently, sodium-ion batteries (SIBs) have attracted considerable interest because of similarities in components and basic operating principles with lithium-ion batteries, making it easy to adapt existing manufacturing facilities. Furthermore, elemental sodium is abundant, allowing for production of low-cost SIBs and addressing resource constraints.⁴ The development of low-cost energy storage devices promotes renewable energy generation, leading to significant reductions in CO₂ emissions.

In general, electrolytes for SIBs consist of organic solvents and sodium salts, with a concentration of approximately 1.0 mol kg⁻¹ commonly reported.⁵ Conventional electrolytes require several performance parameters such as high ionic conductivity, and thermal and electrochemical stability. To address the challenge of low energy density in SIBs, it is essential to focus on developing electrode materials for high voltage, ranging from 4 to 5 V vs. Na/Na⁺. Simultaneously, to suppress the decomposition of electrolytes caused by high operating voltages, it is necessary to design electrolytes with a wide electrochemical window, particularly on the positive electrode side.⁵ In recent years, concentrated electrolytes with intentionally increased salt concentration have attracted attention as compatible electrolytes with high electrochemical stability and activity.⁶ In concentrated electrolytes, almost all of the solvent molecules coordinate with Na⁺ ions, and free solvent molecules are nonexistant.⁷ This unique solution structure contributes to improvements in the electrochemical stability and rate capabilities of electrochemical cells.^8,9 However, issues remain regarding the slow transport of Na⁺ ions, owing to the high viscosity of concentrated electrolytes, and high operating temperatures caused by solvent crystallization in solvent mixtures of Na salt and conventional solvents.¹⁰ Therefore, in this study, we investigated and evaluated the physical properties, electrochemical characteristics, and battery performance of a binary mixture of ethylene carbonate (EC) and sodium (fluorosulfonyl) (trifluoromethylsulfonyl) amide (NaFTA). NaFTA inhibits the crystallization of concentrated electrolytes because of its asymmetric anion structure. In EC-based concentrated electrolytes, even a mixture of EC/NaFTA = 1.5 exhibited a liquid state at room temperature, and their bulk properties and basic Na battery performance are experimentally examined.

2. Experimental

2.1 Preparation of liquid electrolytes

Battery-grade ethylene carbonate (EC, Kishida Chemical Co., Ltd., F_w: 88.06), NaN(SO₂F)(SO₂CF₃) (NaFTA, Fujifilm Wako Pure Chemical, F_w: 253.13), NaN(SO₂F)₂ (NaFSA, Kishida Chemical Co., Ltd., F_w: 203.12), and NaN(SO₂CF₃)₂ (NaTFSA, Kishida Chemical Co., Ltd., F_w: 303.14) were used as the solvent and sodium species, respectively. These materials were stored in a dry-Ar filled glovebox ([O₂] < 10 ppm, [H₂O] < 0.5 ppm, Miwa MFG Co., Ltd.). Appropriate amounts of EC and Na salts (EC-NaX) were weighed and mixed in a sample bottle, resulting in a homogeneous liquid electrolyte obtained at 323 K. The salt concentration of each sample was denoted by molar ratio x = EC/NaX, ranging from 1 to 10.

2.2 Preparation of SMO positive electrode sheet

In this study, all electrode fabrication processes were carried out in an Ar-filled glove box because of the instability of the positive electrode in atmosphere conditions. The Na_0.44MnO₂ (SMO, NEI Corporation) composite positive electrode was prepared by mixing SMO, acetylene black (AB, Denka), and poly(vinylidene difluoride) (PVdF, Kureha) in an 80 : 10 : 10 weight ratio. A quantity of 1-Methyl-2-pyrrolidinone (NMP, Fujifilm Wako Pure Chemical Co.) was added to obtain a homogeneous slurry. The slurry was applied onto Al foil, to a thickness of 50 µm each, and dried for 2 days at 353 K. The resulting SMO composite positive electrode sheets were punched into ϕ16 mm disks and compressed at 1.5 t for 40 s.

2.3 Thermal properties

The thermal properties of prepared EC-NaX electrolytes were evaluated using differential scanning calorimetry (DSC; Thermo Plus EVO2 analyzer, Rigaku). The electrolyte samples were hermetically sealed in Al pans within an Ar-filled glove box. Thermograms were recorded during a cooling scan (303 to 173 K) followed by a heating scan (173 to 353 K), with both scans conducted at a consistent rate of 5 K min⁻¹.

2.4 Transport properties

Viscosity (η) and density (ρ) measurements for the EC-NaX electrolyte were carried out using a Stabinger-type viscometer/density measurement system (SVM 3000/G2, Anton Paar). The temperature range for measurements was 353.15 to 283.15 K, with measurements taken at 5 K intervals while cooling the samples. Prior to each measurement, the samples were thermally equilibrated at each temperature for at least 15 min.

Ionic conductivity (σ) was evaluated using a two-electrode [stainless steel (SUS) | electrolyte | SUS] sealed cell (MiCLab). All sample cells were assembled in an Ar-filled glove box. Electrochemical impedance spectroscopy (EIS) measurements were performed using a Bio-logic VSP instrument, covering a frequency range from 500 kHz–50 mHz with an alternating current (AC) amplitude of 100 mV. The measurements were taken while cooling the samples from 353 K to 263 K at 5 K intervals. Prior to each measurement, the samples were thermally equilibrated at each temperature for at least 90 min.

2.5 Raman spectroscopy

Raman spectra of electrolyte samples were measured using an RFT-6000 (Jasco) with 520 nm laser excitation, which was calibrated against a polypropylene standard at room temperature. Glass tubes (ϕ = 5 mm, l = 60 mm) were filled with the prepared electrolytes. Raman spectra were normalized using the integrated intensity of the CH₂ scissoring vibration peak of the EC molecule in the range of 1450–1500 cm⁻¹.

2.6 Electrochemical evaluations

The battery performance was investigated using 2032-type coin cells (Hohsen), and Na metal negative electrodes were prepared by melting Na ingot (Sigma-Aldrich, 99 %) at 453 K, then dripping it into dehydrated heptane (Fujifilm Wako Pure Chemical) in an Ar-filled glove box. The resulting spherical Na metal droplets formed in heptane were subjected to pressure, flattened, and punched into ϕ 16 mm discs for use as the Na metal electrode. Constant-current charge-discharge measurement tests of [Na metal negative electrode | electrolyte + separator | SMO positive electrode] cells were performed in galvanostatic mode using a charge–discharge measurement system (HJ1020mSD8, Hokuto Denko) at 2.0–3.8 V and a current density of approximately 6.0 µA cm⁻² (5.8 mA g⁻¹) at 303 K.

3. Results and Discussion

3.1 Phase behavior of NaX - EC electrolytes

To investigate the effect of anion structure on the thermal properties of concentrated electrolytes, Figs. 1a through 1c show the DSC profiles of liquid electrolytes composed of Na salts (a: NaFSA, b: NaFTA, c: NaTFSA) and EC, respectively (see Fig. S1). The melting temperature (T_m) and glass transition temperature (T_g) were also determined from the onset temperature of the endothermic peaks and the onset temperature of the heat capacity change in the DSC profiles, respectively (see Table S1). Endothermic peaks were observed for the NaFSA and EC mixture near room temperature and up to a molar ratio of x_EC > 3.5, as shown in Fig. 1a. Furthermore, mixtures in the range 1 < x_EC < 3 exhibited endothermic peaks in the range of 333 K to 353 K. The sharp endothermic peak at approximately 340 K suggests the formation of stable solvent crystals NaFSA-(EC)_x.¹¹ Gradual peaks of T_m were observed at approximately room-temperature in the mixtures of NaFTA or NaTFSA within the range of 6 < x_EC < 10. However, only the thermal transition attributed to T_g was observed in the mixtures with x_EC < 5. These mixtures existed in a liquid state at room-temperature, indicating a region of “crystalline gap.” In this region, the structure of the amorphous and solution phase is entirely different, resulting in either significantly slow growth of the crystalline phase or suppression of crystallization.^12,13 Notably, the highly concentrated composition (x_EC = 1.5) in the NaFTA and EC mixture remained stable in the liquid state at room temperature. Furthermore, the x_EC = 1 electrolyte also temporarily exhibited a liquid state at room temperature, with some of it solidified after a few days. NaTFSA and EC mixture electrolytes exhibited a liquid state up to a composition of x_EC = 2.5 at room temperature. However, in higher concentrations, NaTFSA and EC mixtures did not form uniformly dissolved solutions. The significant difference between the thermal properties of EC- and anion-based electrolytes arises from the high rotational ability of the SO₂CF₃ groups in the FTA and TFSA anions or the asymmetry characteristics of the FTA anion, which impede close packing of anion and cation and suppress crystallization.¹⁴

Figure 1.

Differential scanning calorimetry profiles of (a) NaFSA+xEC, (b) NaFTA+xEC, (c) NaTFSA+xEC electrolytes at a heating rate of 5 K min⁻¹.

3.2 Liquid structure of NaFTA - EC electrolytes

The coordination structures of Na⁺ in NaFTA+xEC electrolytes were investigated using Raman spectroscopy in the wavenumber range of 400–1650 cm⁻¹, as depicted in Figs. 2a and 2b. Figure 2a shows the obtained spectra corresponding to the symmetric stretching vibrations mode of EC in the range of 850–950 cm⁻¹.^15,16 The peaks observed at 895 cm⁻¹ and 902 cm⁻¹ correspond to EC molecules either free or bound to Na⁺, respectively. In the case of low-concentration electrolyte (x_EC = 10), peaks representing EC molecules both free and bound to Na⁺ were clearly confirmed. However, in concentrated electrolyte (x_EC = 1.5), the peak corresponding to free EC molecules was not observed, indicating nearly complete coordination of EC molecules with Na⁺. Furthermore, with increasing NaFTA concentration, the peak representing EC bound to Na⁺ shifted to higher wavenumbers, suggesting the formation of Na⁺-EC complexes.¹⁷ The Raman spectrum in the wavenumber range of 650–800 cm⁻¹ is shown in Fig. 2b. The peak observed at 716 cm⁻¹ could be assigned to the symmetric ring deformation mode of EC, and the peak intensity decreases and shifts to higher wavenumber (725 cm⁻¹) with NaFTA concentration because of enhanced interaction between EC and Na⁺.^11,17–20 Additionally, the peak at 722 cm⁻¹ in NaFTA+10EC, corresponding to the stretching and bending vibration of the S-N-S bond in free FTA⁻ and FTA⁻ within a solvent-separated ion pair (SSIP). The largest peak of 744 cm⁻¹ in NaFTA+1.5EC is assigned to contact ion pairs (CIPs), aggregates (AGGs).^11,19,21 Fard et al. proposed solvated geometric structures of Na⁺ in various solvents using molecular dynamics simulations and optimized theory at the M06-2X/6-311++G(d,p) level. They reported that in EC, Na⁺ is surrounded by five EC molecules. In low-concentration electrolytes (x_EC = 10), an excess of EC molecules compared with the coordination number of Na⁺ results in the presence of free EC molecules, EC molecules coordinated with Na⁺, SSIP.²² However, in concentrated electrolytes (x_EC = 1.5), where there is an insufficient number of EC molecules compared with the solvation number, a specific solution structure characterized by CIP and AGG should be formed to compensate for the stable coordination number.

Figure 2.

Raman spectra of NaFTA+xEC electrolytes (x = 1.5, 10) in the range of (a) 850–950 cm⁻¹, (b) 650–800 cm⁻¹.

3.3 Transport properties of NaFTA - EC electrolytes

To investigate the transport properties of NaFTA+xEC electrolytes, the temperature dependencies of ionic conductivity (σ) and viscosity (η) were measured (see Table S2). Figure 3 shows Arrhenius-type plots of σ for NaFTA+xEC electrolytes. The σ values decreased with NaFTA concentration—attributed to the dominance of aggregate structures with strong Coulombic interactions between Na⁺ and anions such as CIP and AGG—leading to an increase in viscosity.²³ Figure 4 shows log-log plots of molar conductivity (Λ_M) vs. reciprocal viscosity (η⁻¹) (Walden plots) for NaFTA+xEC electrolytes at 318 K. The ideal line in Walden plots is depicted based on the η⁻¹ and Λ_M value of 1 M KCl aqueous solution, representing a fully dissociated strong electrolyte.²⁴ In the low concentration region, Λ_M lies below the ideal line, indicating an associated state with formation of ion pairs (SSIP or CIP) with insufficient contribution from NaFTA to ionic conduction.¹¹ However, with an increase in NaFTA concentration, Λ_M approaches the ideal line. This indicates that Na⁺ act as effective charge carriers and the degree of dissociation is gradually elevated with the NaFTA salt concentration. Although, much ionic association (CIP and AGG) are formed in NaFTA+1.5EC by Raman spectroscopy and the association states of NaFTA+1.5EC contradict the high degree of dissociation. These results suggest behavior where the ionic conductivity of Concentrated electrolytes (NaFTA+1.5EC) disconnected from viscosity, indicating that the transport mechanism of Na⁺ is similar to H⁺ hopping or the Grotthuss mechanism.^11,20,25

Figure 3.

Arrhenius-type plots of ionic conductivity (σ) for NaFTA+xEC electrolytes.

Figure 4.

Walden plots for NaFTA+xEC electrolytes at 318 K.

3.4 Na⁺ charge-discharge behavior of Na_0.44MnO₂

To confirm the compatibility of NaFTA+1.5EC (obtained physicochemical properties at 303 K were shown in Table 1) as an electrolyte for SIBs, constant charge–discharge tests were conducted in a half-cell configuration, using a 4 V-class positive electrode material for SIBs. Figure 5a shows the charge-discharge profiles of the [Na | NaFTA+1.5EC | SMO] cell at 303 K. The initial charge capacity was 47 mAh g⁻¹, corresponding to 65 % of the theoretical capacity of SMO (72 mAh g⁻¹) according to the following reaction:

  

\begin{equation} \text{Na$_{0.44}$MnO$_{2}$} \to \text{Na$_{0.18}$MnO$_{2}$} + \text{0.26Na$^{+}$} + \text{0.26e$^{-}$} \end{equation} (1)

After the initial charging, the following charge-discharge reaction occurs:

  

\begin{equation} \text{Na$_{0.18}$MnO$_{2}$} + \text{0.46Na$^{+}$} + \text{0.46e$^{-}$} \rightleftarrows \text{Na$_{0.64}$MnO$_{2}$} \end{equation} (2)

The initial discharge capacity was 82.8 mAh g⁻¹, which is 65 % of the theoretical capacity of 127 mAh g⁻¹.²⁶ Several plateau regions were observed in the charge-discharge profiles, attributed to Na⁺ intercalation into various occupied sites of the SMO structure, followed by significant Na⁺ adsorption on the electrode surface.^27,28 The coulombic efficiency was 96.1 % in the second cycle, and reached 99.3 % after 50 cycles (Fig. 5b). A capacity retention of 92 % was achieved after 50 cycles, and sufficient charge-discharge reversibility was also confirmed. Moreover, the average voltages of the charge and discharge processes remained relatively stable, with values of 2.93 V at the 2nd charge, 2.85 V at the 50th charge, 2.95 V at the 2nd discharge, and 2.84 V at the 50th discharge. Therefore, notable polarization (resistance increase) of this cell and degradation of NaFTA+xEC were not observed. The formation of an effective solid electrolyte interphase (SEI) at negative electrode,²⁹ with low solubility, derived from anions (mainly, FSA), suppressed the decomposition of electrolytes, while the scarcity of free EC molecules in the NaFTA+xEC inhibited elution of the formed cathode (positive electrode) electrolyte interface (CEI) films.³⁰ However, despite conducting constant-current charge-discharge tests for [Na | NaFTA+10EC | SMO] cells using low-concentration electrolyte, the cells could not reach charging cut-off voltage in the 1st cycle. Generally, sodium-organic components formed from free solvent in low concentration electrolytes exhibit high solubility, potentially leading to the dissolution of CEI. As a result, electrolyte decomposition reactions progressed, hindering sufficient charge-discharge reaction.³¹ To accurately determine the interfacial properties of a [Na | SMO] cell, it is essential to separate the interfacial resistance of the negative and positive electrodes. We will investigate the impedance spectroscopic measurements to conduct a precise analysis of these interfacial properties by careful definition of measurement conditions, including the state-of-charge of SMO and the temperatures at which measurement are examined.

Table 1. Viscosity (η), density (ρ), molar concentration (c), ionic conductivity (σ) and molar conductivity (Λ_M) for NaFTA+1.5EC at the 303 K.

η/mPas	ρ/g cm−3	c/mol L−1	σ/mS cm−1	ΛM/Scm2 mol−1
910	1.757	4.560	0.431	9.46 × 10−2

Figure 5.

(a) Charge-discharge profiles of [Na | NaFTA+1.5EC | SMO] cell at a current density of 6.0 µA cm⁻² at 303 K and (b) cycle number dependencies of obtained charge-discharge capacity and calculated coulombic efficiency.

4. Conclusions

We systematically investigated the phase behavior of liquid electrolytes consisting of Na salts (NaFSA, NaFTA, NaTFSA) and EC. Mixtures of NaFSA and EC readily formed solvate crystals over a wide compositional range. The mixtures of NaFTA with EC exhibited stable liquid states across a wide composition range (1.5 < x). Asymmetric salts are expected to suppress the crystallization of concentrated electrolytes. As the salt concentration increased, the free EC decreased, and a unique coordination structure of Na⁺ was formed to ensure stable coordination numbers. Furthermore, the stable charge-discharge performance of the [Na | NaFTA+1.5EC | SMO] cell suggest the formation of a satisfactory CEI that suppresses electrolyte decomposition.

Acknowledgments

This work was partially supported by the Green Technologies for Excellence (GteX) program JPMJGX23S4 of the Japan Science and Technology Agency (JST), Japan.

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.26094583.

• 1) The phase behavior, Na⁺ coordination structure, Na⁺ conduction and transport properties, and battery performance of liquid electrolytes comprising sodium (fluorosulfonyl) (trifluoromethylsulfonyl) amide (NaFTA) and ethylene carbonate (EC) are investigated. A highly concentrated electrolyte with a molar ratio of EC/NaFTA = 1.5 is shown to exhibit a stable liquid state at room temperature. It is revealed that Na⁺ in the concentrated electrolyte coordinates—not only with EC but also with anions—to satisfy stable coordination numbers. From this specific coordination structure, dynamic ligand exchange of Na⁺ is shown to be enhanced, and [Na | NaFTA+1.5EC | Na_0.44MnO₂] cells are shown to achieve stable cycling performance by constant charge-discharge tests at 303 K.

CRediT Authorship Contribution Statement

Yoshiki Yokoyama: Conceptualization (Equal), Data curation (Lead), Formal analysis (Lead), Investigation (Lead), Writing – original draft (Lead)

Reita Furui: Formal analysis (Supporting), Investigation (Equal), Writing – original draft (Supporting)

Shiro Seki: Conceptualization (Lead), Funding acquisition (Lead), Investigation (Equal), Project administration (Lead), Writing – review & editing (Lead)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

Japan Science and Technology Agency: JPMJGX23S4 (GteX)

Footnotes

S. Seki: ECSJ Active Member

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