Systematic Comparison of the Physical Properties and Transport Characteristics of Na-Based and Conventional Li-Based Electrolytes

The global demand for lithium-ion batteries (LIBs) has been increasing due to the transition toward low-carbon societies. However, concerns about the sustainability of lithium resources has been arisen due to their low crucial abundance and uneven regional distribution.¹ As a result, sodium-ion batteries (SIBs) have gained attention due to the abundance of sodium as a charge carrier. In this study, we focus on Na-based electrolytes, which play a crucial role in charge transport between electrodes. In aqueous solutions, Na⁺ is reported to diffuse faster than Li⁺. 2 However, few studies have directly compared the diffusion behavior of Li⁺ and Na⁺ in organic solvents. Furthermore, due to the considerable nuclear quadrupole moment of Na⁺ and its extremely short relaxation time, measuring its self-diffusion coefficient ( D ) using pulsed-field gradient (PFG)-NMR is challenging.² The aim of this study is the systematic comparison of (Li, Na)-based electrolytes by measuring the D of various nuclei and evaluating their transport and physical properties, including viscosity ( η ) and density ( ρ ). All electrolyte preparations were conducted in an Ar-filled glovebox. LiN(SO₂F)₂ (LiFSA) and NaN(SO₂F)₂ (NaFSA) were used as electrolyte salts. Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were used as solvents. Electrolytes were prepared at concentrations ranging from 1.0 to 3.0 mol kg⁻¹ in EC and EMC with a 3:7 weight ratio. Additionally, mixed salt electrolytes (1.0 mol kg⁻¹) were prepared by dissolving equimolar ratios of (Li 0.5 Na 0.5 )FSA in EC and EC:EMC (3:7) to evaluate the transport properties of Li and Na in electrolytes under identical conditions. To evaluate their transport properties, η and ρ were measured from 283 to 323 K using a Stabinger viscometer/density measurement system. The D of each component (Li + , Na + , EC, EMC) was determined at 333 K using PFG-NMR, measuring 1 H, 7 Li, 19 F, and 23 Na nuclei. Addtionally, solvation structures and salt-solvent interactions were analyzed by Raman spectroscopy and excess density ( E ρ ) analysis. To determine E ρ , the salt density (ρₛₐₗₜ) was obtained by melting (Li, Na)FSA in a volumetric flask at 150-200 ℃ and measuring its mass. The densities at 10-50 ℃ were estimated using calibration lines derived from these measurements. Fig. 1 shows the relationship between the inverse viscosity ( η -1 ) and D for the electrolytes at 303 K. In electrolytes consisting only EC as a solvent, D Na was larger than D Li . According to the Stokes–Einstein equation, this behavior is attributed to differences in the Stokes radii of Na + and Li + . In contrast, in electrolytes containing EC:EMC (3:7), D Na and D Li were nearly indentical. This behavior suggests that the Stokes radii of Li⁺ and Na⁺ become similar in the presence of EMC as duluent. To further explore this phenomenon, we conducted Raman spectroscopy and E ρ analysis. Fig. 2 shows the Raman spectra of the carbonyl-stretching vibrations of EC and EMC in the 1500-1800 cm⁻¹ region for (Li, Na)-based electrolytes with EC:EMC (3:7). 3 The intensity of bound-state EC and EMC increased with salt concentration. To confirm the ratios of free and bound states of EC and EMC with cations, peak area ratios were calculated from Fig. 2 and are presented in Fig. 3. In Figure 3(a), Li⁺ exhibited a stronger coordination with EC than Na⁺, likely due to its higher charge density. Consequently, Li + attracted more EC molecules in the electrolyte. In Fig. 3(b), no significant difference was observed in the peak area ratios of free and bound states of EMC between Li + and Na + . This behavior is likely due to the low dielectric constant of EMC, which results in weak coordination with cations. To further investigate salt-solvent interactions, Fig. 4 shows the E ρ values for (Li, Na)-based electrolytes with EC:EMC (3:7). The equation used to calculated E ρ is also shown in Fig. 4. E ρ decreased with increasing salt concentration for both (Li, Na)-based electrolytes. Moreover, Na-based electrolytes exhibited more negative E ρ values than Li-based electrolytes. These differences are attributed to variations in the coordination numbers and coordination propensities of Li⁺ and Na⁺. We will also report the coordination numbers of Li⁺ and Na⁺ determined by Raman spectroscopy and discuss how EMC addition alters their solvation structures. (1) N. Yabuuchi, et.al., Chem. Rev. , 114 , 11636 (2014). (2) K. Hayamizu, et.al., RSC Adv. , 33 , 20252−20257 (2021). (3) Y. Zhang, e t.al., Energy Environ. Sci ., 13 , 183–199 (2020). Figure 1