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Batteries 2022, 8, 127 8 of 13 The use of SO2 as an additive significantly reduces risks when compared to the use of SO2 as a solvent. However, questions regarding a relatively high SO2 partial pressure in equilibrium with the solution derived from the fact that SO2 is a gas under normal conditions have motivated us to research on a closely related alternative. Specifically, we have considered the use of sulfolane as an additive, aiming again to enhance the Na plating/stripping process. Sulfolane is a dipolar aprotic solvent with a melting point of 27.5 ◦C, density of 1.261 g·cm−3, and viscosity of 0.01007 Pa·s at 30 ◦C. It is very stable; even at temperatures as high as 200 ◦C, the rate of decomposition is 0.009%/h [22]. In addition, sulfolane has been employed in the context of rechargeable Li-ion batteries, being used both as a solvent and as an additive in the electrolyte [23–25]. Sulfolane molecules can adsorb on the working electrode modifying the electrode–solution interface. Xing et al., through molecular simulation studies for solutions consisting of sulfolane, dimethyl carbonate and LiPF6, reported that the oxygen atoms of the sulfolane molecule were strongly adsorbed on the substrate, while the carbonyl groups (coming from the solvent) were repelled when potential reaches sufficiently negative values, as is the case for sodium plating/stripping [26]. In addition, the behavior of the electrolyte was also reported to be a result of the strong interaction of sulfolane with the sodium cations. Another characteristic of sulfolane is its high electrochemical stability, reaching potentials above 5.8 V without decomposing [27–29]. However, sulfolane-based electrolytes show high viscosity, and, as a result, the wettability of the cathode is strongly affected. This is also a limitation for the transport of Na+ ions, particularly relevant for fast battery charging/discharging [24,28]. To minimize these deleterious effects, the use of sulfolane as an additive in Li electrolytes has been described. Cai et al. found that 2% sulfolane in the electrolyte increased, for a LiNi1/3Co1/3Mn1/3O2/graphite lithium-ion cell, the capacity of the initial discharge and the capacity retention, which changed from 53% to 63% after 100 cycles [30]. As far as we know, the use of sulfolane as an additive has not been described for Na- based batteries. Several mole fractions of sulfolane have been added to 1 M NaClO4/PC and the Na plating/stripping process has been studied on a copper substrate in a three- electrode cell. Figure 6 shows ten successive cycles for the Na+/Na process in contact with electrolytes containing mole fractions of sulfolane ranging from 0.02 to 0.10. As observed in Figure 6a, in the absence of sulfolane, Na can be deposited on Cu, but the Coulombic efficiency of the process is below 80% (Q− > Q+). However, when a 0.02 mole fraction of sulfolane is added to the organic electrolyte, the voltammetric behavior undergoes a significant change. During the first cycles the reversibility of the process is very poor, and it shows low Coulombic efficiency. However, as the number of cycles increases, the current densities also increase, achieving a quasi-stationary behavior in the last three cycles, as shown in Figure 6b. It is worth noting that the attained current densities are higher (almost twofold) for a sulfolane mole fraction of 0.02. However, if the concentration of sulfolane is increased up to a mole fraction of 0.10, the shape of the voltammogram changes drastically and the Na+/Na process is barely discernible. This is likely related to a partial gelation of the electrolyte. When the sulfolane mole fraction reaches a value of 0.20, the electrolyte is fully gelled (see a picture of the electrolyte in an inverted vial in Figure 6d) and no electrochemical experiments were carried out.PDF Image | Sulfur Dioxide and Sulfolane Sodium Batteries
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