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Batteries 2022, 8, 127 4 of 13 from 0 to −300 mV vs. Li+/Li, and they considered that tracking such a resistance was a good gauge of lithium cycling performance [18]. Analogously, we define here the plating- stripping resistance for the Na+/Na process as: Electrolyte 2 M NATf/ DOL:DME * 2 M NaSCN/ DOL:DME Rps(Cycle 1)/kΩ 0.62 1.1 Rps(Cycle 10)/kΩ 1.28 1.72 ∆Rps/kΩ XSO2 Rps(SO2)/kΩ 0.02 2.48 0.66 0.05 10 0.10 10 0.02 2.0 0.62 0.05 2.27 0.10 2.50 0.20 7.69 Rps = ∂E ∂I I=0 (1) where E stands for the potential vs. Na+/Na and I for the current, as well as its change over 10 voltammetric cycles (∆Rps). The plating-stripping resistance contains contributions from more fundamental magnitudes, including the liquid electrolyte resistance, the SEI resistance, and the charge transfer resistance. Changes in the magnitude of the plating resistance in the case of lithium were attributed to changes in the SEI composition [18]. Table 1 gathers the resistance values before the addition of sulfur dioxide, and it also shows the effect of the mole fraction of sulfur dioxide on Rps. These values are in the same order of magnitude as those found for lithium deposition [18]. The addition of sulfur dioxide triggers two main effects: (i) an increase in the plating-stripping resistance values and (ii) a significant stabilization of the CVs. Table 1. Plating-stripping resistance for sodium anodes in contact with three different electrolytes. Values for cycles 1 and 10 are given, together with the corresponding variation (∆Rps). Plating- stripping resistance (cycle 10) is also given upon the addition of different mole fractions of sulfur dioxide. * NaTf concentration was 1.6 M in the presence of SO2. 1M 0.34 0.39 0.05 NaClO4 /PC 0.02 1.05 0.05 1.05 0.10 0.68 0.20 0.41 The decrease in the current density associated with Na plating/stripping (increase in plating-stripping resistance) is likely related to the formation of a specific SEI on the sodium surface when it is in contact with an electrolyte containing SO2. Some authors have described such a process by means of Equations (2)–(5) [14,19]. Similar reactions have been proposed for a system based on Li-SO2 [20,21]. 6Na + SO2 → 2Na2O + Na2S (2) Na2O + SO2 → Na2SO3 (3) Na2O + 2 SO2 → Na2S2O5 (4) 2Na + 2 SO2 → Na2S2O4 (5) Equation (5) is the simplest one that more straightforwardly accounts for the formation of an SEI in which the transport of sodium ions is expected to be efficient based on the low melting points reported for Na2S2O4. However, the contribution of the other reactions cannot be discarded. XPS and in-situ Raman spectroscopy measurements are underway to clarify this question. Examination of Figure 1 and Table 1 shows that, as the mole fraction of sulfur dioxide grows, there is a significant increase in the plating-stripping resistance, except in the case of the 1 M NaClO4/PC electrolyte for which Rps decreases as the SO2 mole fraction increases.PDF Image | Sulfur Dioxide and Sulfolane Sodium Batteries
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