1,3,2-Dioxathiolane 2,2-Dioxide (DTD) as an Effective Electrolyte Additive for Layered Oxide/Hard Carbon Sodium-Ion Cells

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This work presents an evaluation of 1,3,2-dioxathiolane 2,2-dioxide (DTD) as an electrolyte additive for sodium layered oxide/hard carbon pouch cells. Several DTD concentrations (1–4 wt.%) were examined at 55 °C with cycling, open-circuit storage, gas generation, and symmetric cell impedance experiments. Increasing the DTD concentration suppresses gas evolution but also shortens cycle life and accelerates impedance growth. X-ray Photoelectron Spectroscopy analysis of the cycled electrodes reveals that higher concentrations of DTD directly correlate with an increased deposition of sulfate species on both the positive and negative electrode surfaces. Post-mortem electrolyte characterization by gas chromatography–mass spectrometry and nuclear magnetic resonance shows that DTD suppresses the formation of alkyl dicarbonates during long-term cycling, consistent with its effective passivation of the hard carbon electrode. In addition, ethylene carbonate (EC) formation in the initially EC-free solvent scales with the DTD content, suggesting a H 2 O-induced decomposition pathway of DTD that generates EC. The proposed reaction pathway was validated using the hydrolysis-resistant derivative 4,4′-bi-1,3,2-dioxathiolane 2,2,2′,2′-tetraoxide (bis-DTD). Electrolyte analysis of post-cycled electrolytes confirmed the absence of EC formation. However, suppressing this reaction did not translate to improved cell longevity as DTD-containing cells exhibited enhanced capacity retention and lower impedance growth compared to their bis-DTD counterparts.

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