Ethylene Sulfate Derivatives as General-Purpose Electrolyte Additives for NMC Li-Ion, LFP Li-Ion and Na-Ion Cells

In this work, a series of novel derivatives of ethylene sulfate, formally called 1,3,2-dioxathiolane-2,2-dioxide (DTD), were synthesized and evaluated. DTD itself is a very effective electrolyte additive but suffers from poor chemical stability during storage and transport of premixed electrolyte. Electrolyte storage experiments showed that DTD degraded by ~50% over 15 weeks at room temperature, whereas 4,4′-Bi-1,3,2-dioxathiolane, 2,2,2′,2′-tetraoxide (bis-DTD) in the same electrolytes remained chemically stable throughout the same period. Bis-DTD also demonstrated wide electrochemical applicability in NMC442/graphite, Ni65/graphite, and LFP/graphite cells. In particular, using bis-DTD as an additive in Ni65/graphite cells enabled long term cycling at 40 °C to an upper cutoff of 4.4 V with excellent capacity retention and minimal impedance growth reaching 2500 cycles to end of life. In addition, electrolytes for LFP/graphite cells including bis-DTD as an co-additive, enabled effective operation from room temperature to 70 °C while maintaining low charge transfer resistance which is important for fast charge. Some of the DTD derivatives also show effectiveness in Na-ion cells, most importantly reducing the rate of impedance growth. These and other results highlight bis-DTD as a high-performing additive that demonstrates effectiveness in different Li-ion and Na-ion chemistries, over wide temperature and voltage ranges.