Researchers from Hyundai have found that the use of a new sulfone-based electrolyte greatly improved the capacity and reversible capacity retention of a Li-sulfur battery compared to the performance of ether-based electrolytes.

In a paper presented at the SAE 2104 World Congress in Detroit, they reported that use of the sulfone-based electrolyte increased capacity by 52.1% to 715 mAh/g and capacity retention by 63.1% to 72.6%.Lithium-sulfur systems are of great interest as a “beyond Li-ion” solution with increased energy densities that would enable much greater electric vehicle range. The Li/S system has a high theoretical specific energy of 2600 Wh kg-1; however, rapid fading of charge capacity is a well-known issue.

The poor long-term performance has been associated with both the shuttling of polysulfides dissolved into the electrolyte, in addition to irreversible deposition of solid lithium sulfide (Li2S) and other mixtures of insoluble discharge products on the cathode.

Chemical processes in Li/S battery include Li ion dissolution from Li metal anode and sulfur reduction to Li polysulfides (PS, the series of sulfur reduction intermediates) on the sulfur cathode while discharging (S8?Li2S8?Li2S6?Li2S4?Li2S), and reversible chemical reactions occur during charging. Among the PS formed in this mechanism, Li2S6 and Li2S4 are soluble in electrolytes. PS solubility plays an important role to improve cyclability as increasing the sulfur utilization.

Ether type solvent has been considered as a suitable electrolyte for Li/S battery because it has good PS solubility and chemical stability. Meanwhile, dissolved PS causes redox shuttle resulting in a low COlumbic efficiency, poor cycle life and self-discharge. Therefore, this work aimed at developing a new electrolyte in order to prevent redox shuttle and improve cyclability.

In their study, the Hyundai researchers compared the performance of 5 single component ether-based systems (DME, DEGDME, Triglyme, TEGDME and DIOX); one binary system (TEGDME:DIOX); and three versions of a ternary system with sulfone (TEGDME:DIOX:Sulfolane at ratios of 1:1:1, 1:1:2, and 1:1:3).

They assembled coin cells for electrochemical testing using a sulfur cathode and Li metal foil as an anode, with a polyethylene separator between them. Cycling test were performed between 1.5V and 2.65V and room temperature at C/20 rate.

Among the single component ether systems, use of DME resulted in the highest capacity of 878 mAh g-1, with DEGDME a close second at 857 mAh g-1. However, the cells with these electrolytes showed drastic capacity fade after 6 and 2 cycles, respectively.

While cyclic ether, DIOX, showed 1,040 mAh g-1 at first cycle, this dropped to 640 mAh g-1 at 12 cycles. The high initial discharge capacity showed that DIOX appeared to be effective on developing high capacity; however, after 13 cycles it evidenced a drastic capacity decrease.

TEGDME showed a low initial capacity of 200 mAh g-1, but it did not show drastic capacity fade.

The researchers combined TEGDME and DIOX into a 1:1 binary system to investigate the synergy of the good cyclability of TEGDME and the high capacity of DIOX.

The cell with the binary ether electrolyte showed first discharge capacity of 1057 mAh g-1 and 470 mAh g-1 after 20 cycles. Compared to the single component results, this cell showed good cyclability. However, the issues large drop in capacity after the first cycle and the low reversible capacity retention of 44.5% after 20 cycles remained.

The researchers then inserted a glass filter between the electrodes to restrain the high resistance around the electrodes in the cell with the binary electrolyte. (The glass filter absorbs electrolyte, thereby preventing a deficiency of electrolyte next to the electrode.) This served to increase overall capacity to 605 mAh g-1 after 20 cycles, lowered the capacity decrease after first cycle, and improved capacity retention.

Chemical analysis suggested that sulfone solvent could form a protective layer on the anode surface, and prevent the PS shuttle by blocking the reaction between the Li anodes and PS. In addition, the protective layer can mitigate the crack formation on the surface observed with the other electrolyte systems, the researchers determined.

The Hyundai team used Sulfolane (a sulfone-based solvent), as it is already known as a suitable Li battery electrolyte. They prepared three ternary compositions of electrolyte, adding different amounts of sulfolane to the binary TEGDME : DIOX mixture.

They found that the 1:1:2 (TEGDME:DIOX:Sulfolane) mixture (TDS2) showed the best cyclability, as noted above with capacity of 715 mAh g-1. TDS1 also showed improved capacity and retention: 674 mAh g-1 and 68%. Cycle performance worsened in TDS3.

The researchers also found that cracks on the anode surface diminished significantly.