Lithium-sulfur batteries can pack up to five times as much energy by weight as some lithium-ion batteries, in principle. In practice, experimental versions of the batteries have fallen short of expectations because of unwanted chemical reactions between the electrodes and the electrolyte solution that alter the electrodes, thereby reducing energy storage and shortening battery life.
Rather than trying to solve these problems by modifying the electrodes, as many researchers have done, a team led by Quanquan Pang and Linda F. Nazar of the University of Waterloo have come up with a promising alternative approach. The team designed a customized electrolyte solution that limits these destructive reactions and suppresses battery degradation (Nat. Energy 2018 DOI: 10.1038/s41560-018-0214-0).
Lithium polysulfides are well-known bad actors in Li-S battery chemistry. The species, which dissolve in electrolyte solutions, shuttle between the cathode and anode, removing electrochemically active cathode material and corroding the anode. So the Waterloo team designed an electrolyte that barely dissolves polysulfides, and they used it sparingly in a Li-S battery. The electrolyte they arrived at contains the solvent diglyme and a lithium salt.
The team found that the electrolyte solution mediates the ionic reactions needed to drive the battery steadily for over 100 charge cycles but confines the sulfur chemistry to a quasi-solid state. The team also found that upon repeated charging, the electrolyte facilitates smooth deposition of lithium on the anode, avoiding formation of lithium dendrites, which can cause safety and battery degradation problems.