Lithium-sulfur batteries: from liquid to solid cells

Lithium-sulfur (Li-S) batteries supply a theoretical specific energy 5 times higher than that of lithium-ion batteries (2500 vs. similar to 500 W h kg(-1)). However, the insulating properties and polysulfide shuttle effects of the sulfur cathode and safety concerns of the lithium anode in liquid electrolytes are still key limitations to practical use of traditional Li-S batteries. In this review, we start with a brief discussion on fundamentals of Li-S batteries and key challenges associated with conventional liquid cells. We then introduce the most recent progress in liquid systems, including sulfur positive electrodes, lithium negative electrodes, and electrolytes and binders. We discuss the significance of investigating electrode reaction mechanisms in liquid cells using in situ techniques to monitor the compositional and morphological changes. We also discuss the importance of this game-changing shift, moving from traditional liquid cells to recently developed solid cells, with positive advances in both solid electrolytes and electrode materials. Finally, the opportunities and perspectives for future research on Li-S batteries are presented.