Solid/Quasi‐Solid Phase Conversion of Sulfur in Lithium–Sulfur Battery

The lithium–sulfur (Li–S) battery is considered as one of the most promising options because the redox couple has almost the highest theoretical specific energy (2600 Wh kg−1) among all solid anode‐cathode candidates for rechargeable batteries. The “solid–liquid–solid” mechanism has become a dominating phase transformation process since it was first reported, although this cathode mode suffers from a tough “shuttle” phenomenon due to the dissolution of the soluble intermediate polysulfides generated during the charging‐discharging process, which causes rapid loss of energy‐bearing material and shortened lifespan. For decades, tremendous efforts have been made to restrict the shuttle effect. Changing sulfur conversion to “solid–solid” mode or “quasi‐solid” mode, which successfully exceed the limit of the dissolution of the intermediates, and may address the root of the problem. In this review, the main focus is on the fundamental chemistry of the “solid‐solid” and “quasi‐solid” phase transformation of the sulfur cathode. First, the strategies of sulfur immobilization in “solid–liquid–solid” multi‐phase conversions as well as the pivotal influence factors for the electrochemical conversion process are briefly introduced. Then, the different routes are summarized to realize the “solid–solid” and “quasi‐solid” redox mechanisms. Finally, a perspectives on building high‐energy‐density Li–S batteries are provided. This review emphasizes the recent advances on two emerging strategies under increasing attention, namely “solid‐solid” and “quasi‐solid” conversion, the fundamental mechanisms, representative characteristics, and implementation methods of these conversion pathways in Li–S batteries are systematically summarized. The perspectives toward further mechanism studies and strategies on building high‐performance Li–S batteries for practical applications are also provided.