Rechargeable metal-SO2 batteries: Recent progress, current challenges and future prospects

To date, a large number of research programs have been carried out on metal-SO2 (M-SO2) batteries due to their promising applicability in space exploration, electric vehicles, and military purposes. Impressively, the nonflammability of SO2-based electrolytes is the exciting feature of M-SO2 batteries, which could be a solution for safe battery exploration. The primary M-SO2 batteries, especially primary Li-SO2 batteries (LSBs), have an energy density of about 330 W h kg-1, while the secondary LSBs are characterized by their high specific energy density (> 1100 W h kg(-1)) and long shelf-life. Likewise, inspired by the secondary LSBs, rechargeable Na-SO2 batteries (NSBs) have also emerged. Besides, M-SO2 batteries would be more attractive for next-generation metal-gas batteries over metal-air batteries due to their lower charge polarization and higher cell voltage. Therefore, we are inspired to systematically discuss the electrochemistry, performances, and challenges of current M-SO2 batteries. Afterward, as a future direction, this review will provide the improvement strategies for current challenges associated with M-SO2 batteries. While only rechargeable LSBs and NSBs have been reported so far, an outlook is given on promising multivalent metals (such as Mg, Zn, and Al)-SO2 batteries for future development. Likewise, as photo-assisted batteries have recently attracted huge research attention, we will also provide a perspective on photo-assisted rechargeable M-SO2 batteries for future development.