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 non-flammability 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.