Improving the uniformity of lithium deposition via ionic liquid additives for long-cycling lithium–sulfur batteries

Lithium–sulfur (Li–S) battery is considered as a promising next-generation high-energy-density battery. However, the cycle life of Li–S batteries is severely plagued by the instability of Li metal anodes. Improving Li deposition uniformity can mitigate the formation of inactive Li and the side reactions between Li polysulfides and Li metal anodes. Herein, 1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)imide (Py13FSI) is proposed as an additive to improve the uniformity of Li deposition in Li–S batteries. At a low concentration, Py13⁺ exhibits a lower reduction potential than Li ions. Py13⁺ can be adsorbed and accumulate on the surface protuberances of Li metal anodes, inducing the deposition of Li towards non-tip sites and improving the uniformity of Li deposition. With a high-loading cathode (4.1 mg_S/cm²) and an ultrathin Li metal anode (50 µm), the cycle life of Li–S batteries is prolonged from 61 to 120 cycles via Py13FSI additives. Furthermore, a 401 Wh/kg Li–S pouch cell with Py13FSI additives undergoes 14 cycles. This work demonstrates the potential of improving Li deposition uniformity in Li–S batteries by appropriate electrolyte additives.