Recent progress in developing Li₂S cathodes for Li–S batteries

With its unique features, lithium sulfide (Li₂S) has been investigated as the cathode material for next-generation rechargeable batteries. Even though Li₂S itself cannot solve all the problems faced by lithium-sulfur batteries (LSBs) and it may also introduce new issues, it does provide new opportunities. As the fully lithiated state of sulfur, Li₂S offers the prospect of lithium-metal-free anodes and will also alleviate the volume expansion issues otherwise occurred in the sulfur cathode. Perhaps a most radical change when substituting sulfur with Li₂S lies at the high-temperature process ability of the latter, thus opening new avenues to construct rationally designed electrodes. Despite sharing certain similarities with sulfur-based LSB, Li₂S-based has its own opportunities and challenges in term of material synthesis, electrode fabrication, cell construction, and electrochemical behavior. To advance its state of the art, this review article discusses the current understandings on the initial Li₂S activation process, which plays a crucial role in guiding Li₂S nanostructure design and fabrication. With this leading thread, the article surveys impactful works on producing Li₂S nanoparticles, encapsulating Li₂S nanoparticles, simultaneously producing and encapsulating Li₂S nanoparticles, and fabricating Li₂S cathodes, followed by constructing lithium-metal-free LSBs. The pros and cons of different methods and the associated electrochemical behaviors are highlighted. Throughout, we call out the important research opportunities and challenges, both scattered out in the survey and aggregated in our conclusion perspective on future works, towards the fundamental understanding and practical development of Li₂S-based LSBs.