The Design of Transition Metal Sulfide Cathodes for High-Performance Magnesium-Ion Batteries

Conspectus Despite the widespread use of lithium-ion battery (LIB) technology, conventional LIB suffer from severe limitations (e.g., low energy density, flammable electrolytes) that have prompted much research interest for alternative battery technologies. To overcome the limitations of lithium-ion batteries, magnesium-ion batteries (MIBs) have been proposed as promising alternative energy storage devices, with advantages of high volumetric energy density, high safety, low cost, and environmental benignity. However, the high charge density of Mg 2+ in MIBs leads to sluggish electrochemical kinetics, owing to the strong electrostatic interactions between the host material and Mg 2+ . To mitigate this problem, transition metal sulfides (TMS) have been proposed as a solution and intensively researched as cathodes in MIBs, given that the soft features of sulfur can weaken undesirable electrostatic interactions (the low charge density of sulfur and high theoretical capacity). Nevertheless, TMS suffer from large volume variation, poor electronic conductivity as well as detrimental side reactions that all lead to degraded cycling performance. To this end, many solutions have been proposed to resolve these issues. Herein, we present the latest research on the design of nanostructured TMS (e.g., NiS 2, FeS 2, and Co 3 S 4 /CoS 2 ) and their electrochemical storage performances when used as cathodes in MIBs. We highlight and discuss important findings that include: (1) different synthetic methods for preparing TMS nanostructures, (2) nanostructures (hollow and hierarchical spheres) effectively alleviating the volume variation in the insertion/extraction of Mg 2+, (3) sulfur anions enhancing the electrochemical properties, (4) the TMS cathode having a shuttle effect in the electrochemical process that can be retarded by well-designed crystalline structures. (5) density functional theoretical calculations and ab initio molecular dynamics being extensively used to support the experimental results to guide the design of high-performing TMS cathodes, and (6) advanced characterization technologies (e.g., cryogenic transmission electron microscopy, X-ray absorption spectroscopy) being effective tools to investigate the dynamic evolution of TMS cathodes during the discharge/charge process. Moreover, we also evaluate other conventional strategies for designing TMS cathodes. To advance the realization of MIBs as an energy system, recent studies of MIBs in pouch cells are reviewed and discussed with reference to the challenges faced in industrial-scale production. To satisfy increasing demand for cathodes with high energy densities, we demonstrate our prospect in machine learning-driven TMS-based cathode research, given that machine learning is highly suited for discovering new materials and reducing the time taken for developing a technology from the laboratory to commercialization. Our Account will thus guide fabrication of other transition metal chalcogenide-based cathodes for high-performance multivalent-ion batteries.