Defect-Mediated Mg²⁺ Transport and Structural Stability in the Sulfide Cathodes for Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) are promising next-generation energy storage systems due to high volumetric capacity of Mg anode, dendrite-free deposition, and high natural abundance. However, their development is largely restricted by sluggish Mg²⁺ diffusion, strong electrostatic interactions within host lattices, and severe interfacial desolvation barriers. Sulfide-based cathodes, featuring soft anion frameworks and enhanced ionic mobility, have emerged as attractive candidates, yet they suffer from limited conductivity, structural instability, and a voltage-kinetics trade-off. This review comprehensively summarizes recent progress in defect engineering strategies for sulfide cathodes in RMBs. The fundamental mechanism of Mg²⁺ migration is discussed, followed by a systematic classification of defect types and their roles in regulating diffusion pathways, electronic structure, redox activity, and interfacial behavior. Synthetic approaches for controlled defect construction and their electrochemical implications are critically evaluated. Finally, key challenges including quantitative defect characterization, interfacial instability, structural degradation, and limited practical validation are identified, and future design strategies for high-performance defect-engineered sulfide cathodes are proposed.