Anion-Defect Engineering in Transition Metal Compounds for Lithium–Sulfur Batteries: Current Progress, Mechanistic Insights, and Future Directions

Lithium–sulfur (Li–S) batteries are promising next-generation energy storage, yet their practical deployment is severely hindered by the polysulfide shuttle effect, sluggish sulfur redox kinetics, and uncontrolled lithium dendrite growth. Recent studies have demonstrated that defect engineering in functional host and interfacial materials offers an effective pathway to overcome these challenges. Specifically, introducing anion vacancies into transition metal compounds (TMCs) can optimize electronic structures, enhance polysulfide adsorption, and accelerate charge transfer, thereby improving catalytic activity and lithium-ion affinity. Despite these advances, a comprehensive understanding of how anionic defect-rich TMCs (AD-TMCs) modulate electrochemical processes remains elusive, particularly for solid-state Li–S batteries. This review systematically summarizes recent progress in the design principles, synthesis strategies, and electrochemical characteristics of AD-TMCs in Li–S systems. Defect-driven mechanisms that govern sulfur redox kinetics and lithium interfacial stability are highlighted, and further advanced characterization and computational approaches for probing vacancy structures and catalytic behavior are discussed. Furthermore, perspectives on precise defect regulation, remaining challenges, and future research directions are presented, with emphasis on bridging the gap between fundamental studies and practical applications. This review aims to provide a roadmap for leveraging anion vacancy engineering to enable high-performance and commercially viable Li–S batteries.