Electron Spin as a Descriptor for Sulfur Electrochemistry: Principles, Characterization, and Regulation Strategies in Sulfur-Based Batteries
Sulfur-based batteries are promising for next-generation energy storage due to high theoretical capacity, natural abundance, and low cost of sulfur cathodes. However, practical implementation is impeded by sluggish sulfur redox kinetics, dissolution and migration of intermediate polysulfides, and formation of insulating discharge products. Conventional catalyst design focuses on charge distribution, adsorption energetics, and structural confinement, yet these approaches incompletely describe the complex electronic processes governing sulfur conversion. Electron spin, an intrinsic quantum degree of freedom, offers an additional dimension for modulating catalytic behavior via its influence on electronic structure and orbital interactions at catalytic interfaces. In spin-polarized systems, changes in occupation and splitting of transition-metal d orbitals can regulate d-p hybridization with sulfur species, affecting interfacial charge transfer and energetics of sulfur redox reactions. This review summarizes recent progress in elucidating and manipulating electron spin in sulfur-based battery systems. Fundamental principles connecting spin states with electronic structure and catalytic behavior are outlined, followed by experimental approaches for probing spin-related electronic properties using spectroscopic and magnetic characterization techniques. Emerging strategies for spin regulation are highlighted, including heteroatom doping, defect engineering, coordination environment modulation, chirality-induced spin selectivity, and external magnetic-field control. Remaining challenges in identifying spin effects under realistic electrochemical conditions are addressed, along with opportunities for integrating spin-related descriptors into catalyst design. Establishing quantitative relationships between spin polarization, orbital hybridization, and sulfur reaction pathways may provide new perspectives for high-performance sulfur-based batteries.