Spin-State-Coupled Redox Chemistry in Rechargeable Battery Materials

Rechargeable battery reactions are commonly interpreted through transition-metal valence changes, anion redox, and ion insertion/extraction. However, formal valence descriptions alone cannot fully capture how electronic configuration, lattice response, and interfacial chemistry evolve during electrochemical cycling. In this Review, we discuss spin-state transitions as functional electronic degrees of freedom that couple redox compensation, structural distortion, oxygen activity, charge-transfer kinetics, and interfacial reactions in rechargeable battery electrodes. We first outline the electronic-structure origin of spin states, emphasizing the competition among ligand-field splitting, electron-pairing energy, Hund exchange, and metal-ligand covalency. We then discuss representative strategies for spin-state regulation, including external-field manipulation, chemical-environment engineering, interfacial spin effects, and spin-dependent chemistry in carbon-based electrodes, followed by emerging spin-state engineering beyond conventional electrochemical limits. Advanced magnetic and spectroscopic probes, including magnetic measurements, EPR, Mössbauer spectroscopy, XMCD, XAS, XES, and RIXS, are discussed for correlative identification of spin-state evolution and its distinction from conventional valence-state changes. This Review provides a spin-state-aware perspective on rechargeable battery chemistry by considering the coupled evolution of spin configuration, redox activity, lattice response, and interfacial processes.