Harnessing and Suppressing Electron Spin-State Transitions: From Decoding to Rational Design of High-Performance Cathodes for Alkali-Ion Batteries.

The electron spin state of transition metal ions represents a fundamental quantum property that is increasingly recognized as a pivotal design dimension for tuning the performance of cathode materials in Li/Na/K‑ion batteries. This review begins by consolidating the foundational principles through which spin states govern electrochemical properties, establishing a robust theoretical framework that bridges atomic-scale coordination environments with macroscopic electrode behavior. It further discusses advanced experimental and computational techniques for probing complex spin states and, critically, for establishing clear structure-spin-performance relationships. A central focus is placed on the rational design of spin configurations, whether via proactive engineering or suppression of unfavorable transitions, to optimize key electrochemical processes: modulating cationic vs. anionic redox competition, enhancing structural stability by mitigating Jahn-Teller distortions and magnetic frustration, and improving charge and ion transport. The review also highlights the emerging role of spin‑sensitive machine learning as an accelerated pathway for discovering superior cathode materials. By integrating theoretical insights, methodological advances, and application‑oriented studies, this work provides a comprehensive mechanistic framework and practical guidelines for the design of next‑generation high‑performance cathodes through deliberate spin‑state control.