Fundamentals and advances of ligand field theory in understanding structure-electrochemical property relationship of intercalation-type electrode materials for rechargeable batteries

The ion-intercalation-based rechargeable batteries are emerging as the most efficient energy storage technology for electronic vehicles, grids, and portable devices. These devices require rechargeable batteries with higher energy-density than commercial Li-ion batteries, which are intrinsically limited by specific capacities and electrochemical potentials of transition-metal (M) electrode materials. Over the past decades, a significant number of studies have focused on exploring coordination environments and electronic origins of these materials based on ligand field theory (LFT). However, studies to understand and manipulate the relationship between their local-structural characteristics and electrochemical properties are limited. In this review, we comprehensively discussed how the combining of LFT and first-principles calculations can be used to derive Fermi levels that determine electrochemical potential, crystal field stabilization energy, and anionic redox activity. Based on this, a series of strategies are proposed to improve the phase-stability and energy-density of intercalation-type electrode materials, such as ion -intercalation potential tuning of rigid-band systems and electrode phase stability regulations with different M periods. Two high energy-density cathode materials, M-free LiBCF2 and Li-free group-VB/VIB MX2 (X = S, Se), are successfully designed from the aforementioned principles derived. Finally, we also highlight further directions for designing better intercalation-type ma-terials based on LFT and their opportunities/challenges.