High Spin‐State Modulation of Catalytic Centers by Weak Ligand Field for Promoting Sulfur Redox Reaction in Lithium‐Sulfur Batteries

The spin state of transition-metal compounds in lithium-sulfur batteries (LSBs) significantly impacts the electronic properties and the kinetics of sulfur redox reactions (SRR). However, accurately designing the spin state remains challenging, which is crucial for understanding the structure-performance relationship and developing high-performance electrocatalysts. Herein, the CoF₂, specifically the Co²⁺ with 3d⁷ electrons in a high-spin state distribution (t_2g ⁵e_g ²), were tailored predictably for the first time through the weak coordination field effect of the F element. Both DFT calculations and experimental results confirm that the spin state of Co²⁺ transitions from low- to high-spin configurations and strongly interacts with sulfur species through Co-S and Li-F bonds during the SRR process. This interaction weakens the S-S bond, promoting its facile cleavage from both ends while also facilitating the rapid and uniform nucleation of Li₂S₂/Li₂S, thus resulting in LSBs with a capacity of 447.7 mAh g^-1 at 10 C rates and stable cycling for 1000 cycles, with an acceptable practical capacity of 585 mAh g^-1 at a high sulfur loading mass of 10 mg cm^-2. This work achieves rational control of the active Co²⁺ d electron state through the field effect and enriches the application of spin control to accelerate SRR in LSBs.