Motivation of low-energy d orbital from an enhanced intermediate spin state in Fe-doped 2D monolayers boosting electrocatalysis of Li-O₂ batteries

The electronic structure of cathode catalysts has a significant influence on the electrocatalytic performance of Li-O₂ batteries (LOBs), which are considered a promising next-generation energy storage system. The precise modulation of spin state and orbital interaction at the active metal centers is pivotal to optimizing catalytic performance. Herein, Fe and Co are introduced into Ti vacancies in 2D lepidocrocite-type Ti_0.87O₂ monolayers to explore the effect of varying spin states on the catalytic behavior of LOBs. Both FeTiO and CoTiO exhibit orbital coupling between low-energy t_2g orbitals and reactants. Unlike the high-energy e_g orbital filling for efficient d-p coupling at a high spin state, the Fe site presents an enhanced intermediate spin state and motivates the low-energy d orbital with poor catalytic activity. It is demonstrated that this enhanced intermediate spin state promotes the charge transfer efficiency and redox kinetics of FeTiO by generating more unpaired electrons for the fully occupied d_yz orbital with low energy, which exquisitely matches with the O 2p orbital of the LiO₂ key intermediate. As cathode catalysts in LOBs, FeTiO delivers a stable cycling performance (325 cycles at 1 A g⁻¹) and an ultralong operating lifespan of 3400 h. This work highlights the critical role of spin-state manipulation for dopants in the rational design of highly efficient cathode catalysts in LOBs.