Rational Construction of Co‐Fe 3 C@Fe 3 O 4 /C via Spin state Modulation of the D z 2 Orbital Occupancy Toward High‐performance Capacitive Deionization

Herein, a spin‐state regulation strategy is introduced as an electronic structure modulation approach to enhance the electron occupancy of the metal d z 2 orbital and is applied for the first time to capacitive deionization. Specifically, Co‐doped Fe 3 C@Fe 3 O 4 nanocubes assembled on porous carbon are synthesized via a multi‐step calcination‐impregnation‐calcination procedure, in which the increased d z 2 orbital electron occupancy accounts for the enhanced sodium ion adsorption capacity. Cobalt doping withdraws electrons from the low‐energy d x 2 ‐y 2 orbitals of iron, which reduces electron‐pair repulsion, lowers crystal field splitting, and thus promotes orbital degeneracy and stabilizes the high‐spin state. This, in turn, populates the higher‐energy d z 2 orbital, whose electron‐rich lobe electrostatically attracts Na + , and rationally enhances sodium adsorption capacity. Consequently, the electrode delivers a NaCl adsorption capacity of 307.9 mg g −1 at 1.4 V in a 2000 mg L −1 saline solution, with a corresponding deionization rate of 51.9 mg g −1 min −1 . Notably, the prepared electrode delivers an ultrahigh energy efficiency of nearly 98% in 2000 mg L −1 NaCl solution, outperforming most state‐of‐the‐art electrodes. Overall, this work establishes a fundamental design principle by linking spin state modulation to increased d z 2 orbital occupancy and ultimately to enhanced salt adsorption capacity and energy utilization efficiency for advanced desalination materials.