Planar Hypercoordinate FeB3 Monolayer for Universal Alkali-Ion Storage with Ultrahigh Capacity and Ultralow Diffusion Barriers

Developing a single anode host capable of high-capacity and fast storage of Li, Na, and K remains a major challenge for next-generation alkali-metal-ion batteries. Herein, the electrochemical performance of a planar hypercoordinate FeB3 monolayer is systematically investigated using first-principles calculations. The FeB3 monolayer features a metallic electronic structure and an exposed boron-rich surface, providing favorable conditions for alkali-metal adsorption. Li, Na, and K preferentially occupy the B–B–B hollow regions and undergo substantial charge transfer to the substrate. The calculated migration barriers are only 0.041, 0.008, and 0.025 eV for Li, Na, and K, respectively, indicating ultrafast surface diffusion. Open-circuit-voltage analysis gives accessible theoretical capacities of 944.2, 809.3, and 606.9 mAh g−1 for Li-, Na-, and K-ion storage, respectively. Moreover, the Fe–B framework remains largely intact under high alkali-metal loading. These results identify planar FeB3 as a promising universal anode combining metallicity, ultrafast ion transport, high capacity, and structural compatibility.