Iron carbodiimide as a High-reactivity anode for potassium ion batteries

Exploring highly reactive anode materials is crucial for further development of potassium ion batteries (PIBs). Conversion-type materials have the merit of high energy density as anode for PIBs, while suffering from inferior stability and poor rate capability. Herein, we propose to utilize iron carbodiimide (FeNCN) as the anode for PIBs to achieve stable and high-rate potassium storage performance for the first time. The smaller electronegativity and strong covalent bonding structure of FeNCN facilitate improving the charge transfer kinetics of battery systems. Consequently, the FeNCN anode deliver a high reversible capacity (515 mAh/g at 50 mA g-1) and remarkable long-life span over 350 cycles with capacity retention of 90 % (133 days, current density 100 mA g-1). Moreover, the mechanism for potassium ion storage of FeNCN was revealed by in-situ XRD, ex-situ XPS, and FT-IR. The feasibility of FeNCN as an anode for PIBs paves the way for transition metal (Cr, Mn, Fe, and Co) carbodiimides to be investigated as potential energy storage materials.