Covalently bonded metal-organic groups anodes for high-performance potassium-ion batteries

A key challenge in potassium ion batteries (PIBs) is the design of anode materials with advanced structures that can enable fast charge transport to enhance potassium storage performance. The use of iron carbodiimide (FeNCN) as the anode, containing a considerable number of covalent bonds and a stable structure at the molecular level, enables the achievement of excellent electrochemical performance of potassium storage systems. The FeNCN anode exhibits high electrical conductivity with a band gap close to 0 eV and decent structural stability owing to its covalently bonded structure. In addition, the amorphous reaction products provide multiple pathways to facilitate ion diffusion. Consequently, the FeNCN anode demonstrated a high reversible specific capacity (600 mA h g⁻¹ at a current density of 50 mA g⁻¹), remarkable rate capability, and long cycle life (a reversible specific capacity of 400 mA h g⁻¹ at a current density of 500 mA g⁻¹ over 300 cycles). The conversion mechanism between Fe²⁺ and K⁺ was revealed by theoretical simulation, in situ X-ray diffraction analysis, and X-ray photoelectron spectroscopy. Moreover, the as-assembled FeNCN//perylene-3,4,9,10-tetracarboxylic dianhydride full cell demonstrated a high energy density of 184.7 W h kg⁻¹ (the highest among all iron-based PIBs) with a power density of 198.6 W kg⁻¹, which is superior to the previously reported values for PIBs or potassium-ion hybrid capacitors.[Figure not available: see fulltext.]