Enhanced phase-transfer kinetics achieved by atomic iron catalysts on hard carbon toward low-temperature sodium-ion batteries

Hard carbons with desirable pore morphologies are the most promising anode materials for sodium-ion batteries, but experience undesirable cycling stability and depressed capacity owing to sluggish reaction kinetics of sodium-related species through or across the carbon layer. Herein, based on closed pore morphology engineering using carbon dioxide physical activation method, the hard carbon composite (SAFe@HC) was prepared by coating a polydopamine capping layer with uniformly distributed iron single atoms on the closed-pore-rich hard carbon, where the introduction SAFe help to expand the layer spacing distance, providing additional channels for solvated Na⁺ diffusion. It is innovatively proposed that SAFe catalysts provide abundant active catalytic sites for accelerating the interfacial desolvation and the phase-transfer kinetics of Na⁺ is lowered by decreasing the sodium ion transport energy barrier, as confirmed by theoretical simulation and various electrochemical demonstration. In addition, the closed pore morphology also enhances the surface area and adsorbs more sodium ions. As a result, the SAFe@HC electrode exhibits a high reversible capacity of 340.7 mAh g⁻¹ and a remarkable rate capacity up to 306.2 mAh g⁻¹ and a high capacity-retention of 96.0 % after 500 cycles at 5C. Decreasing the environmental temperature to 0 °C, an excellent robust electrochemical performance of 172.2 mAh g⁻¹ at 5C, offering promising avenues of single atomic catalysts achieving high-rate hard carbon electrode for sodium-ion batteries.