Metastable FeCN 2 @nitrogen-doped carbon with high pseudocapacitance as an anode material for sodium ion batteries.

Pseudocapacitive materials are good candidates for fast charging anodes of sodium ion batteries (SIB). However, pseudocapacitive materials with a high surface area face the severe problem of low initial coulombic efficiency. In this work, micro-sized nitrogen-doped carbon (NC) coated and supported polyhedron FeCN 2 networks are designed and synthesized by a facile in situ gel-swelling technique. Impressively, FeCN 2 @NC as an SIB anode exhibits excellent rate performance with highly reversible rate capacities of 466 and 303 mA h g -1 at 0.2 and 10.0 A g -1 , respectively. Furthermore, the FeCN 2 @NC anode shows a high initial coulombic efficiency (ICE) of 86% due to a low surface area. Electrochemical tests and density functional theory (DFT) calculation indicate that the metastable character enables the low intercalation/conversion reaction energy for FeCN 2 and further greatly promotes the fast pseudocapacitive storage mechanism for FeCN 2 @NC. This work provides evidence that FeCN 2 is a new type of metastability induced pseudocapacitive material with high initial coulombic efficiency.