Insights into the Phase and Structure Transformation of a VN/C Anode Material in Sodium-Ion Batteries

Vanadium nitrides (VNs) have been considered as one of the most promising energy storage materials due to their good conductivitiy and high theoretical specific capacities. When used as the anode materials in sodium-ion batteries (SIBs), VNs usually show a long-life property, resulting in an illusion that VNs are extremely stable during the charge-discharge process, while ignoring their structure and phase changes. To deeply reveal its electrochemical performance, a composite of VN immerged in carbons was synthesized by simple calcination and used as the anode materials in SIBs. Through various characterizations including ex situ XRD, SEM and TEM analyses, a new phase transformation process of VN was found, which for the first time is a quite different discovery from the commonly de/intercalation process. Under the long-life stability test, the VN particles gradually transformed into rod-shaped NaVO 3 (including a small part of Na 2 V 6 O 16 ), accompanied by a drastic change in morphology. Meanwhile, the surface of carbon framework became rough and lots of pores occurred after cyclic test. This work reported a deep study on the structure and phase transformations of VN anode materials in SIBs, which we hope will contribute some efforts to the explanation of mechanisms in energy storage and conversion.