Synthesis of Vanadium Nitride–Hard Carbon Composites from Cellulose and Their Performance for Sodium-Ion Batteries

A promising material for sodium-ion battery anodes has been developed through the controlled formation of a thin, uniformly dispersed layer of vanadium nitride (VN) nanoparticles onto a high-performance hard carbon. Hard carbon is the standard (pre)commercial material for sodium-ion negative electrodes, and our hard carbon electrodes exhibit an electrochemical performance comparable to the state-of-the-art. The introduction of VN produces an increased capacity: with addition of 8.6 wt % VN, the hard carbon-based electrode achieves a first cycle reversible (oxidation, desodiation) capacity of 354 mA h g–1 at 50 mA g–1, while with pure hard carbon it is 302 mA h g–1. The additional specific capacity achieved upon addition of VN, compared with the pure hard carbon, is 605 mA h g–1 when referred to the mass of VN only, which is the highest capacity of VN materials in sodium-ion batteries reported to date. In addition, VN also improves the capacity retention with cycling: after 50 cycles the reversible capacity of hard carbon electrodes with 8.6 wt % VN is 294 mA h g–1, while with pure hard carbon it is 239 mA h g–1. This promising new material is obtained via a new and easily scalable synthesis method in which cotton wool is reacted with a vanadium source (VOCl3), followed by a single firing step in N2. Insights into the reaction mechanism are obtained by ex situ characterization of the discharged and charged electrodes.