Rapid and durable electrochemical storage behavior enabled by V4Nb18O55 beaded nanofibers: a joint theoretical and experimental study

Abstract

A relatively thermodynamically stable phase in the V₂O₅–Nb₂O₅ system, namely, V₄Nb₁₈O₅₅, was prepared and then structurally and electrochemically characterized. Theoretical calculations show that the crystal structure of V₄Nb₁₈O₅₅ allows the rapid diffusion of lithium ions in three directions in the open structure, which ensures a high diffusion coefficient at crystal structure horizon. V₄Nb₁₈O₅₅ synthesized by a sol–gel method exhibited a reversible specific capacity of 226.8 mA h g⁻¹. A significant enhancement in electrochemical properties can be delivered by the electrospun V₄Nb₁₈O₅₅ beaded nanofibers due to the shorter pathways in the spheres, leading to an improved capacity of 251.6 mA h g⁻¹ between 1 V and 3 V with 92.54% capacity retention. According to the results of the theoretical calculations, we can find that electrochemical energy storage in V₄Nb₁₈O₅₅ arises from the occupation by lithium ions in 8q, 4g, 4h and 4j sites in the structure. The in situ X-ray diffraction study further confirmed that the open structure of V₄Nb₁₈O₅₅ not only ensures the highly reversible storage and transport of lithium ions in these cavities, but also provides a stable framework during repeated charge/discharge cycles.