Synchronized ion and electron transfer in a blue T-Nb2O5-x with solid-solution-like process for fast and high volumetric charge storage

Abstract The practical application of supercapacitor is largely hindered by insufficient volumetric energy density. Herein, a pseudocapacitive material with high volumetric specific capacity is achieved by introducing oxygen deficiency into T-Nb2O5, which produces a blue T-Nb2O5-x material with simultaneously low barrier for ion and electron transfer. By carrying out over-discharge experiments combined with in-situ X-ray diffraction and transmission electron microscopy, we further revealed that a solid-solution like electrochemical process without substantial structural change and internal stress provides the structural foundation for the reversible electrochemical process. The blue T-Nb2O5-x could achieve a particle-level volumetric specific capacity of 907.9 mAh/cm3 at 10 mA/g, much higher than traditional pseudocapacitive materials that only work with their shallow surfaces. The intrinsic bulk lithium storage mechanism endows the material with an excellent electrode-level volumetric specific capacity of 141.0 mAh/cm3 within 1.2-minute discharge. An asymetric lithium-ion capacitor was assembled using active carbon as cathode to demonstrate its performance in practical device.