Pseudocapacitive material with 928 mAh cm-3 particle-level volumetric specific capacity enabled by continuous phase-transition

The low active ratio of traditional (only surface-active) pseudocapacitive materials limits the fabrication of electrochemical energy storage/ conversion devices with high volumetric energy densities. This work focuses on the T-Nb2O5 phase that show bulk pseudocapacitance. The capacitor-like electrochemical kinetic of T-Nb2O5 was found to exsit in particles without nano structuring, thus providing possibility to design core@shell T-Nb2O5@C particles with larger core and thinner shell. By scaling the capacity to the volume of the particle, a T-Nb2O5@C with 85 nm core and 1.5 nm shell is able to provide a particle-level volumetric specific capacity of 928 mAh cm(-3), which is much larger than any other traditional pseudocapacitive materials that work with only their surfaces. Advanced in situ XRD and in situ TEM tools reveal that the T-Nb2O5 experiences a small and continuous phase change process without phase-separation and energy barrier for the formation of phase boundary. We propose the unique structural behavior of T-Nb2O5 is due to the existance of large amounts of intermediate phases between the original and lithiated state of T-Nb2O5.