Unraveling the Nature of Anomalously Fast Energy Storage in T-NbO.

While T-NbO has been frequently reported to display an exceptionally fast rate of Li-ion storage (similar to a capacitor), the detailed mechanism of the energy storage process is yet to be unraveled. Here we report our findings in probing the nature of the ultrafast Li-ion storage in T-NbO using both experimental and computational approaches. Experimentally, we used in operando Raman spectroscopy performed on a well-designed model cell to systematically characterize the dynamic evolution of vibrational band groups of T-NbO upon insertion and extraction of Li ions during repeated cycling. Theoretically, our model shows that Li ions are located at the loosely packed 4g atomic layers and prefer to form bridging coordination with the oxygens in the densely packed 4h atomic layers. The atomic arrangement of T-NbO determines the unique Li-ion diffusion path topologies, which allow direct Li-ion transport between bridging sites with very low steric hindrance. The proposed model was validated by computational and experimental vibrational analyses. A comprehensive comparison between T-NbO and other important intercalation-type Li-ion battery materials reveals the key structural features that lead to the exceptionally fast kinetics of T-NbO and the cruciality of atomic arrangements for designing a new generation of Li-ion conduction and storage materials.