Three-Dimensional Cross-Linked NbO Polymorphs Derived from Cellulose Substances: Insights into the Mechanisms of Lithium Storage.

Niobium pentoxide (NbO)-based materials have been regarded as promising anodic materials for lithium-ion batteries due to their abundant crystalline phases and stable and safe lithium storage performances. However, there is a lack of systematic studies of the relationship among the crystal structures, electrochemical characteristics, and lithium storage mechanisms for the various NbO polymorphs. Herein, pure pseudohexagonal NbO (TT-NbO), orthorhombic NbO (T-NbO), tetragonal NbO (M-NbO), and monoclinic NbO (H-NbO) with three-dimensional interconnected structures are reported, which were synthesized a hydrothermal method using the commercial filter paper as the structural template followed by specific annealing processes. Impressively, the TT- and T-NbO species both possess bronze-like phases with "room and pillar" structures, while M- and H-NbO ones are both in the Wadsley-Roth phases with crystallographic shear structures. Among the pristine NbO materials, H-NbO exhibits the highest initial specific capacity (270 mA h g), while T-NbO performs with the lowest (197 mA h g) at 0.02 A g, meaning that crystallographic shear structures provide more lithium storage sites. TT-NbO realizes the best rate capability (207 mA h g at 0.02 A g and 103 mA h g at 4.0 A g), indicating that the "room and pillar" crystal structures favor fast lithium storage. Electrochemical analyses reveal that the TT- and T-NbO phases with "room and pillar" crystal structures utilize a pseudocapacitive intercalation mechanism, while the M- and H-NbO phases with the Wadsley-Roth shear structures follow a typical battery-type intercalation mechanism. A fresh insight into the further understanding of the intercalation pseudocapacitance on the basis of the unit cells of the electrode materials and a meaningful guidance for crystalline structural design/construction of the electrode materials for the next-generation LIBs are thus provided.