Three-Dimensional Cross-Linked Nb2O5 Polymorphs Derived from Cellulose Substances: Insights into the Mechanisms of Lithium Storage

Niobium pentoxide (Nb2O5)-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 Nb2O5 polymorphs. Herein, pure pseudohexagonal Nb2O5 (TT-Nb2O5), orthorhombic Nb2O5 (T-Nb2O5), tetragonal Nb2O5 (M-Nb2O5), and monoclinic Nb2O5 (H-Nb2O5) with three-dimensional interconnected structures are reported, which were synthesized via a hydrothermal method using the commercial filter paper as the structural template followed by specific annealing processes. Impressively, the TT- and T-Nb2O5 species both possess bronze-like phases with room and pillar structures, while M- and H-Nb2O5 ones are both in the Wadsley-Roth phases with crystallographic shear structures. Among the pristine Nb2O5 materials, H-Nb2O5 exhibits the highest initial specific capacity (270 mA h g(-1)), while T-Nb2O5 performs with the lowest (197 mA h g(-1)) at 0.02 A g(-1), meaning that crystallographic shear structures provide more lithium storage sites. TT-Nb2O5 realizes the best rate capability (207 mA h g(-1) at 0.02 A g(-1) and 103 mA h g(-1) at 4.0 A g(-1)), indicating that the room and pillar crystal structures favor fast lithium storage. Electrochemical analyses reveal that the TT- and T-Nb2O5 phases with room and pillar crystal structures utilize a pseudocapacitive intercalation mechanism, while the M- and H-Nb2O5 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.

Automated summary

Niobium pentoxide (Nb2O5)-based materials with different crystal structures exhibit varying lithium storage performances, with crystallographic shear structures providing more storage sites and room and pillar crystal structures favoring fast lithium storage. The study sheds light on the intercalation pseudocapacitance mechanism and provides valuable guidance for the design of electrode materials for next-generation LIBs.