Three-Dimensional Cross-Linked Nb<sub>2</sub>O<sub>5</sub> Polymorphs Derived from Cellulose Substances: Insights into the Mechanisms of Lithium Storage
Ming Yang, Shun Li, Jianguo Huang
Abstract
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.