Nanoarchitectured Nb ₂O ₅ hollow, Nb ₂O ₅@carbon and NbO ₂@carbon Core-Shell Microspheres for Ultrahigh-Rate Intercalation Pseudocapacitors

Abstract

Li-ion intercalation materials with extremely high rate capability will blur the distinction between batteries and supercapacitors. We construct a series of nanoarchitectured intercalation materials including orthorhombic ( o-) Nb ₂O ₅ hollow microspheres, o-Nb ₂O ₅@carbon core-shell microspheres and tetragonal ( t-) NbO ₂@carbon core-shell microspheres, through a one-pot hydrothermal method with different post-treatments. These nanoarchitectured materials consist of small nanocrystals with highly exposed active surface, and all of them demonstrate good Li ⁺ intercalation pseudocapacitive properties. In particular, o-Nb ₂O ₅ hollow microspheres can deliver the specific capacitance of 488.3 F g ⁻¹, and good rate performance of 126.7 F g ⁻¹ at 50 A g ⁻¹. The o-Nb ₂O ₅@carbon core-shell microspheres show enhanced specific capacitance of 502.2 F g ⁻¹ and much improved rate performance (213.4 F g ⁻¹ at 50 A g ⁻¹). Furthermore, we demonstrate for the first time, t-NbO ₂ exhibits much higher rate capability than o-Nb ₂O ₅. For discharging time as fast as 5.9 s (50 A g ⁻¹), it still exhibits a very high specific capacitance of 245.8 F g ⁻¹, which is 65.2% retention of the initial capacitance (377.0 F g ⁻¹ at 1 A g ⁻¹). The unprecedented rate capability is an intrinsic feature of t-NbO ₂, which may be due to the conductive lithiated compounds.