Ultrahigh-energy sodium ion capacitors enabled by the enhanced intercalation pseudocapacitance of self-standing Ti2Nb2O9/CNF anodes

Abstract

In order to increase the capacity and improve the sluggish Na⁺-reaction kinetics of anodes as sodium ion capacitors (SICs), a Ti₂Nb₂O₉/CNF self-standing film electrode comprised of Ti₂Nb₂O₉ nanosheets and carbon nanofibers has been fabricated via electrospinning HTiNbO₅ nanosheets with PAN and subsequent carbonization treatment. The as-prepared Ti₂Nb₂O₉/CNF film electrode possesses fast Na-ion intercalation kinetics and high conductivity during Na-ion storage, and it displays a high reversible capacity of 324 mA h g⁻¹ at 0.1 A g⁻¹. Additionally, it also delivers a superior rate capability of 204 mA h g⁻¹ at a high current density of 4 A g⁻¹, as well as an excellent cycling stability of 97% retention after 2000 cycles at 1 A g⁻¹ in a half-cell test. A prototype Ti₂Nb₂O₉/CNF//AC SIC full device was assembled by employing the presodiated Ti₂Nb₂O₉/CNF anode and AC cathode, and it exhibits an high energy density of 129 W h kg⁻¹ at a power density of 75 W kg⁻¹ and a high power density (7560 W kg⁻¹ with 63 W h kg⁻¹), a good cycling performance of 85% capacitance retention after 10 000 cycles at 1 A g⁻¹, suggesting that the Ti₂Nb₂O₉/CNF electrode with excellent performance would be a very promising candidate as the anode for high-performance SICs.