Ti2Nb2O9/graphene hybrid anode with superior rate capability for high-energy-density sodium-ion capacitors

Highlights: • Ti₂Nb₂O₉/RGO anode was prepared via flocculation method and annealing treatment. • The graphene promotes electron transport and prevents Ti₂Nb₂O₉ nanosheets stacking. • Ti₂Nb₂O₉/RGO anode shows superior rate capability and excellent cycling life. • Ti₂Nb₂O₉/RGO//AC SIC delivers high energy density and long-term cycling life. -- Abstract: Ti₂Nb₂O₉/reduced graphene oxide (Ti₂Nb₂O₉/RGO) hybrid anode is fabricated by flocculation of HTiNbO₅ nanosheets and graphene oxide (GO) nanosheets, followed by annealing HTiNbO₅/GO composite at 500 °C in Ar for 1 h, in which HTiNbO₅ nanosheets are prepared by ion-exchange and followed by exfoliating process using KTiNbO₅ as precursor. HTiNbO₅ nanosheets are topotactically transformed into Ti₂Nb₂O₉ nanosheets and GO is reduced into RGO by calcining the HTiNbO₅/GO composite, causing enhanced Na⁺-intercalation pseudocapacitive behavior and significantly improved conductivity for Ti₂Nb₂O₉/RGO hybrid anode. Ti₂Nb₂O₉/RGO hybrid anode delivers superior rate capability (206 mAh g⁻¹ at 4 A g⁻¹) and excellent cycling performance with 95% capacity retention at 1 A g⁻¹ over 1000 cycles. A hybrid device is assembled using Ti₂Nb₂O₉/RGO as the anode and activated carbon (AC) as the cathode, it shows a maximum high energy density of 96 Wh kg⁻¹ at a power density of 75 W kg⁻¹, and the energy density still maintains 39 Wh kg⁻¹ when the power density achieves as high as 10029 W kg⁻¹. Moreover, the assembled device displays good cycling stability with a capacity retention of 79% after 10,000 cycles. This work can provide new insights on fabrication of Ti-Nb oxides-graphene based composite anode for sodium ion capacitors.