Revisiting Li+ intercalation into various crystalline phases of Nb2O5 anchored on graphene sheets as pseudocapacitive electrodes

Herein, polycrystalline Nb2O5 nanoparticles including pseudo-hexagonal (TT-), orthorhombic (T-), tetragonal (M-) and monoclinic (H-) phases are uniformly anchored onto graphene sheets through a polyol-mediated solvothermal reaction and post heat-treatment, enabling us to discuss the effect of Nb2O5 crystalline phases on Li+ intercalation process. Electrochemical results show that Li+ intercalation into M- and H-Nb2O5 are also capacitive behavior, exhibiting higher capacity of 650 C g(-1) and 615 C than T- and TT-phases of 585 C g(-1) and 530 C g(-1) at 0.5 A g(-1). The higher capacity-retention is also obtained for M- and H-phases (509 and 452 C g(-1)) compared to T- and TT-phases (305 and 207 C g(-1)) at 20 A The high rate performance of H-phase can be explained by higher Li+ diffusion coefficients (D-Li) of 1.6 x 10(-12) cm(2) s(-1), which is almost two order of magnitude higher than TT-phase of 4.7 x 10(-14) cm(2) s(-1). This results indicate that Nb2O5 with various crystal structures shows similar Li+ capacitive intercalation behavior, but more ordered arrangement of unit cell may provide more vacancy for Li+ with the lower diffusion barriers, thus leading to higher rate performance. (C) 2016 Elsevier B.V. All rights reserved.