Impact of the Morphology of V2O5 Electrodes on the Electrochemical Na+-Ion Intercalation

Abstract

The development of high performance electrodes for Na-ion batteries requires a fundamental understanding of the electrode electrochemistry. In this work, the effect of the morphology of vanadium oxide on battery performance is investigated. First, the phase transitions upon sodiation/de-sodiation of NaxV2O5 cathodes in standard battery solvents are explored by cyclic voltammetry and X-Ray diffraction. At potentials 1.5 V positive of Na/Na+ the insertion of the first Na+ into pristine V2O5 is completed and α’-NaV2O5 is formed. A discharge to 1.0 V results in the introduction of a second Na+ and after a deep discharge to 0 V a third Na+ is intercalated. When cycled as an intercalation electrode, the Na-content x in NaxV2O5 varies between x = 1 (charged) and x = 2 (discharged). For studying the effect of electrode morphology on the battery performance, several types of V2O5 (hollow V2O5 microspheres, V2O5 nanobundles and V2O5 nanobundles blended with 10%wt TiO2) were prepared and compared to a commercially available V2O5-micropowder. The nanobundles were prepared by a facile sonochemical process. In comparison to the microsized V2O5 morphologies, the potential plateaus in the charge/discharge curves of the V2O5 nanobundles are at more positive potentials and the capacity loss in the first cycle is suppressed. The V2O5 nanobundles showed the best battery performance with a reversible capacity of 209.2 mAh g−1 and an energy density of 571.2 mWh kg−1 (2nd cycle). After an initial capacity fading, which can be slightly suppressed by blending the V2O5 with TiO2, the pure V2O5 nanobundles have a practical capacity of 85 mAh g−1, an operation potential of 2.4 V, an energy density of 266.5 mWh kg−1 and a capacity retention of 83% after 100 cycles. The best battery performance of the nanomaterial is ascribed in this study to the amorphous character of the electrode, favoring faster electrode kinetics due to a (pseudo-) capacity dominated charging/discharging, reducing diffusion lengths and preventing further amorphization, which all is beneficial in terms of lifetime, capacity, operation voltage, energy density and energy efficiency.