Impact of the morphology of V₂O₅ electrodes on the electrochemical Na⁺-ion intercalation

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 Na_xV₂O₅ 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 Na_xV₂O₅ 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 (2^nd 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.