Comparative Electrochemical Charge Storage Properties of Bulk and Nanoscale Vanadium Oxide Electrodes

Vanadium oxide nanostructures have been widely researched as a cathode material for Li-ion batteries due to their layered structure and shorter Li⁺ diffusion path lengths, compared to the bulk material. Some oxides exhibit charge storage due to capacitive charge compensation, and many materials with cation insertion regions and rich surface chemistry have complex responses to lithiation. Herein, detailed analysis by cyclic voltammetry was used to distinguish the charge stored due to lithium intercalation processes from extrinsic capacitive effects for micron-scale bulk V₂O₅ and synthesized nano-scale vanadium oxide polycrystalline nanorods (poly-NRs), designed to exhibit multivalent surface oxidation states. The results demonstrate that at fast scan rates (up to 500 mV/s), the contributions due to diffusion-controlled intercalation processes for micron-scale V₂O₅ and nanoscale V₂O₃ are found to dominate irrespective of size and multivalent surface chemistry. At slow potential scan rates, a greater portion of the redox events are capacitive in nature for the polycrystalline nanorods. Low dimensional vanadium oxide structures of V₂O₅ or V₂O₃, with greater surface area do not automatically increase their (redox) pseudocapacitive behaviour significantly at any scan rate, even with multivalent surface oxidation states.