The Influence of a Binder in a Composite Electrode: The Case Study of Vanadyl Phosphate in Aqueous Electrolyte

Layered VOPO₄·2H₂O is synthesized by the sonochemical method. An X-ray powder diffraction is used to examine the crystal structure, while scanning electron microscopy is used to reveal the morphology of the powder. The crystal structure refinement is performed in the P4/nmmZ space group. The electrochemical intercalation of several cations (Na⁺, Mg²⁺, Ca²⁺, and Al³⁺) in saturated nitrate aqueous solutions is investigated. The most notable reversible activity is found for the cycling in aluminium nitrate aqueous solution in the voltage range from -0.1 to 0.8 V vs. SCE. During the preparation of the electrode, it is observed that the structure is prone to changes that have not been recorded in the literature so far. Namely, the use of conventional binder PVDF in NMP solution deteriorates the structure and lowers the powder's crystallinity, while the use of Nafion solution causes the rearrangement of the atoms in a new crystal form that can be described in the monoclinic P2₁/c space group. Consequently, these structural changes affect electrochemical performances. The observed differences in electrochemical performances are a result of structural rearrangements.