Effects of phosphate additives on the stability of positive electrolytes for vanadium flow batteries

Highlights: • A series of phosphates is investigated as additives for vanadium flow battery. • Superior V(V) thermal stability and improved electrochemical performance. • Enhanced battery efficiency and slower capacity fading. • Mechanism for the stabilization and performance improvement is put forward. • NH₄H₂PO₄ indicates a promising candidate for additive of the positive electrolyte. - Abstract: A series of phosphates is investigated as additives to improve the stability of the electrolyte for vanadium flow battery (VFB). Two selected additives show positive effect on the stability of electrolytes under ex-situ stability tests and in situ flow cell experiments. The effects of additives on electrolyte are studied by Nuclear magnetic resonance (NMR), X-ray diffraction (XRD), Raman spectroscopy, Cyclic voltammetry (CV), Electrochemical impedance spectroscopy (EIS) and charge–discharge test. The results show that a VFB using the electrolyte with NH₄H₂PO₄additive demonstrates significantly improved redox reaction reversibility and activity, and higher energy efficiency. In addition, the cell employing the electrolyte with NH₄H₂PO₄ exhibits a charge capacity fading rate much slower than the cell without additives during the cycling at high temperature. These results indicate that the phosphate additives are highly beneficial to improving the stability and reliability of VFB