On the Degradation of Vanadium-Based Phosphate Framework Electrode Materials in Aqueous Environments

Abstract

Na3V2(PO4)3 and Na3V2(PO4)2F3 are among the most studied and applied positive electrode materials in non-aqueous sodium-ion batteries due to their relatively high capacities and redox potentials. However, the stability of these materials in aqueous environments is relatively low limiting their applications in aqueous batteries or deionization cells. In this study, we provide a comprehensive analysis of Na3V2(PO4)3 and Na3V2(PO4)2F3 degradation in aqueous media using a number of techniques such as standard electrochemical methods, elemental analysis, powder X-ray diffractometry, and rotating ring-disc electrode. The latter allows for real time in situ/operando degradation analysis during electrochemical operation. The results show that Na3V2(PO4)3 suffers from chemical vanadium dissolution when immersed even in neutral pH electrolytes, whereas Na3V2(PO4)2F3 is significantly more stable. The results obtained by the rotating ring-disc electrode technique explicitly show that at pH ∼7 Na3V2(PO4)3 and Na3V2(PO4)2F3 generate most of the soluble V(V) species during the electrochemical charging process. Whereas in acidic pH, there is also additional electrochemically-induced generation of soluble V(IV) species during the discharging process. The overall results suggest that fluoride ions significantly increase the structural stability of phosphate materials in aqueous environments. Potentially, a careful electrolyte design with controlled proton and water activity could enable the use of Na3V2(PO4)2F3 in aqueous electrochemical devices.