Highly stable electrolyte enables wide temperature vanadium flow batteries

Abstract

Vanadium flow batteries (VFB) offer an ideal solution to the issue of storing massive amounts of electricity produced from intermittent renewables. However, the historical challenge of high thermal precipitation of V₂O₅ from VO₂⁺ (∼50 °C for 1 day) represents a critical concern. Temperature control can alleviate the problem to a certain extent, however, at the expense of the cost of system design and operation. Herein, we report stable electrolyte chemistry at high temperature. By introducing Cr³⁺ as a stabilizer, it bridges with VO₂⁺ to form a CrOV^Ⅴ structure, which reduces the electron cloud density of V. Therefore, it combines more tightly with H₂O and prevents its dehydration process. In addition, the dimerization process of VO₂⁺ is also inhibited due to the occupancy of Cr³⁺. As a result, a formed 1.5 M VO₂⁺ electrolyte demonstrates a high stability for over 30 days at 50 °C (v.s. blank for < 1 day at 50 °C). Additionally, the low-temperature precipitation temperature of V²⁺ on the negative side has been reduced from 0 °C of commercial electrolytes to −5 °C. As a proof of concept, a VFB assembled with Nafion 115 membrane demonstrates an energy efficiency (EE) of 80% at 120 mA cm⁻² for 1000 cycles (50 °C). Most importantly, a 4 kW stack can continuously run for ∼1000 cycles with EE of 80% at 120 mA cm⁻² without any heat management. Combined with high thermal stability and excellent performance, our design will certainly provide new impetus for the further commercialization of VFB batteries.