A Na3PO4/CTAB Composite Additive for a High-Temperature Stable Positive Electrolyte of Vanadium Redox Flow Batteries

Abstract Although vanadium redox flow batteries (VRFBs) hold significant promise for large-scale, long-duration energy storage, their practical deployment at elevated temperatures is severely hindered by the poor thermal stability of the positive electrolyte. In this work, a Na3PO4/CTAB composite additive was introduced into vanadium electrolytes to enhance high-temperature stability. Compared with single-additive systems, the composite additive exhibited a distinct synergistic effect by suppressing precipitation of the positive electrolyte while maintaining or improving interfacial redox kinetics. As a result, the precipitation rate of vanadium at 50 °C decreased by 11.67% relative to that of the pristine electrolyte. At 55 °C and a current density of 80 mA cm–2, the cell containing the composite additive delivered an energy efficiency of 85.72% after 60 cycles, and its discharge capacity reached 876.73 mAh, significantly higher than that of the cell with the pristine electrolyte (497.70 mAh). Mechanistic analyses indicate that phosphate enhances bulk electrolyte stability, whereas CTAB mitigates the interfacial aggregation and deposition of vanadium species. These findings demonstrate that the rational design of composite additives offers an effective and practical strategy for broadening the operating temperature window of VRFB electrolytes.