**4. Conclusions**

In this study, five phosphate-containing scale inhibitions, including HDTMPA, HEDP, DTPMP, ATMPA, and EDTMPS, were employed as additives for the VRFB positive electrolyte and their effects on the electrolyte thermal stability and electrochemical performance were investigated. HDTMPA has a great positive effect on the thermal stability of the electrolyte, extending the time at which the electrolyte begins to precipitate to 15 days, while it has a lesser effect on the electrochemical performance; HEDP, DTPMP, and EDTMPS effectively improve the thermal stability of electrolyte and simultaneously accelerate its electrochemical reaction kinetics, but they have a greater effect on the cyclic reversibility of electrolyte, among which HEDP extended the time at which the electrolyte began to precipitate to 30 days, and the charge transfer resistance of the electrolyte decreased to 9.40 Ω cm2; and the addition of ATMPA greatly improves the mass transfer kinetics of the electrolyte, increasing the electrolyte diffusion coefficient on the electrode surface to 6.80 × <sup>10</sup>−7, simultaneously causing poor thermal stability of the electrolyte. In addition, the compounding effect of additives was studied. It was verified that the compounding combination of ATMPA + HDTMPA and ATMPA + HEDP had a good effect on the electrochemical performance of the electrolyte. In conclusion, most of the additives selected in this study have positive effects on the positive electrolyte of VRFB in terms of thermal stability and electrochemical performance; the diffusion coefficients of the electrolyte were 4.34 × <sup>10</sup>−<sup>7</sup> and 5.43 × <sup>10</sup>−7, respectively. The necessity of combined utilization of these additives should be recognized, which may work better and has great potential for future development.

**Author Contributions:** Conceptualization, X.Z. and L.W.; Data curation, X.Z.; Formal analysis, F.M.; Project administration, Z.Z. and D.C.; Resources, L.W.; Writing—original draft, X.Z.; Writing—review and editing, X.Z., F.M., L.S. and Z.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** National Key R&D Program of China (2018YFC1900500), the National Natural Science Foundation of China (51804289), General project of Beijing Municipal Natural Science Foundation (2202053), Strategic Priority Research Program of the Chinese Academy of Sciences (XDC04010100), Special Project for Transformation of Major Technological Achievements in Hebei Province (19044012Z), and Province Key R&D Program of Hebei (20374105D).

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
