Regulating solvation structure and interfacial chemistry via multifunctional acetylurea for highly reversible neutral zinc-manganese flow batteries

Abstract

Neutral zinc-manganese flow batteries (ZMFBs) have emerged as promising candidates for large-scale energy storage because of their safety, low cost, and the abundance of Zn and Mn resources. Nevertheless, their performance is severely limited by Mn3+ disproportionation and uncontrolled MnO2 deposition, which result in inactive manganese species, sluggish reaction kinetics, and rapid capacity fading. Herein, acetylurea (Ace) is proposed as a multifunctional additive to regulate both the solvation chemistry of Mn2+ and the cathode/electrolyte interface in neutral ZMFBs. Owing to the coordination interaction between Ace and Mn2+, the manganese solvation environment is stabilized, Mn-related side reactions are alleviated, and MnO2 deposition becomes more uniform and reversible. Consequently, the Ace-modified ZMFB delivers markedly improved electrochemical performance, including a cycle life extended from 80 to 380 cycles, an energy efficiency (EE) increased from 74% to 86%, and a coulombic efficiency (CE) enhanced from 83% to 98%. Moreover, Ace lowers the charge-transfer resistance and accelerates redox kinetics. SEM, XPS, and FT-IR analyses verify the optimized interfacial chemistry and the uniform MnO2 morphology induced by Ace, while theoretical calculations confirm the strong coordination of Ace with Mn2+. This work demonstrates a robust additive engineering strategy for neutral ZMFBs and provides new insights into designing durable manganese-based flow batteries for grid-scale energy storage.