Building a High-Concentration Zn²⁺ Cation Reservoir of Zn Anode for Long-Cycling and High-Efficiency Zinc–Bromine Flow Batteries

Zinc–bromine flow batteries (ZBFBs) are highly competitive for large-scale energy storage due to their safety and low cost. However, unstable Zn2+ distribution within the inner Helmholtz plane (IHP) of the Zn anode often leads to dendrite growth and severe polarization, especially under high-rate and long-duration conditions. In this work, we introduce MXene nanosheets with strong Zn2+ cation hosting capability onto carbon felt (MXene@CF), which form a "Zn2+ reservoir" at the electrode–electrolyte interface, stabilizing the IHP and preventing dendrite formation. The high electric conductivity, low activation energy barrier, and Zn2+-enriched IHP of the MXene@CF electrode collectively alleviate the Ohmic polarization, activation polarization, and concentration polarization, respectively, thus reducing overall voltage polarization and improving energy efficiency. ZBFBs with MXene@CF maintain stable cycling for over 1000 h at 20 mAh cm–2 and 20 mA cm–2 with an average energy efficiency of nearly 85%.