Aqueous Zn-metal batteries have been recognized as promising energy storage devices due to their high theoretical energy density and cost-effectiveness. However, side reactions and Zn dendrite growth during cycling limit their practical application. Herein, we investigated methylammonium acetate as an electrolyte additive to enhance the reversibility and stability of the Zn anode. The results revealed that the acetate anions would competitively engage the Zn 2+ solvation structure to reduce the water reactivity and promote the anion-enriched structure in the electrolyte, which can efficiently suppress the byproducts and dendrite formation. These occurs thanks to the formation of an anion-derived, robust solid electrolyte interphase with an inorganic/organic hybrid structure. Such an electrolyte enables a long cycle life over 2000 h in the Zn||Zn cell and a high Coulombic efficiency of >99.5% for 700 cycles in the Zn||Ti cell. Therefore, both Zn||Na 3 V 2 (PO 4 ) 3 batteries and Zn||activated carbon capacitors in this electrolyte exhibit improved cycling performance.

Sodium superionic conductor (NASICON)-structured compounds with a robust polyanionic framework, e.g., Na 3 V 2 (PO 4 ) 3 , have been considered as promising cathode candidates for rechargeable batteries due to their open 3D structure and high thermal stability. However, their practical implementation in aqueous batteries is hindered by their structural instability during the charge/discharge process. Herein, Na 3 V 2-x Cr x (PO 4 ) 3 is investigated as a cathode material for aqueous zinc-metal batteries. It is identified that the Cr substitution has a significant effect on improving its rate capability and cycling stability. As a result, the optimal Na 3 V 1.5 Cr 0.5 (PO 4 ) 3 electrode delivers an ultra-stable cycling performance (68% capacity retention after 10,000 cycles at 1000 mA g −1 ). A two-electron reaction mechanism between V 4+ /V 3+ and V 5+ /V 4+ redox couples has been revealed during the electrochemical process for Zn-ion storage. This work verifies the feasibility of multi-electron reactions in NASICON-type cathodes for aqueous zinc batteries and sheds light on designing advanced cathode materials for other aqueous batteries.

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