Aqueous　Zn-ion　batteries　(AZIBs)　have　been　recognized　as　promising　energy　storage　devices　due　to　their　high　theoretical　energy　density　and　cost-effectiveness.　However,　side　reactions　and　Zn　dendrite　generation　during　cycling　limit　their　practical　application.　Herein,　ammonium　acetate　(CH3COONH4)　is　selected　as　a　trifunctional　electrolyte　additive　to　enhance　the　electrochemical　performance　of　AZIBs.　Research　findings　show　that　NH4+　(oxygen　ligand)　and　CH3COO–　(hydrogenligand)　with　preferential　adsorption　on　the　Zn　electrode　surface　can　not　only　hinder　Zn　anode　directly　contact　with　active　H2O,　but　also　regulate　the　pH　value　of　the　electrolyte,　thus　suppressing　the　parasitic　reactions.　Additionally,　the　formed　SEI　is　mainly　consisted　of　Zn5(CO3)2(OH)6　with　a　high　Zn2+　transference　number,　which　could　achieve　a　dendrite-free　Zn　anode　by　homogenizing　Zn　deposition.　Consequently,　the　Zn||Zn　symmetric　batteries　with　CH3COONH4-based　electrolyte　can　operate　steadily　at　an　ultrahigh　current　density　of　40 mA　cm–2　with　a　cumulative　capacity　of　6880　mAh　cm–2,　especially　stable　cycling　at　−10 °C.　The　assembled　Zn||MnO2　full　cell　and　Zn||activated　carbon　capacitor　also　deliver　prominent　electrochemical　reversibility.　This　work　provides　unique　understanding　of　designing　multi-functional　electrolyte　additive　and　promotes　a　long　lifespan　at　ultrahigh　current　density　for　AZIBs.　©　2022　The　Authors.　Advanced　Science　published　by　Wiley-VCH　GmbH.