Aqueous rechargeable ammonium-ion (NH₄⁺) batteries (AAIBs) with ammonium ions as charge carriers possess many advantages, yet the relatively low discharge capacities (e.g., < 200 mAh·g⁻¹) of the reported NH₄⁺ host materials hinder the development of AAIBs. Herein, we study the NH₄⁺ storage properties of an electrochemically activated NiCo double hydroxide (A-NiCo-DH) in neutral ammonium acetate electrolyte for the first time. The activation process extracts the interlayer anions (NO₃⁻) from the host material, providing additional cation accommodation sites for charge storage. The introduced H vacancies in A-NiCo-DH could activate the O sites, leading to the enhanced cation adsorption capability for the electrode. Therefore, A-NiCo-DH exhibits a high discharge capacity of 280.6 mAh·g⁻¹ at 0.72 A·g⁻¹ with good rate capability. Spectroscopy studies suggest A-NiCo-DH experiences a NH₄⁺/H⁺ coinsertion mechanism. A NH₄⁺-Zn hybrid cell is assembled using A-NiCo-DH as the cathode and Zn foil as the anode, respectively. The device delivers an energy density of 306 Wh·kg⁻¹ at the power density of 745.8 W·kg⁻¹ (based on the active mass of A-NiCo-DH). This work provides a new NH₄⁺ storage material and would push forward the development of aqueous NH₄⁺-based batteries.