NiMn ₂ O ₄ Microsphere as an Efficient Cathode for Aqueous Zinc‐Ion Batteries

Abstract Microspherical mixed‐valence spinel‐type NiMn ₂ O ₄ cathode host is synthesized via a simple two‐step process involving hydrothermal treatment followed by calcination, and is employed as a high‐performance cathode host for aqueous zinc‐ion battery (ZIB) applications. The Zn//NiMn ₂ O ₄ cell delivers high initial discharge capacities of 146.4 and 108.1 mAh g ⁻¹ at 1 and 6 C, respectively (1 C = 115 mA g ⁻¹ ). Excellent cycling stability is observed, with capacity retention of 95.8%, 86.1%, 85.3%, and 84.5% at 1, 2, 4, and 6 C after 550 galvanostatic charge‐discharge (GCD) cycles. Long‐term cycling at 1 C demonstrates 88.5% capacity retention after 2050 GCD cycles with stable Coulombic efficiency. A two‐stage ion insertion mechanism involving co‐insertion of H ⁺ and Zn ²⁺ is identified through GCD, cyclic voltammetry (CV), and galvanostatic intermittent titration technique (GITT) analyzes. CV and GITT reveal high H ⁺ diffusion coefficients and dominant H ⁺ kinetics in Zn//NiMn ₂ O ₄ , while scan rate‐dependent CV confirms a hybrid capacitive‐diffusion charge storage mechanism. Self‐discharge test shows 95.5% of open circuit voltage retention after 50 days, indicating remarkable shelf life for Zn//NiMn ₂ O ₄ battery. These findings demonstrate the potential of spinel‐type NiMn ₂ O ₄ as a cobalt‐free, high‐performance cathode for next‐generation aqueous ZIB storage systems.