A stable ZnMn2O4 cathode for aqueous Zn-ion batteries via Ni doping to suppress Mn dissolution

ZnMn ₂O ₄ (ZMO) emerges as a promising cathode for aqueous Zn-ion batteries due to its high theoretical capacity of 224 mAh g ⁻¹ and operating voltage of ∼1.9 V (vs. Zn ²⁺/Zn). However, it suffers from capacity degradation over cycling, attributed to the dissolution of redox-active Mn through Mn ³⁺ disproportionation. In this study, we propose a strategy to stabilize ZMO cathodes by transforming Mn ³⁺ into Mn ⁴⁺ via Ni doping, aiming for charge balance. The ZnMn _2−xNi _xO ₄ ( x = 0, 0.5, 1.0, and 1.5) with different Ni amounts was prepared by straightforward precipitation and calcination. The mitigation of redox-active Mn dissolution enhanced the specific capacity of all Ni-doped ZMO compared to 201 mAh g ⁻¹ of ZMO. ZnMn _1.5Ni _0.5O ₄ and ZnMnNiO ₄ exhibit 277 and 278 mAh g ⁻¹, respectively, surpassing ZMO’s theoretical capacity (224 mAh g ⁻¹) of ZMO. This additional capacity was attributed to MnO _x deposition on the cathode and the charge storage reaction with Zn-ion within the MnO _x deposit. Furthermore, Ni doping induces a structural transition of the tetragonal into a cubic spinel, expanding the unit cell volume. This structural modification combined with suppressed Mn dissolution contributes to improved cycling stability. ZnMnNiO ₄ exhibits 80 % retention of initial capacity after 1000 cycles, outperforming ZMO (57 % retention). The expanded unit cell reduces repulsion between inserted Zn ions, enabling a higher rate performance than pure ZMO. The change in Mn valence states induced by Ni doping enhanced the lifespan and rate capability of ZMO cathodes, underscoring their potential as cathodes for Zn-ion batteries.