Mn-deficient ZnMn₂O₄/Zn_0.5Mn_0.5Fe₂O₄ cathode for enhancing structural reversibility and stability of zinc-ion batteries

Aqueous zinc-ion batteries have surfaced as a viable energy storage technology due to their safety, eco-friendliness, and cost-effectiveness. Binary zinc manganite oxide (ZnMn₂O₄ or ZMO) stands out as the potential cathode material owing to its considerable theoretical capacity and high operating voltage. However, high electrostatic repulsion of Zn²⁺ ions within ZMO leads to sluggish diffusion and poor reversibility of Zn²⁺ ions insertion/extraction that can reduce charge storage and overall cycling performance. This study addresses this challenge by introducing FeCl₃ during the synthesis to form ZMO/Zn_0.5Mn_0.5Fe₂O₄ (ZMFO) cathode material. This cathode features Mn-deficient structures, which can reduce the electrostatic repulsion via low Mn occupancies and lattice parameters enlargement, thus facilitating Zn²⁺ ions diffusion. Ex-situ X-ray diffraction analyses further show that the cathode is able to maintain good structural reversibility during the charge-discharge cycles. Additionally, the ZMO/ZMFO cathode exhibits smaller particle sizes than pristine ZMO, providing a wider surface area. The presence of ZMFO with a large unit cell volume can also enhance Zn storage capacity and accelerate Zn²⁺ ions kinetics. Highlighting those advantages, aqueous zinc-ion batteries with ZMO/ZMFO cathode show a capacity of ~230 mAh g⁻¹ at 0.05 A g⁻¹ and retain 99 % of its initial capacity at 0.2 A g⁻¹ after 200 cycles of charge-discharge.