Synthesis and characterization of Co₃O₄ nanosphere and its charge storage characteristics in aqueous Zn-ion batteries

In this work, Co₃O₄ spinel is synthesized and exposed as cathode material because of its high stability in alkaline electrolytes, high working voltage window in full cell (vs. Zn) > 1.75 V, high specific capacity (446 mAh g⁻¹), having a high theoretical energy density of 516 Wh kg⁻¹, and good electrochemical stability. Here, the Co₃O₄ sample was synthesized using a simple co-precipitation process followed by calcination. X-ray diffraction was used to confirm the structure and phase purity of the samples. The surface morphology of the sample was examined using FESEM and HR-TEM analysis. The oxidation state of Co in the sample was evaluated using XPS analysis. The Zn||PVA-KOH||Co₃O₄ electrochemical cell was fabricated, and the electrochemical performance was investigated. Here, we used gel polymer electrolyte to demonstrate the practical feasibility of the flexible Zn||Co₃O₄ cell. Polyvinyl alcohol (PVA) gives excellent chemical stability and electrochemical inertness, is easy to fabricate, and is non-toxicity; in this work, it is used as a host polymer, and potassium hydroxide (KOH) is used as an ion source in the gel electrolyte medium. Here, PVA is used as a polymer host and separator to separate the anode and cathode in the full cell fabrication. The hydrogel-based pouch-type full cell retained good capacity over 1000 cycles with the maximum tested cell capacity of 10 mAh cm⁻² at 2 mA cm⁻². The cell showed above > 95 capacity retention even at 1000 cycles, which indicates that the Co₃O₄ can be used as a potential cathode in 1.6 V aqueous Zn cells.