An Electrochemically Prepared Mixed Phase of Cobalt Hydroxide/Oxyhydroxide as a Cathode for Aqueous Zinc Ion Batteries

Abstract

Cobalt hydroxide is a widely studied electrode material for supercapacitor and alkaline zinc ion batteries. The large interlayer spacing of Co(OH)2 is also attractive to store Zn ions. However, Co(OH)2 is quite unstable in the acidic ZnSO4 electrolyte due to its amphoteric nature. Herein, we synthesized a mixed phase of Co(OH)2/CoOOH via a two-step electrochemical preparation. As the cathode material for an aqueous zinc ion battery (AZIB), Co(OH)2/CoOOH delivered a maximum capacity of 164 mAh g−1 at 0.05 A g−1 and a high energy density of 275 Wh kg−1. Benefiting from the low charge-transfer resistance, a capacity of 87 mAh g−1 was maintained at 1.6 A g−1, showing a good rate performance of the mixed phase. Various spectroscopy analyses and simulations based on the density functional theory (DFT) suggested a higher thermal stability of the mixed phase than pure Co(OH)2, due to its less local structural disorder. The reduced Co-Co and Co-O shells increased the mechanical strength of the mixed phase to accommodate Zn2+ ions and endure the electrostatic repulsion, resulting in an enhanced cycling stability. The mixed phased also delivered a good stability at the current density of 0.05 A g−1. After 200 cycles, a capacity retention of 78% was retained, with high Coulombic efficiencies. These results provide a new route to synthesize high-performance LDH for aqueous zinc ion batteries.