Synthesis of Amorphous Nickel-Cobalt Hydroxides for Ni-Zn Batteries

In this work we developed a hydrothermal method for synthesizing amorphous Ni-Co hydroxide (NC(OH)) and in the subsequent step crystalline NiCo₂O₄ (NCO) has been produced using water as the solvent. For nickel-zinc batteries, NC(OH) was found to have superior performance to its NCO prepared by two-step process. The thermal stability analysis exhibited the optimum temperature to obtain the NC(OH), and NCO electrode materials. The XRD pattern showed mixed phases containing both Ni and Co hydroxides (during the initial step) and in the subsequent step (calcined) the formation of cubic spinel structure was noticed. For NC(OH), aggregated particles with irregular morphology were observed while clustered nanorod-like shapes were noticed for NCO samples. To be noted, that the nanorod morphology was obtained through a facile approach without employing any structure-directing agent. Both NC(OH) and NCO were employed as cathodes for Ni-Zn battery studies against Zn foil anode with a polyamide-based separator soaked in 6 M KOH saturated with ZnO additive was used as electrolyte. The Ni-Zn cell was fabricated in CR2032 coin cell configuration. The electrochemical studies such as cyclic voltammetry (CV) showed the characteristic redox peaks for NC(OH) sample exhibiting high peak current compared to its NCO counterpart. The NC(OH) had a capacity of 268 mAh g^-1 against 120 mAh g^-1 for NCO at a current density of 1 Ag^-1. The cell was able to retain 85 % of the capacity at the end of 500 cycles and showed remarkable rate capability. The Ni-Zn battery presents energy and power densities of 428.8 Wh Kg^-1 and 2.68 kW Kg^-1, respectively surpassing the normal values reported for aqueous rechargeable batteries. Owing to the presence of Ni and Co in hydroxide form (reduced crystallinity) the NC(OH) sample showed improved electrochemical activity. This work provides a facile approach and effective strategy for developing bimetallic hydroxides for optimal energy storage performance.