α/β-Ni(OH)2 phase control by F-ion incorporation to optimise hybrid supercapacitor performance

Abstract

We have controlled the formation of flower-like nanostructured pure alpha phase and pure beta phase nickel hydroxide (α-Ni(OH)₂, β-Ni(OH)₂), as well as mixtures of phases (αβ-Ni(OH)₂). Through a range of experiments, we prove that the addition of fluoride ions controls the phase formed and we suggest two possible mechanisms for this effect. Incorporated fluoride ions in the layers disfavour the insertion of other anions and water molecules, resulting in β-Ni(OH)₂ which has a smaller interlayer distance; and we suggest that the high electronegativity of F results in less nickel oxidation to Ni³⁺ also disfavouring extra anions between layers. The nickel hydroxide materials were characterised by a suite of techniques including powder X-ray diffraction, electron microscopy, thermal analysis and in situ X-ray absorption spectroscopy, in combination with electrochemical studies. The redox cycle stability and conductivity were improved through control of phase formation, such that αβ-Ni(OH)₂ exhibits 108% capacity retention after 2000 continuous charge–discharge cycles at 20 mA cm⁻¹ while α-Ni(OH)₂ displays only 50% capacity retention after 2000 cycles. Finally, the assembled αβ-Ni(OH)₂//activated carbon hybrid supercapacitor displays maximum energy density of 32 W h kg⁻¹ at a power density of 900 W kg⁻¹, and retains 86% capacity after 5000 continuous charge–discharge cycles.