Capacity and phase stability of metal-substituted α-Ni(OH)2nanosheets in aqueous Ni-Zn batteries

Abstract

Batteries that offer high specific energy and energy density coupled with improved safety and lower cost will affect applications ranging from electric vehicles, portable electronic devices, and grid-level energy storage. Alkaline nickel-zinc (Ni-Zn) batteries use nonflammable aqueous electrolyte and nonstrategic, low-cost electrode materials; however with a two-electron anode, a cathode that stores more than one electron per Ni atom would increase energy density. Herein, we report the effect of substituting metal ions (aluminium, cobalt, manganese, or zinc) into α-Ni(OH)2, a phase that can accommodate more than one-electron charge storage, but which typically converts to lower-capacity β-Ni(OH)2 upon cycling in alkaline electrolytes. We adapt a microwave-assisted process that expresses α-Ni(OH)2 as a high surface-area nanosheet morphology and find that we retain this morphology with all metal-ion substituents. The series is characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. Metal-ion substitution influences aggregate growth, interlayer distance, and vibrational frequencies. We test powder-composite cathodes prepared using the substituted α-Ni(OH)2 series versus zinc sponge anodes in alkaline electrolyte under device-relevant mass loadings and using an intentionally aggressive charging protocol to determine onset voltage for oxygen evolution. The electrochemical charge-storage behaviour is established using galvanostatic cycling and differential capacity analysis. The substituents significantly influence both Ni-centred redox and oxygen-evolution voltages (vs. Zn/Zn2+). The incorporation of Al3+ within α-Ni(OH)2 nanosheets provides higher capacity and phase stability compared to the divalent substituents and unsubstituted α-Ni(OH)2. The presence of ordered free nitrates in the interlayer of Al3+-substituted α-Ni(OH)2, not seen with Co2+ or Mn2+ substituents, correlates with the improved electrochemical performance.