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

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.