Calcium-ion batteries (CIBs) are in the framework for providing next-generation energy storage initiatives due to their theoretically high operating voltages and the natural abundancy of calcium adhering with cost efficiency. Considerable efforts have been made towards the concept of multivalent ions in energy storage systems as alternatives to conventional lithium-based technologies, of which pose limited potential to reach higher energy densities and capacities. Despite calcium’s promise in delivering for energy storage devices, cathode materials remain limited, with very few cathode materials (e.g., Prussian Blue Analogues and particular metal oxides) being of high voltage. This project aimed to assess for the first time the suitability of a widely reported (NixMnyCoz)O2 (NMC) cathode material for sodium-ion batteries in calcium-ion battery technologies. Calcium (Ca2+) and sodium (Na+) ions both exhibit a similar ionic size (0.99 Å and 0.95 Å, respectively), pinpointing the potential for calcium ions to intercalate within reported NMC host structures in sodium-ion cells. Completing this project involved investigating the electrochemical behaviour and the suitability of these unreported calcium-intercalated NMC cathode materials for intercalation of calcium ions during the discharge step of a cell. In parallel, a manganese-based cathode material (CaMn2O4) reference was also synthesised and characterised against the NMC counterparts with varying nickel and cobalt contents to assess the structural stability of the NMC materials. All materials were confirmed to have good crystallinity and uniform particle size using X-Ray diffraction and scanning electron microscopy. In detail, crystal structures revealed a Buckyball morphology, coinciding with NMC cathode material reported in sodium-ion battery technologies. Energy-dispersive X-ray spectroscopy (EDX), inductively coupled plasma – optical emission spectrometry (ICP-OES), Fourier-transform infrared spectroscopy (FT-IR), and Raman spectroscopy were also used to characterise the NMC crystal structures obtained and their atomical composition. With the designed materials library, multiple CIBs were fabricated, and their performance evaluated in battery performance testing. Preliminary galvanostatic results revealed discharge capacities of the Ca0.05(Ni0.06Mn0.68Co0.06)O4 cathode material up to ca. 228 mAh g-1 at 2.6 V (Ca2+/Ca) at a fixed current density of 5 mA g-1. This report significantly adds to the literature of CIBs using NMC materials, which remain unreported to date in this field.
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