Molten salt synthesis of disordered spinel CoFe₂O₄ with improved electrochemical performance for sodium-ion batteries

Sodium-ion (Na-ion) batteries are currently being investigated as an attractive substitute for lithium-ion (Li-ion) batteries in large energy storage systems because of the more abundant and less expensive supply of Na than Li. However, the reversible capacity of Na-ions is limited because Na possesses a large ionic radius and has a higher standard electrode potential than that of Li, making it challenging to obtain electrode materials that are capable of storing large quantities of Na-ions. This study investigates the potential of CoFe₂O₄ synthesised via the molten salt method as an anode for Na-ion batteries. The obtained phase structure, morphology and charge and discharge properties of CoFe₂O₄ are thoroughly assessed. The synthesised CoFe₂O₄ has an octahedron morphology, with a particle size in the range of 1.1-3.6 μm and a crystallite size of ∼26 nm. Moreover, the CoFe₂O₄ (M800) electrodes can deliver a high discharge capacity of 839 mA h g⁻¹ in the first cycle at a current density of 0.1 A g⁻¹, reasonable cyclability of 98 mA h g⁻¹ after 100 cycles and coulombic efficiency of ∼99%. The improved electrochemical performances of CoFe₂O₄ can be due to Na-ion-pathway shortening, wherein the homogeneity and small size of CoFe₂O₄ particles may enhance the Na-ion transportation. Therefore, this simple synthetic approach using molten salt favours the Na-ion diffusion and electron transport to a great extent and maximises the utilisation of CoFe₂O₄ as a potential anode material for Na-ion batteries.