Synthesis of ZnFe2O4 spinel nanoparticles at varying pH values and application in anode material for lithium-ion battery

Spinel materials have attracted increasing attention as anode materials for lithium-ion batteries (LIBs) owing to their unique structures. ZnFe2O4 spinel was synthesized by a co-precipitation method at different pH values - pH 11, pH 12, and pH 13 - followed by annealing in air at 750 degrees C for 3 h. The pH of the medium significantly influences the properties of ZnFe2O4. At pH 11, ZnFe2O4 particles of sizes 25-70 nm were covered by amorphous layers with thicknesses of approximately 2 nm, whereas higher degrees of crystallinity and larger particle sizes were obtained at higher pH values (12 and 13). Additionally, at pH 12, the Zn:Fe molar ratio in the prepared sample was 1:1.3, which deviated significantly from the nominal formula (1:2). Possibly, owing to the presence of amorphous layers that allow an isotropic volume change during the lithiation/delithiation steps, ZnFe2O4 prepared at pH 11 exhibited the best performance when employed as an anode material in LIBs. Namely, its reversible specific capacity at the 1st and the 160th cycles at 0.1 A g-1 were 0.69 Ah g-1 and 0.72 Ah g-1, respectively. Although ZnFe2O4 prepared at pH 12 gave an initial reversible specific capacity up to 0.89 Ah g-1, its capacity retention after 160 cycles was only 20%. Therefore, adjusting the pH of the coprecipitation step can improve the final lithium storage performance by controlling the degree of crystallinity, particle size, and chemical composition of the anode material ZnFe2O4.