Magnetic and magnetostrictive properties of non-stoichiometric cobalt ferrite synthesized from spent Li-ion batteries

A series of Co1+xFe2-2x/3O4 (x = 0, 0.05, 0.1, 0.15, 0.2) samples, in which Fe3+ was progressively replaced by Co2+, were prepared through the sol-gel auto-combustion method using spent Li-ion batteries as raw materials. The structure, morphology, magnetic and magnetostrictive properties of the samples were studied. The purpose of the present work is to make cobalt ferrite more suitable for its application in non-contact sensors and actuators. X-ray diffraction (XRD) patterns show that all the samples have a pure single-phase cubic structure. The non-stoichiometric cobalt ferrite has different crystallite size and lattice constant compared with pure CoFe2O4. Field emission scanning electron microscopy (FE-SEM) was carried out to investigate the amount and size of pores in the sintered samples. The cationic oxidation state and cation distribution of the spinel structure was investigated by means of X-ray photoelectron spectroscopy (XPS). The variations in structure and morphology were found to lead to significant changes in the magnetic and magnetostrictive properties. With the increase of cobalt content, the saturation magnetization and the maximum magnetostrictive coefficient of the samples initially increase and then decrease. Given the different behaviors of the magnetostriction coefficient and strain sensitivity of the samples, their magnetostrictive performance was quantified through a "figure of merit" defined as the product between the two above mentioned parameters.