Physico-Chemical and Electrochemical Features of Nanometric ZnFe₂O₄, Anode Material for Libs

Ferrites are a broad class of ceramic oxides, possessing intriguing physico-chemical properties, mainly due to their unique structural features, that, during the last 50–60 years, made them the materials of choice for many different applications. They are, indeed, applied as inductors, high-frequency materials, for electric field suppression, as catalysts and sensors, in nanomedicine for magneto-fluid hyperthermia and magnetic resonance imaging, and, more recently, in electrochemistry. Ferrites are commonly broadly divided into three groups based on the crystal structure: garnets, hexaferrites and cubic spinels with AB2O4 stoichiometry, that are undoubtedly the most known and exploited ferrites, so that worldwide the term ferrite is synonymous for the AB2O4 compounds. In particular, ZnFe2O4 (ZFO) and its solid solutions attracted the researchers’ attention for the application as anode materials in lithium-ion batteries (LIBs). ZFO is a normal cubic spinel, with Zn2+ ions on tetrahedral sites and Fe3+ on octahedral ones, with an anti-ferromagnetic character. The reasons of the interest for electrochemical applications can be found in the low cost, abundance, and environmental friendliness of both Zn and Fe precursors, high surface-to-volume ratio, relatively short path for Li-ion diffusion, low working voltage of about 1.5 V for lithium extraction, and high theoretical specific capacity (1072 mAh g-1). However, some drawbacks are represented by fast capacity fading and poor rate capability, resulting from a low electronic conductivity, severe agglomeration, and large volume change during lithiation/delithiation processes. ZnFe2O4, unlike other oxides, has a lithium insertion mechanism that involves both conversion and alloying reactions. After the conversion reaction of ZFO with lithium ions and the formation of metallic Zn, Fe, and Li2O, the resulting Zn can further react with lithium to form a LixZn alloy, thus contributing additional capacity. ZFO experiences first-cycle irreversibility and fast decay in capacity with cycling, mainly resulting from poor electrical conductivity and large volumetric changes associated with the conversion reaction. However, thanks to the downsizing of the particles, the addition of proper carbon sources, and to peculiar morphologies, the long-term cycling and the capacity values of ZnFe2O4 are, nowadays, very appealing. The peculiar ferrite properties are mainly related to the cation distribution (affected by the eventual substitutions of other elements onto tetrahedral and/or octahedral sites) and nano-dimensions. In particular, this is true for magnetic, optical, and electrical properties but also for the electrochemical ones. The determination of sample purity is also mandatory, because even small amount of iron oxides such as hematite Fe2O3 or magnetite Fe3O4, easily stabilized during the synthesis processes, heavily influence the intrinsic ZFO properties. To determine all the cited characteristics, the combined use of diffraction, morphological and spectroscopic techniques is required for the whole sample characterization. In this work, ZnFe2O4 was synthesized by two different methods, co-precipitation route and by using a template synthesis from MOF, to obtain samples with different crystallite sizes. Then, the obtained samples were thoroughly characterized: structural, morphological and vibrational properties were detected by combining X-ray powder diffraction (XRPD) with Rietveld structural refinement, SEM analysis with EDS and micro-Raman spectroscopy. In this way, the samples’ purity, the lattice parameters and crystallite sizes, the particles’ morphology and the eventual spinel inversion degree were determined. Furthermore, Mössbauer and EPR spectroscopies were carried out to verify the possible presence of impurity phases, particularly iron oxides, hard to be determined by XRPD only. The electrochemical properties were measured by using galvanostatic cycling at different C-rates and cyclic voltammetry. Then, in-situ diffraction measurements were applied on electrochemical cells and the results were discussed on the base of the physico-chemical characterizations.