## 1 Introduction  
The term ferrite indicates a broad class of ceramic oxide materials, joined by the presence of iron ions and largely applied during the last five decades [1]. They are well-known and -studied materials with an impressively wide range of applications (Figure 1) extending from millimeter wave integrated circuitry to power handling, simple permanent magnets and magnetic recording [2], catalysis [3], sensors [4], energy [5], and in nanomedicine from magnetic hyperthermia to drug delivery [6] and imaging in magnetic resonance [7] up to electrochemistry (for rechargeable ion batteries) [8].  
A unique combination of properties makes ferrites so versatile: significant saturation magnetization, high electrical resistivity, low electrical losses, and very good chemical and structural stabilities [1]. Ferrites are commonly broadly divided into three groups based on the crystal structure: cubic spinels with AB\({}_{2}\)O\({}_{4}\) stoichiometry, garnets, and hexa-ferrites. The key factor for tailoring their properties for so many different applications is the virtually unlimited number of solid solutions that can be obtained. In addition, the possibility to produce ferrites in the form of nanoparticles has opened a new and exciting research field, with revolutionary applications not only in electronic technology but also in the field of biotechnology. The crystal structures of garnets and hexa-ferrites are relatively complex. The first ones have a cubic structure and derive from the garnet mineral, Mn\({}_{3}\)Al\({}_{2}\)Si\({}_{3}\)O\({}_{12}\). The magnetic garnets include Fe\({}^{3+}\) instead of Al and Si and a rare earth cation (R) substitute Mnion, to give the general formula R\({}_{3}\)Fe\({}_{5}\)O\({}_{12}\) for ferromagnetic garnets [9]. Hexa-ferrites are a family of synthetic compounds with hexagonal or rhombohedral symmetry, also forming extended solid solutions. For example, barium hexa-ferrite (BaFe\({}_{12}\)O\({}_{19}\)) possesses the same crystal structure of the natural mineral magneto-plumbite. The magnetic structure of hexa-ferrites is very complex due to the crystal structure complexity: their most interesting feature is the high coercivity [10].  
However, cubic spinels are undoubtedly the most known and exploited ferrites, so that worldwide the term ferrite is synonymous for the AB\({}_{2}\)O\({}_{4}\) compounds, that will be the subject of this review.  
Firstly, a brief description of the crystal structure of spinels and their main application fields will be provided. Then, we will focus on the zinc ferrite ZnFe\({}_{2}\)O\({}_{4}\) (ZFO) that, in these last years, has aroused a renewed interest in very different fields, especially nanomedicine, with an intriguing use in magnetic hyperthermia and electrochemistry, as anode material for lithium-ion batteries (LIBs). In particular, the application in LIBs will be deepened: the most promising synthesis methods for producing pure or composite/hybrid-based ZnFe\({}_{2}\)O\({}_{4}\) nanomaterials, with structural and morphological properties particularly suitable for the electrochemical long-term cycling, will be analyzed and thoroughly discussed. The main aim of the review is filling up a lack of inclusive papers on the electrochemical properties of ZnFe\({}_{2}\)O\({}_{4}\) by summarizing the most promising electrochemical findings of the state of the art of the last years.