## 1 Introduction
This method requires long heat treatments (typically 12-24 h [16; 33; 34; 35]) at high temperatures (1000-1300 \({}^{\circ}\)C [16; 33; 34; 35]) to crystallize the material, previously homogenized by grinding process(es) [34; 35]) to obtain a random distribution of the cations in the HEO lattice and prevent the formation of secondary phase(s). These preparation conditions usually involve heavy sintering effects and particle agglomeration. Very recently, some authors have demonstrated that HEOs can be produced via simpler methods under milder conditions (shorter heat treatments at lower temperatures) than solid-state [36; 37; 38; 39], thus reducing not only the effects of sintering on the size of the oxide particles, but also the environmental impact of the material production. [PERSON] et al. [36] have produced pure single-phase _rock-salt_ (Mg\({}_{0.2}\)Co\({}_{0.2}\)Ni\({}_{0.2}\)Cu\({}_{0.2}\)Zn\({}_{0.2}\))O via the solvothermal method and electrospinning. [PERSON] et al. [37] have synthesized single-phase _perovskite_ Ba(Ti\({}_{0.2}\)Zr\({}_{0.2}\)Sn\({}_{0.2}\)Hf\({}_{0.2}\)Ce\({}_{0.2}\))O\({}_{3}\) via the sol-gel method. Mesoporous _spinel_ (Ni\({}_{0.2}\)Co\({}_{0.2}\)Cr\({}_{0.2}\)Fe\({}_{0.2}\)Mn\({}_{0.2}\))\({}_{3}\)O\({}_{4}\) nanospheres have also been obtained using the same method [40]. The preparation of single-phase _fluorite_ (La\({}_{0.2}\)Nd\({}_{0.2}\)Sm\({}_{0.2}\)Dy\({}_{0.2}\)Yb\({}_{0.2}\))\({}_{2}\)Zr\({}_{2}\)O\({}_{7}\)[38] and bixbyite (Y\({}_{0.2}\)Yb\({}_{0.2}\)Sm\({}_{0.2}\)Eu\({}_{0.2}\)Er\({}_{0.2}\))\({}_{2}\)O\({}_{3}\)[39] electrospun nanofibers has been also reported. This work demonstrates that a large set of high-entropy oxides with _spinel_ structure (HESOs) can be obtained using the sol-gel method and, for the first time, electrospinning. Being solution-based, both synthesis techniques allow for the easy preparation of homogeneous multi-component oxides. Besides, they are suitable for large-scale material manufacturing since they are scalable and consist of two sequential steps (namely, sol-gelpreparation/fibrous film deposition and heat treatment) that, in an industrial production chain, can progress simultaneously. In syntheses via electrospinning, a technique particularly suitable for the manufacturing of fibrous nanostructured materials with high aspect-ratio and surface-area [41], two polymers (polyvinylpyrrolidone and polyacrylamide-triple) are used to investigate the influence of the polymer on the formation of fibers and on their microstructure. Ten different equimolar combinations are considered, each containing five metals selected from Mg, Ti, Cr, Mn, Fe, Co, Ni, Cu and Zn. Their choice is operated by selecting cations with a similar ionic radius and a good chemical compatibility. The produced HESOs are analyzed by complementary techniques to investigate how the synthesis method and conditions influence their structure, morphology and crystalline phase. Since the results of their preliminary evaluation as active anode materials in LIBs indicate that the materials need to be improved by optimizing their preparation conditions, herein, two strategies are proposed to enhance the rate capability and cyclability of two representative samples. The first strategy is to add lithium into the HESO lattice, as the multi-valence nature of the HESO constituent elements promotes the formation of oxygen vacancies, known to favor Li\({}^{+}\) transport [32]. The second (conventional) strategy is to embed the HESO particles in a carbonaceous matrix [42; 43; 44]. Both the strategies are successfully implemented.