## 1 Introduction  
The development of advanced materials with tailorable properties is a cornerstone of materials science and new technologies. Very recently, high-entropy materials (HEMs), a new class of materials obtained by using a configurational entropy-based design approach inherited from that of high-entropy alloys, have undergone an explosion of interest thanks to their unique structure and their novel and often unexpected functional properties [1, 2, 3, 4]. HEMs are single-phase solid solutions stabilized in a multi-component system (generally, five or more elements in equimolar or near-equimolar ratio) by high configurational entropy. Numerous high-entropy compounds have been successfully synthesized and reported,including nitrides [5; 6], carbides [7; 8] diborides [9], sulfides [10] and oxides [3; 11; 12; 13; 14]. In particular, starting with pioneering work by [PERSON] et al. [11] published in 2015, high-entropy oxides (HEOs) have attracted growing interest in both academic research and industry for their potential to obtain intriguing multifunctional properties (such as thermal, electrochemical, dielectric, magnetic, catalytic and mechanical) resulting from high-entropy, lattice distortion, sluggish diffusion and cocktail effects, by exploiting a great number of possible element combinations [12; 15; 16; 17]. The pioneering system was a single phase (Mg\({}_{0.2}\)Co\({}_{0.2}\)Ni\({}_{0.2}\)Cu\({}_{0.2}\)Zn\({}_{0.2}\)O) solid solution with the rock-salt structure (\(Fm\)-\(3m\) symmetry), synthesized from five binary oxides (MgO, CoO, NiO, CuO, and ZnO) in equimolar proportions via a solid-state reaction route in the 850-900 \({}^{\circ}\)C temperature range [11]. Lower temperatures lead to a multiphase structure, rock-salt and tenorite (CuO), confirming that the increased configurational entropy was responsible for the thermodynamic stability of the rock-salt structure due the random distribution of cations on the specific sublattice sites. So far, various HEOs with different crystal structures, such as spinel type [13; 14; 18], fluorite type [19; 20], layered O\({}_{3}\)-type [21], pyrochlore type [22] and perovskite type [23; 24; 25] structures have been synthesized using different synthetic routes. As previously mentioned, compared to conventional binary or doped transition metal oxide systems, HEOs exhibit amazing functional properties that can be finely tuned by varying their composition, such as \"colossal dielectric constant\" [26], very narrow band gap [27], high room temperature Li-ion conductivity [28], high temperature stability [29] and superior catalytic performance [30], making them potentially suitable for a wide range of applications, including energy storage, large-k dielectric materials, water splitting, catalysts, thermal protection and insulation [2; 3]. For example, works from [PERSON] [31] and [PERSON] [16] reported a higher reversible capacity and excellent cycling performance of the first rock-salt structured (Mg\({}_{0.2}\)Co\({}_{0.2}\)Ni\({}_{0.2}\)Cu\({}_{0.2}\)Zn\({}_{0.2}\))O HEO with respect to individual metal oxides, when it was used as anode materials in lithium-ion batteries (LIBs). The good electrochemical performance is ascribed to high configuration entropy that preserves the stability of the original crystal structure of the HEO during the charge/discharge processes. (Ni\({}_{0.2}\)Co\({}_{0.2}\)Mn\({}_{0.2}\)Fe\({}_{0.2}\)Ti\({}_{0.2}\))\({}_{3}\)O\({}_{4}\) with a spinel-structure, synthesized via solid-state sintering method by [PERSON] et al. [32], as an anode material for LIBs, delivered a high capacity (\(\sim\)-560 mA\({}^{-1}\)) at a current density of 100 mA\({}^{-1}\) and exhibited an excellent capacity retention of 100% after 100 cycles, making it interesting for energy storage applications with high capacity and stability. HEOs are most frequently prepared through the solid-state reaction.