## 3 Results
### Electrochemical Properties  
The superior lithium-storage properties of rock-salt (Mg,Co,Ni,Cu,Zn) HEO, produced by solid-state reaction from an equimolar mixture of MgO, CoO, NiO, CuO and ZnO [11], as anode material have been pointed out by many groups [16; 31]. For example, it was shown that such an HEO can deliver a reversible capacity of 920 mAh g\({}^{-1}\) after 300 cycles at 100 mA g\({}^{-1}\) and an excellent rate capability (600 mAh g\({}^{-1}\) at 2 A g\({}^{-1}\)) [16]. Furthermore, electrospun (Mg\({}_{0.2}\)Co\({}_{0.2}\)Ni\({}_{0.2}\)Cu\({}_{0.2}\)Zn\({}_{0.2}\))O NFs exhibit excellent cycling stability (390 mAh g\({}^{-1}\) after 300 cycles at 500 mA g\({}^{-1}\)) [36]. HEOs with spinel structures are particularly interesting because the unoccupied crystallographic sites should allow for the storing of additional Li ions, while the crystal structure should permit their fast diffusion. They have been the focus of in-depth studies [45; 47; 51]. (Cr,Mn,Fe,Co,Ni)\({}_{3}\)O\({}_{4}\) was the first to be reported among them [47]. (Cr,Mn,Fe,Co,Ni) HESO prepared by solid-state reaction delivers specific capacities of 735, 586, 478, 361, 269 and 180 mAh g\({}^{-1}\) at 0.05, 0.1, 0.2, 0.5, 1 and 2 A g\({}^{-1}\), respectively [45]. (Cr,Mn,Fe,Ni,Zn) HESO prepared by the same method retains a capacity of 387 mAh g\({}^{-1}\) after 185 cycles at 0.5 A g\({}^{-1}\)[70]. Here, two representative HESOs with the same nominal composition, (Cr\({}_{0.2}\)Mn\({}_{0.2}\)Fe\({}_{0.2}\)-Co\({}_{0.2}\)Zn\({}_{0.2}\))\({}_{3}\)O\({}_{4}\), are considered: SMF-sg, produced by SG route and; 5 MF-b, prepared by ES using PAN as a polymer. Their electrochemical behavior as active anode materials in LIBs is preliminarily evaluated by carrying out measurements in the potential window of 0.01-3.0 V (versus Li/Li\({}^{+}\)). Figure 5 displays the results obtained. Figure 5: Electrochemical performance of HESOs (**a**,**c**,**c**) SMF-b and (**b**,**d**,**f**) SMF-sg in Li-ion half cells. (**a**,**b**) CV curves (scanning rate of 0.1 mV s\({}^{-1}\)), (**c**,**d**) galvanostatic charge–discharge curves measured at 20 mA g\({}^{-1}\) and (**e**,**f**) galvanostatic charge–discharge curves measured at different current density. (**g**) Cycling stability at a current density of 500 mA g\({}^{-1}\). Figure 5a,b shows the cyclic voltammetry (CV) curves of the electrodes based on (Cr,Mn,Fe,Co,Zn) HESO NPs (5 MF-sg) and (Cr,Mn,Fe,Co,Zn) HESO NPs (5 MF-b) in the initial five cycles at a scan rate of 0.1 mV s\({}^{-1}\). These electrode materials display similar CV patterns, suggesting similar electrochemical processes of the two HESOs. The multiple anodic and cathodic peaks are associated with the Li\({}^{+}\) insertion-deinsertion process. In the first cycle, there is a strong cathodic peak located at around 0.4 V; this can be ascribed to the formation of solid electrolyte interphase (SEI) due to the decomposition of electrolyte [32]. The weaker cathodic peak at higher potential arises from the reduction of the HESO cations [71]. In the following anodic scan, the oxidation peaks may refer to the re-oxidation of metal elements into oxides. In the second cycle, the cathodic peak shifts to a higher potential and its intensity is significantly reduced, which might be caused by the irreversible reaction of the first lithiation process and pulverization of the electrode materials [16; 72]. In the subsequent cycles, both the reduction and oxidation peaks are not overlapped very well, suggesting that the HESO anodes do not exhibit good stability and reversibility for the Li\({}^{+}\) insertion and extraction reactions. To evaluate the capacity of the considered HESOs, galvanostatic charge-discharge tests were carried out at a current density of 20 mA g\({}^{-1}\) (Figure 5c,d). In the first lithiation process, there is a clear potential plateau at around 0.45 V, corresponding to the reduction reaction of transition metals and the formation of the SEI layer, which is consistent with CV curves.