## 5 Electrochemistry of ZnFe2O4
### Doping  
As discussed, some critical challenges associated with ZFO anodes in LIBs are represented by the large irreversible capacity loss at the initial discharge-charge cycles and poor cycling performance due to the low electronic conductivity and large volume variation. The doping is well recognized as an easy way to improve the electronic conductivity and/or the crystal structure stability of ferrites [37; 38; 39; 40]. The dopant elements that can satisfy both these requests are obviously the transition metals ions. ZFO nanofibers doped with Mn ions [37] exhibited a good capacity of \(\sim\)612 mAh g\({}^{-1}\) at the 50 th cycle (at 60 mAh g\({}^{-1}\)) for the Zn\({}_{0.3}\)Mn\({}_{0.7}\)Fe\({}_{2}\)O\({}_{4}\) composition, the best result among the solid-solution series, through the beneficial conversion reaction and alloy --dealloy mechanism (Figure 7). The use of ex situ characterizations such as high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) allowed us to study the involvement of Mn in the battery performance. This paper offers interesting insights on how to flexibly adjust the electrochemical performance by tuning the transition metals' ratio within the spinel [37].  
An appropriate Mn doping and CNTs' intertwining actively affect the formation of uniform morphology and improve the cycling stability and rate capability [39]. Zn\({}_{0.5}\)Mn\({}_{0.5}\)Fe\({}_{2}\)O\({}_{4}\) @CNT composites exhibit excellent electrochemical performances, with enhanced reversible capacity (1375 mAh g\({}^{-1}\) after 100 cycles at the current density of 100 mA g\({}^{-1}\)) and good rate capability (933.5 mAh g\({}^{-1}\) at 500 mA g\({}^{-1}\), 810 mAh g\({}^{-1}\) at 1000 mA g\({}^{-1}\). and 634.2 mAh g\({}^{-1}\) at 1500 mA g\({}^{-1}\)).  
The co-doping is another successful strategy to improve the lithium storage properties of ZFO. ZnFe\({}_{2}\)O\({}_{4}\) and Mg\({}_{x}\)Cu\({}_{0.2}\)Zn\({}_{0.82-x}\)Fe\({}_{1.98}\)O\({}_{4}\) (x = 0.20, 0.25, 0.30, 0.35, and 0.40) nanoparticles were synthesized by the sol\(-\)gel-assisted combustion method. The as-synthesized samples showed large capacity fading, but the addition of a thermal treatment at 800 \({}^{\circ}\)C allowed us to obtain better cycling stability [40]. Undoped ZFO exhibited a high reversible capacity of 575 mAh g\({}^{-1}\) after 60 cycles at a current density of 100 mA g\({}^{-1}\). Mg\({}_{0.2}\)Cu\({}_{0.2}\)Zn\({}_{0.62}\)Fe\({}_{1.98}\)O\({}_{4}\) showed a similar capacity after 60 cycles but better capacity re tention. Therefore, though the Mg, Cu co-doped compounds exhibited lower theoretical capacity, the co-doping improved the reversible capacity and capacity retention (Figure 7), especially at higher current rates. The nickel ion, with high redox potentials, as a dopant has great potential; in fact, NiFe\({}_{2}\)O\({}_{4}\) is deemed as a promising anode material for LIBs. Ni\({}_{x}\)Zn\({}_{1\cdot x}\)Fe\({}_{2}\)O\({}_{4}\) (x = 0, 0.25, 0.5, 0.75, 1) compounds [38] were prepared by hydrothermal method and subsequent heat treatment. Ni\({}_{0.25}\)Zn\({}_{0.75}\)Fe\({}_{2}\)O\({}_{4}\) showed good electrochemical performance with a discharge capacity of 1488 mAh g\({}^{-1}\) in the first cycle and 856 mAh g\({}^{-1}\) after 100 cycles. Compared with ZFO, nickel-doped samples, except NiFe\({}_{2}\)O\({}_{4}\), increased to some extent the specific discharge capacity. This improvement was attributed to good crystallinity, low resistance, and small particle sizes that could shorten the diffusion length of the lithium ion, accommodate volume expansion, and lead to better electrochemical performance.  
Figure 7: (**a**) FESEM images of as-spun Zn\({}_{1\cdot x}\)Mn\({}_{x}\)Fe\({}_{2}\)O\({}_{4}\) with various manganese content nanofibers [37]; (**b**) first galvanostatic charge—discharge cycle of ZFO and Mg\({}_{x}\)Cu\({}_{0.2}\)Zn\({}_{0.82\cdot x}\)Fe\({}_{1.98}\)O\({}_{4}\) (x = 0.20, 0.25, 0.30, 0.35, and 0.40) calcined at 600 \({}^{\circ}\)C [40].