## 5 Electrochemistry of ZnFe2O4
### Studies on SEI and Mechanism of Conversion/Allowing  
The processes involved in lithium intercalation/deintercalation of ZFO are complex and their understanding requires different but combined experimental techniques [117, 118, 119, 120, 121, 122, 123, 124]. A crucial part for the cell functioning is related to the solid electrolyte interface (SEI), formed during the first lithium uptake, usually considered as a passivation layer preventing the degradation of the electrodes while not participating in the lithium storage process. The investigation of SEI was fundamental for lithium metal batteries, allowing us to understand that the SEI film is the major reason for the failure of the lithium metal anode because it is unstable against the corrosive electrolyte owing to the formation of unstable SEI; but without SEI, the battery operation would not be feasible [125].  
The formation of the SEI layer takes place during the first steps of the charging process, and its thickness reaches about 40 nm at about 1/3 of the full capacity, with a stable total thickness up to 20 working cycles, as demonstrated by XAS studies [117, 118]. The structural analysis of the distinct components forming the SEI gave detailed information on the SEI evolution at different charge\(-\)discharge stages [119]. The experimental results showed that the electrode reduction process, resulting in the formation of Li\({}_{2}\)O in proximity of the electrode active material and the formation of the passivation layer on the electrode core, was accompanied by the formation of a partly reversible layer as the uppermost SEI layer (typical thickness of about 5-7 nm). This mostly consisted of alkyl lithium carbonate (ROCO\({}_{2}\)Li), being partly replaced by Li\({}_{2}\)CO\({}_{3}\) during delithiation. This layer acts as a lithium reservoir and seems to give a significant contribution to the extra capacity of the electrodes. This result further extends the role of the SEI layer in the functionality of Li-ion batteries.  
The use of sophisticated techniques, such as X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) [120], impedance spectroscopy [121], in situ TEM [122], in situ X-ray diffraction data, synchrotron-based powder diffraction, and ex situ extended X-ray absorption fine structure [123, 124], allowed us to prove the distinctive feature of the ZFO anode. The lithiation of ZFO nanoparticles was demonstrated to be a multistep process involving the presence of intermediate Li-Zn-Fe-O  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Material** & **Synthesis** & **Anode Composition/Electrolyte** & **First Discharge Capacity (mAh g\({}^{-1}\)/C** & **Capacity at the** **(mAh g\({}^{-1}\)/C** & **Long Cycling** **(mAh g\({}^{-1}\)/C** & **Reference** **(mAh g\({}^{-1}\)/C** & **Rate/N Cycles)** & **Reference** \\ \hline
3D cuboid-structured ZFO@C nano-whiskers) & In situ graft polymerization-calcination & Ac:SP:PVdF 80:10:10 1  
phases. In fact, ZFO, during the first discharge, formed a mixture of metallic iron, metallic zinc, and Li\({}_{2}\)O. Instead of the original ZFO spinel, the metallic iron and zinc particles were re-oxidized to Fe\({}_{2}\)O\({}_{3}\) and ZnO during the subsequent de-lithiation (see Equations (1)-(7)). All the cited studies [120; 121; 122; 123; 124] agree with this mechanism (Figure 9).