## 5 Electrochemistry of ZnFe2O4
### Peculiar Morphologies  
The improvement of the electrochemical performances of ferrites starts from the search for particular nanoparticles' morphologies, as well as the formation of hybrid nanocomposites with peculiar carbon sources. Many different and rumpy morphologies, producing interesting electrochemical performances, have been proposed [92; 93; 94; 95; 96; 97; 98; 99]. The 3D, cuboid-structured ZnFe\({}_{2}\)O\({}_{4}\)@C nano-whiskers [85], highly mesoporous carbon nanofibers in free-standing mat form [92], carbon-decorated ZnFe\({}_{2}\)O\({}_{4}\) nanowires [86], and 1D ZnFe\({}_{2}\)O\({}_{4}\) hollow fibers with integrated tubular mesoporous nanostructures [70] are hybrid materials that combine the advantages of nanostructures with the carbon peculiarities.  
Examples of peculiar ZFO morphologies with the corresponding cell performances are reported in Figure 6.  
Porous nanorod-like nanostructures (many tiny nanoparticles with smooth surfaces) of the as-prepared ZFO materials guarantee 1458 mAh g\({}^{-1}\) after 120 cycles and an excellent rate capability (778 mAh g\({}^{-1}\) at 3 Ag\({}^{-1}\)). More importantly, a discharge capacity of 456 mAh g\({}^{-1}\) was maintained at a current density of about 5 C after 200 cycles [95].  
ZnFe\({}_{2}\)O\({}_{4}\) nanoneedles, uniformly dispersed on the surface of the carbonized poly-dopamine porous nanofiber, can align perpendicularly to the nanofibers' surface, whose mesoporous nature may restrict the size of the nucleation sites. The nanoneedle-like mor  
Figure 6: TEM image, voltage profiles, and rate performance of ZFO hollow microspheres [52] and porous hollow octahedral ZnO/ZFO/C nanoparticles [74], reproduced with permission, [PERSON] 2014.  
phology can favour the contact between the nanoneedles themselves and the conductive matrix, with the easy diffusion of electrolyte and lithium ions in the nanochannels leading to excellent cyclability and rate capacity. This peculiar structure allowed obtaining normalized capacities between 1000 and 1700 mAh g\({}^{-1}\) at 0.1 C and 560 mAh g\({}^{-1}\) at 5 C, respectively [92].  
A particularly promising morphology is represented by the thin triple-shelled ZFO hollow microspheres with a complex structure formed by the outer shell, void, inner shell, and inner hollow core along the radial direction from the surface (Figure 6). The thin shell offers a highly specific surface area with a lot of electrochemical active sites and short diffusion lengths. The wide voids between the shells lead to an easy mass transfer by storing abundant electrolyte and protecting against the damage of microspheres by providing large spaces, hindering a severe volume change during lithiation/delithiation. As a result, this peculiar architecture offers a high reversible capacity of 932 mAh g\({}^{-1}\) at 2 C even at the 200 th cycle without obvious decay. Furthermore, it delivers 1235, 1005, 865, 834, and 845 mAh g\({}^{-1}\) at current densities of 0.5, 2, 5, 10, and 20 C after 100 cycles (Figure 6) [52].  
The use of metal-organic framework MOFs as electrode materials for LIBs is growing, due to their inimitable structure, tunable morphology, well-developed porosity, high specific surface area, and excellent lithium storage capacity. The open crystal frameworks of MOFs can deliver efficient and reversible ionic insertion/extraction [100]. Prussian Blue (a kind of metal-organic framework) can react with Zn acetate under microwave irradiation, forming hydroxides' and carbonates' nanosheets on the surface of Prussian Blue itself. After pyrolysis, a composite of ZnO/ZnFe\({}_{2}\)O\({}_{4}\) porous nanoparticles was obtained [101], showing a good capacity of 804 mAh g\({}^{-1}\) for 500 cycles at a current density of 1 C.  
The obvious advantages shown by the nanocomposites with various architectures are, thus, the high capacities at high C rates and prolonged cycle life.