## 5 Electrochemistry of ZnFe2O4
### Composites  
The strategy to synthesize hybrid nanomaterials with conductive carbonaceous sources (e.g., porous carbon, carbon nanotubes, nanofibers, graphene/graphite, etc.) is particularly promising to overcome the main limitation of ferrites for electrochemical application, i.e., the poor electronic conductivity. Carbonaceous materials possess good electronic conductivity and high-rate capability and can preserve the structural integrity of active materials, thereby giving enhanced cyclability and rate capacity of the resulting hybrid composites. A variety of modifications of ZFO with carbonaceous materials have been exploited: surface carbon coating [31], graphene as substrate with active nanomaterials decorating [102; 103], growing, or anchoring on surfaces [25; 55], embedding in carbon matrix [34] or graphene nanosheets-wrapped active material particles [32] until the use of sacrificial templates [104; 105; 106; 107; 108; 109] (Figure 8). Ultrafine nanoparticles with average diameters of about 5 nm homogeneously distributed within a 3D, interconnected, porous carbon framework in the walls of the hollow octahedra work very well (Figure 6). This elaborate nanoarchitecture combines the superior properties of a hollow interior structure, ultrafine building blocks, a conductive elastic buffering framework, and high porosity. High reversible capacities of 1060 mAh g\({}^{-1}\) at 0.5 C, 934 mAh g\({}^{-1}\) at 1 C, 887 mAh g\({}^{-1}\) at 2 C, and 842 mAh g\({}^{-1}\) at 5 C could be achieved, demonstrating a magnificent, high-rate performance of the hybrid electrode. Moreover, a capacity of more than 1200 mAh g\({}^{-1}\) was recovered at 0.5 C [74]. Mesoporous ZnFe\({}_{2}\)O\({}_{4}\) spheres wrapped in graphene sheets provide good performances [104]. These spheres are composed of numerous nanoparticles where the void spaces between the neighboring nanoparticles provide a cushion to alleviate volume variation during the conversion/alloying process as well as large contact areas with the electrolyte to facilitate lithium-ion diffusion and electron transport during cycling. A high specific capacity of 1182 mAh g\({}^{-1}\) at 0.1 C and significantly enhanced rate capability and cycling stability after long-term testing were obtained [104]. [PERSON] et al. [31] prepared well-dispersed ZFO nanoparticles anchored on an acetylene black substrate, further covered and interlinked by amorphous carbon layers, resulting in self-assembly into large hierarchical porous granules. The presence of conductive carbon enables better electron transfer and prevents the aggregation of the ZFO nanoparticles, buffering the large volume change of the active material. The reversible discharge capacity of the AB/ZnFe\({}_{2}\)O\({}_{4}\)-NPs@C decreased from 731 to 229 mAh g\({}^{-1}\) as the current density increased from 0.1 to 2.0 C; but it is worth noting that a reversible discharge capacity of 375 mAh g\({}^{-1}\) after cycling at 1 C was provided, still higher than the theoretical capacity of graphite. Importantly, when the current density returned to 0.1 A g\({}^{-1}\), a discharge capacity of 470 mAh g\({}^{-1}\) was recovered, further increasing to 830 mAh g\({}^{-1}\) after another 100 cycles [31]. ZnFe\({}_{2}\)O\({}_{4}\) nanorods [36], coated with carbon (by self-polymerization of dopamine) on the surfaces followed by carbonization, have better rate capability and cycling stability. When discharged/charged at 0.5 and 2 Ag\({}^{-1}\), the ZFO/carbon nanorod anode can deliver reversible capacities of 805 and 504 mAh g\({}^{-1}\), respectively. Besides, it shows excellent cycling stability and no obvious capacity fading at the current density of 1 Ag\({}^{-1}\) for 100 cycles [36]. The obtained unique morphologies can provide materials with high reversible capacity, enhanced rate performance, and superior cycling stability due to the synergetic effect of carbon matrix and carbon coating (see Table 1).