## 5 Electrochemistry of ZnFe2O4
### Full Cell Based on ZFO Anode  
The papers above dealt with semi-cell configuration, in which the ZFO anode performances were tested towards Li metal. Clearly, to hypothesize the future commercialization of batteries based on ZFO, it is mandatory to study full cells. Papers about this topic are limited: In these cases, the ZFO anode was coupled with LiFePO\({}_{4}\) cathode, a well-known material, commercially available. A carbon-coated, ZnFe\({}_{2}\)O\({}_{4}\) nanoparticle-based spinel was interfaced in a complete cell with a LiFePO\({}_{4}\) multiwalled carbon nanotube-based cathode, both aqueously processed with Na-CMC, obtaining exceptional electrochemical performance [126] (Figure 10). Such a battery showed remarkable rate capabilities, delivering about 50% of its nominal capacity at currents corresponding to about 20 C. The pre-lithiation of the negative electrode, a diffuse strategy offering the possibility of tuning the cell potential, allowed obtaining values of 202 Wh kg\({}^{-1}\) and 3.72 Wkg\({}^{-1}\) of gravimetric energy and power density, respectively, in addition to granting a lithium reservoir. The high reversibility of the system enabled sustaining more than 10,000 cycles at about 10 C with respect to LiFePO\({}_{4}\) cathode, while retaining up to 85% of its initial capacity.  
Figure 9: In situ XRPD measurements of ZnO/ZnFe\({}_{2}\)O\({}_{4}\)/N-doped C micro-polyhedrons: (**a**,**b**) potential profile of the initial (de)lithiation, (**c**) in situ XRPD patterns upon discharge/charge (scans 1–130), (**d**–**k**) details on the evolution of the XRPD patterns [124]. Reproduced with permission, Copyright Elsevier 2017.  
In another study [127], a ZnFe\({}_{2}\)O\({}_{4}\)/G nanohybrid was synthesized by a facile in situ route, showing better cycling stability and rate capability than bare ZrO. At a charge current density of 800 mA g\({}^{-1}\), the hybrid still delivered a first-charge capacity of 701 mAh g\({}^{-1}\), which was maintained at 464 mAh g\({}^{-1}\) after 300 cycles. The excellent high-rate cycling stability was attributed to the three combined effects of graphene: conducting, confining, and dispersing. In addition, the unique 2D sheet-like nanostructure maximized the exposure of the active material to the electrolyte and buffered the volume changes. In the ZnFe\({}_{2}\)O\({}_{4}\)/G-LiFePO\({}_{4}\)/C full cell, ZnFe\({}_{2}\)O\({}_{4}\)/G can yield a high initial discharge capacity of 805 mAh g\({}^{-1}\) at a current density of 100 mA g\({}^{-1}\).  
In general, the evaluation of the performances of a full cell was based on the analysis of specific gravimetric and/or volumetric capacities, while other key parameters were often neglected. Notwithstanding the tempting specific energy (Whkg\({}^{-1}\)) of ZrO in full batteries, other parameters such as the Coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) have to be considered for practical applications in large-sized batteries. In a detailed study [128], ZrO, with its large voltage hysteresis, exhibited lower VE and EE than other anode materials, for instance, graphite, and currently does not seem to be an alarming competitor for the commercialized anodes. However, all these electrochemical parameters particularly depend on electrode characteristics and operating conditions. Therefore, they can be carefully optimized to improve the performances.