## 3 Results and Discussion
### Electrochemical Behaviour
The above reaction (2) takes place due to that Zn\({}^{2+}\) (aq) provides the hydrogen ions needed as follows:  
\[[Zn(H_{2}O)_{\mathrm{x}}]^{2+}\to[Zn(H_{2}O)_{\mathrm{x-1}}(OH)]^{+}+H^{+}(aq) \tag{4}\]  
At the same time, the precipitation of zinc hydroxide sulphate (ZHS) occurs. \[4 Zn^{2+}(aq)+6 OH^{-}(aq)+SO_{4}{}^{2-}(aq)+5H_{2}O(l)\to Zn_{4}(OH)_{6}(SO_{4 }).5H_{2}O(s) \tag{5}\]  
Figure 6: Cyclic voltammetry curves for KMn8, KMn4, and HKMn. XRD data was collected for KMn4 electrode after three cycles, and the XRD profiles of the sample discharged and charged are shown in Figure 8. In the XRD diagram corresponding to the discharged electrode, the peaks located at 12.3\({}^{\circ}\) and 24.8\({}^{\circ}\)(20) are characteristic of birnessite (PDF 1-080-1098). The other maxima that appear in the profile could be assigned to Zn\({}_{4}\)(OH)\({}_{6}\)(SO\({}_{4}\))\(\cdot\)5H\({}_{2}\)O (PDF-044-0673) phase (ZHS). However, at 20 = 36.4\({}^{\circ}\) a new maximum appears, which corresponds to the most intense maximum of the ZnMn\({}_{2}\)O\({}_{4}\) spinel. Therefore, the second plateau at 1.3 V can be related to the reaction between MnO\({}_{2}\) and Zn\({}^{2+}\) giving rise to the spinel-type compound:  
\[2\text{MnO}_{2}+\text{Zn}^{2+}+2\text{e}^{-}\rightarrow\text{ZnMn}_{2}\text{O }_{4} \tag{6}\]  
The XRD pattern of KMn4 electrode charged presents practically the same peaks as those observed in the original diffraction profiles. A slight difference can only be appreciated, the maximum at 20 = 36.4\({}^{\circ}\) is slightly more intense, because of ZnMn\({}_{2}\)O\({}_{4}\) phase which was obtained in the discharge process. Therefore, the second plateau at 1.3 V is only partially reversible. As the number of cycles increases, the amount of spinel phase increases, and this could explain the loss of capacity. Figure 7: (**a**) Charge–discharge profiles at 300 mA g\({}^{-1}\) and (**b**) Specific capacity and efficiency for KMn8, KMn4 and HKMn during 50 cycles. Therefore, the global reaction can be formulated as follows:  
Figure 7.0  
Figure 7.1  
Figure 7.2  
On the contrary, in the charge process the reverse reaction takes place, i.e., the Mn\({}^{2+}\) is oxidized to MnOOH, which in acid environment disproportionates leading to MnO\({}_{2}\) bimessite:  
Figure 7.3  
Figure 7.4  
Figure 7.5  
Figure 7.6  
Taking into account the mechanism indicated above, some conclusions could be established. First, charge-discharge process leads to the ZnMn\({}_{2}\)O\({}_{4}\) spinel, which is created in the discharge process, but it is not reversibly transformed to MnO\({}_{2}\) in the charge process. Thus, ZnMn\({}_{2}\)O\({}_{4}\) seems to be present in charge stage (Figure 8). Therefore, the irreversibility obtained in aqueous Zn-ion batteries is related to the ZnMn\({}_{2}\)O\({}_{4}\) compound [26], which is obtained in the second plateau, i.e., at low potential (1.3 V). At low density current, 30 mA g\({}^{-}\)1, both plateaux appear and when the number of cycles grows up, the second plateau becomes smaller. The order in the length of this plateau is as follows: KMn8 \(<\) KMn4 \(<\) HKMn. The capacity loss for these materials goes on the same sequence. Figure 7.7 XRD profile “ex situ” of KMn4 after three cycles (**a**) discharged (**b**) charged (**c**) KMn4 profile with comparative purpose. Finally, this plateau is hardly seen at high density current (300 mA g\({}^{-1}\)), then after 50 cycles the loss capacity is practically negligible. Moreover, this result has been supported by XRD data (Figure 9) of the HKMn electrode cycled at 30 mA g\({}^{-1}\) rate until the battery stopped working. All maxima that appear were identified as the phase ZnMn\({}_{2}\)O\({}_{4}\), neither maximum of ZHS or birnesite-type structure was observed.