## 4 Conclusions  
Manganese oxides with birnessite-type structure have been synthetized by two different methods: thermal treatment and hydrothermal method. Both syntheses were carried out from a low-cost material (KMnO\({}_{4}\)). The thermal decomposition at temperatures of 400 and 800 \({}^{\circ}\)C made it possible to obtain K\({}_{\rm x}\)MnO\({}_{2}\) birnessite with different crystallinity degree. Moreover, XPS data indicated that the sample obtained at higher temperature presents on the surface a lower amount of Mn\({}^{4+}\) that the other derivatives. In relation to structure characterization obtained by powder X-ray diffraction, KMn4 presents a turbostractic structure whereas KMn8 and HKMn have hexagonal and rhombohedral symmetries, respectively. The morphologies obtained by SEM and HRTEM for KMn8, KMn4, and HKMn samples shown well-defined crystals, cloud-like particles with aggregates and a flower-like nanostructure, respectively. The sample obtained at high temperature (KMn8) exhibits a lower oxidation state as well as a denser morphology, both characteristics are related to the obtained low-capacity value. The Zn\({}^{2+}\) insertion is very small at low density current and negligible at high density current, which seems to be a key factor of capacity retention. On the other hand, in the samples obtained at low temperature (KMn4) or by means of hydrothermal method, Mn cations have an oxidation state close to 4 and their morphologies allow Zinc ions to have a better accessibility towards birnessite-type structure. The incorporation of Zn\({}^{2+}\) in the structure gives rise to the ZnMn\({}_{2}\)O\({}_{4}\) spinel and this process implies that some links must be broken, and at the same time others must be set up. When the spinel-like ZnMn\({}_{2}\)O\({}_{4}\) structure is obtained capacity retention reduces, which is favorable at low speeds. In consequence, the best result is obtained when the sample presents a great accessibility and high density current where ZnMn\({}_{2}\)O\({}_{4}\) compound would not have enough time to form. Figure 9: X-ray diffraction pattern “ex situ” of HKMn post-mortem. **Supplementary Materials:** The following are available online at [[https://www.mdpi.com/article/10.3390/app12031176/s1](https://www.mdpi.com/article/10.3390/app12031176/s1)]([https://www.mdpi.com/article/10.3390/app12031176/s1](https://www.mdpi.com/article/10.3390/app12031176/s1)), Figure S1: (a) Thermogravimetric analysis under N\({}_{2}\) for all samples. (b) XRD profile of KMn8 residue, the maxima were assigned to a mixture of phases: bimessite and K\({}_{1\text{-}33}\)Mn\({}_{8}\)O\({}_{16}\); (c) XRD profile of the HKMn residue, the maxima were indexed to K\({}_{1\text{-}33}\)Mn\({}_{8}\)O\({}_{16}\). Table S1. Weight loss from ATG data (%) and structural formulas for bimessite-type material obtained from EDX and ATG data. **Author Contributions:** Conceptualization, M.L.L. and E.R.-C.; methodology, I.A.-S.; validation, M.L.L., E.R.-C. and I.A.-S.; investigation, P.A. and D.A.G. and E.R.-A. ; writing--original draft preparation, M.L.L. and E.R.-C.; writing--review and editing, M.L.L., I.A.-S. and E.R.-C. visualization, P.A. ; supervision, M.L.L. ; project administration, M.L.L. funding acquisition, I.A.-S. All authors have read and agreed to the published version of the manuscript. **Funding:** This work has been supported by the Spanish Ministry of Economy and Competitiveness (MINECO) through project MAT2017-84118-C2-2-R and Spanish Ministry of Science Innovation and Universities trough project RTI2018-099668-B-C22, which have not played any role in any aspect regarding the study design, the collection, analysis and interpretation of data, the writing of the report, or the decision to submit the article for publication. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The following are available on line Figure S1. (a) Thermogravimetric analysis under N2 for all samples.