In today's market, the dominant batteries employ lithium-ion technology. However, controversy surrounds their sustainability, safety and cost. Hence, the FQM-288 research group at the IUNAN University Institute of Nanochemistry carried out a study focused on rechargeable magnesium batteries as an alternative to lithium ones, to come up with another storage option for energy from renewable sources.
The objective was to study a cathode for these magnesium batteries; that is, the positive pole of an electric battery. An electrochemical cell, a device capable of obtaining electrical energy from chemical reactions, is comprised of several parts: an anode, which is the negative electrode; a cathode, the positive electrode, and an electrolyte. As an anode they used magnesium-metal, and, as a cathode, a polyanionic compound (a robust structure with covalent bonds); specifically, sodium vanadium fluorophosphate. The research team highlighted as its main result the presence of a reversible multielectron reaction; that is, a compound emitting more than one electron. In this case, it is vanadium, giving rise to an energy density slightly less than half that of that of lithium-ion batteries, directly affecting its durability.
The reaction mechanism was analysed during the first battery charging and discharging cycles, with an approximate duration of 20 to 50 hours, respectively. The study concluded examining the use of magnesium batteries to store energy from renewable sources, such as wind or photovoltaic, given the reactions' limitations for use in urban transport and electric vehicles.
The main reasons why they proceeded to study these batteries are the advantages they offer. Among them are their greater energy storage capacity, energy density, the abundance of magnesium in the earth's crust, and their greater safety, and long-term stability, as there is no dendrite (small rigid structures that grow inside a battery formation) as is the case with lithium batteries.
Gregorio Ortiz, coordinator of the study and a professor at the UCO, stated that the key aspects of a battery are its gravimetric capacity and cell voltage. "Based on these two parameters, we are going to talk about energy density, which determines how long a battery, such as those in mobile phones, or electric vehicles, will last," the researcher added.
For the study, the University of Cordoba carried out the theoretical calculations, electrochemistry and characterization by means of electronic paramagnetic resonance. Likewise, the analysis of the surface of these electrodes was carried out using X-ray photoelectron spectroscopy techniques. Meanwhile, Xiamen University (China) performed the structural synthesis and characterization via nuclear magnetic resonance to detect the presence of vanadium in different states of oxidation (V3 + -V5+), as well as the chemical environment of phosphorus, essential parameters that defined its multi-electronic character.
This project was funded by Spain’s Ministry of Science, Innovation and Universities (MCIU), the Junta de Andalucía (FQM-288 Group), China's National Key Research and Development Program, the National Natural Science Foundation of China, and the Ministry of Education and Vocational Training for professors and senior researchers who conduct research at foreign centres, including the Salvador de Madariaga Program.
Reference
Rubio, S., Liang, Z., Liu, X., Tirado, José L., Lavela, P., Stoyanova, R., Zhecheva, E., Liu, R., Zuo, W., Yang, Y., Pérez-Vicente, C., Ortiz, G. F. (2021). Reversible Multi-Electron Storage Enabled by Na5V(PO4)2F2 for Rechargeable Magnesium Batteries. Energy Storage Materials. Vol. 38, pp. 462-472. DOI: https://doi.org/10.1016/j.ensm.2021.03.035