Mecanismo de reação catódica em baterias recarregáveis a base de azul da Prússia: uma abordagem DFT

Abstract

High performance rechargeable batteries are a technological necessity to be used in combination with renewable sources such as eolic and solar. The understanding of the cathodic reaction mechanism is considered crucial for the development of new materials and increasing the performance of rechargeable batteries. The objective of this work was to investigate, based on the electronic structure calculations, the cathodic reaction mechanism and the structural and electronic properties of Prussian Blue (PB) used as the cathode of a rechargeable sodium battery. The NaFIII[FeII(CN)] (PB "solúvel") e FeII[FeII(CN)6]3.nH20 (insoluble PB) structures were calculated at the PBE+U/Plane waves level of theory. The electronic properties of the two structures were characterized, using the analysis of the density of states (DOS)and Bader charges. The results are consistent with the presence of Fe(III)-NC of high spin (FeHS) and Fe(II)-CN of low spin (FeLS), as evidenced by experimental techniques. The cathode was simulated with the insertion of sodium atoms in the PB structures. The reduction process, with the transfer of electrons from sodium to the PB structure, was investigated and its effect on the electronic structure analyzed. The electrochemical potential for the processes FeHS(III) -> FeHS(II) and FeLS(III) -> FeLS(II) using the soluble PB was estimated to be 2.89 V and 3.82 V, respectively. These values should be compared with the experimental values of 2.92 V and 3.58 V. In the case of "insoluble" PB, the structure involves coordinating water molecules and FeHS(III)atoms are not in a perfect octahedral structure. The system is complex and still a challenge for computational chemistry. The data demonstrated that with the insertion of sodium atoms, there is a transfer of electrons mainly to the cyanide ligands and, apparently, the Fe(III) atoms are not reduced. Further investigation of the "insoluble" PB needs to be performed.