Inserção iônica em hexacianoferratos de Cu-Fe: um estudo aplicado à armazenagem de energia em baterias recarregáveis

Abstract

This work presents the study of Cu-Fe hexacyanoferrates applied to rechargeable ion batteries operating in aqueous electrolytes under Na+ou Li+ electroinsertion. The analyzed materials were characterized by atomic absorption spectroscopy, X-ray diffraction, transmission electron microscopy and Mössbauer spectroscopy techniques in order to infer composition, morphology as well as structural and electronic properties. Electrochemical techniques were applied to evaluate energy properties, such as energy capacity and discharge rate and to calculate kinetic and thermodynamic parameters. The as-prepared nanostructured materials, CuIIxFeII3-x[FeIII (CN)6] 2.nH2O (where x correspond to 3.00, 2.60, 2.38 and 1.09) presented a typical facecentered cubic Fm-3m crystalline structure with an average particle size ranging from 8 to 15 nm. Electrochemical characterizations showed good values of energy capacities even when high rates were applied. More specifically, the best results were 60 80 mA.h.g -1 (Na+ insertion) and 90 120 mA.h.g-1 (Li+insertion) under discharge rates from C/2 to 20C. In addition, Mössbauer spectroscopy coupled to in-situ electrochemical experiments was applied to investigate electronic properties under a discharge process (Na+ insertion). A charge transfer associated to different redox pairs, high spin FeIII /FeII, low spin FeIII/FeII, and CuII/CuI were evidenced. A spin crossover process between high spin and low spin states was also observed at the Fe II electronic state. These results confirmed that this class of material is a promising low-cost safety technology for rechargeable ion batteries based on aqueous electrolytes. Our findings will also contribute to the development of new advanced materials with improved energy properties.