Estudos físico-químicos da eletroinserção de íons em hexacionoferratos aplicados a baterias recarregáveis

Abstract

Aqueous Batteries based on insertion electrodes are remarkable energy devices with lower costs and safer than the current commercial Li-ion batteries, which are based on high cost, flammable and toxic organic electrolytes. Such devices are ideal for grid storage applications. However, only a handful of aqueous insertion materials can meet the existing energy demand. Among them hexacyanoferrates (HCFs) stand out. They are Prussian Blue analog compounds which can withstand ion insertion of a range of cations in aqueous solution with high reversibility and good energy performance. Herein, a CuHCF and a VOHCF are synthesized and submitted to chemical, structural, morphological and electrochemical characterizations. Thermodynamic and kinetic properties of CuHCF were evaluated during insertion of Li+, Na+, K+, Mg2+, Al3+ and H3O+. This material shows good reversibility for all applied cations with specific discharge capacities between 47 and 55 mAhg-1 at 0.5 C (30 mAg-1), where the best energy results were delivered by using K+ ions.The capacity retention at high rates are considerably improved in concentrated acid electrolytes, going from 17% (Na+ at pH = 2) to 57% (Na+ in H2SO4 6 M) at 60C (3,6 Ag-1). Notably, both CuHCF and VOHCF can operate reversibly by proton (hydronium ions) insertion/deinsertion. At 0.5C CuHCF shows a capacity of 57 mAhg-1 with 53% retention at 120C (7.2 Ag-1). While VOHCF shows a capacity of 40 mAhg-1 even inan extremely high rate of 160C (14.4 Ag-1). Such high retentions are achieved due to the fast proton transport in the material via hydrogen bonds. Additionally, diffusion coefficients were determined for each of the applied cations and can help understanding CuHCF performance in the various electrolytes. Studying the physicochemical properties of HCFs in aqueous electrolytes with different cations is a key step to understand ion insertion mechanism in these materials towards the development of more efficient and cheaper batteries.