Synthesis and characterization of Al-Si alloys for anode materials of metal-air batteries

  • Synthese und Charakterisierung von Al-Si-Legierungen für Anodenmaterialien von Metall-Luft-Batterien

Aslanbas, Özgür; Eichel, Rüdiger-A. (Thesis advisor); Mayer, Joachim (Thesis advisor); Figgemeier, Egbert (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021

Abstract

In the scope of this PhD thesis, mechanisms taking place on primary metal-air battery cell anodes, which employ pure Al and Al-Si alloys as their negative electrodes, were investigated to gain insights about the correlation between the microstructure and the anode performance. The primary cells were operated with a room temperature ionic liquid as their electrolytes, which is composed of 1-ethyl-3methylimidazolium cation and µ-fluoro-bis(fluorohydrogenate), µ3-fluoro-tris(fluorohydrogenate) anions. The pure Al samples were used as the negative electrode of the batteries as received. Vacuum arc melting is employed as the processing method of Al-Si alloys from pure Al and Si raw materials, also as for the microstructure modification for the pure Al. Processed samples of the alloys were subjected to microstructure characterization for calculating the quantities of the alloy micro-constituents. The grain size of the fine-grained pure aluminium was increased by processing with arc melting process from 10-75 µm to 200-1500 µm, which increased the polarization resistance of aluminium. It was found that the fine-grained aluminium is more prone to corrosion, but also can deliver higher gravimetric capacities. Moreover, the discharge potential is found higher than the coarse-grained aluminium under same discharge conditions. As a result of discharge experiments with various current densities, it was observed that the pure Al anode batteries can deliver capacities up to 2200 math/g which corresponds to the 73% utilization of the theoretical capacity of Al, when discharged with highest current density of 1.5 mA/cm2 A reduction of the discharge current density resulted in low capacity discharges. The open circuit and discharge potentials of the cells were reduced with the increasing Si content of the anodes. The reduction of the open circuit potentials of the cells with increasing Si content was also confirmed by the linear polarization measurement in half-cells vs. gel-based reference electrodes. The electron microscopy study of the Al-Si anode surface morphologies after discharge experiments revealed that, for all alloy anodes, the discharge results mostly from the oxidation of the aluminium rich phase. The corrosion potentials, corrosion currents and the Tafel slopes of the electrodes were obtained by using the Tafel fitting on the linear polarization curve for each electrode composition as well as for the Si reference materials. In addition, mixed potential theory was employed under different scenarios based on the microstructural properties to investigate the galvanic coupling of the alloy components. The results were discussed in terms of Evans diagrams; thereby approaches based on alternative scenarios for the galvanic coupling were examined. It was shown that by employing an extended approach in mixed potential calculations in terms of accepting the physical properties of eutectic-Si and primary-Si different, the potentials and corrosion currents can be estimated for a metal-semiconductor galvanic couple from the electrochemical properties of the parent components of the alloy. Corrosion rates of the pure Al and the Al-Si alloys were investigated via gravimetric measurements in combination with calculations over corrosion currents, which were obtained from linear polarization measurements. The corrosion rates of the anodes were first increased as the aluminium alloyed with silicon up to 50 wt% with Si, then decreased with further alloying. The electrolyte compositions of the batteries operated with Al-Si alloy anodes were analysed with inductively coupled plasma optical emission spectroscopy method after corrosion and discharge experiments. The data indicated that the total mass lost by the anode of the batteries after can be found as dissolved species in the electrolytes, which may lead to the saturation of the electrolyte and consequent termination of cell discharge. In addition, for the cells operated with the anodes containing 75 wt.% and more Si in their compositions, the amount of Si detected in the electrolyte was higher than that of Al. The electron microscopy observations of these anode surfaces after discharge revealed pitting type dissolution patterns on the primary-Si surfaces which may be the proof of the active contribution of primary-Si to the discharge.

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