Fabrication and Characterization of Magnesium Powder Anodes for High Energy Batteries

Abstract

Development of post lithium ion technology plays an important role in meeting challenges of high energy density, safe, and long lasting batteries. Magnesium sulfur batteries are a promising technology to replace lithium ion batteries due to their higher safety and volu-metric energy density. The main focus of this thesis lies with reducing high overpotentials during charging by replacing the magnesium foil (which is currently standard in the labora-tory scale research) with a magnesium powder anode or a magnesium graphite powder anode. Further objectives include gaining a better understanding of the processes at the interface between the metal anode and the electrolyte and to find correlations of these processes with battery discharge capacity and lifetime. Magnesium sulfur cells containing magnesium powder and magnesium foil anodes were cycled in a 2-electrode Swagelok system to examine lifetime, discharge capacity, and coulombic efficiency. The various processes at the magnesium powder anode and the overpotentials were investigated using electrochemical impedance spectroscopy and cy-clic voltammetry in a 3-electrode PAT-Core system with a magnesium reference elec-trode. It was found that when replacing the magnesium foil with magnesium powder anodes the overpotentials were reduced, especially during magnesium plating while charging. The pressed magnesium powder electrode, having a high specific surface area and grain boundaries, had a strong influence on increased cycle life stability. Adsorption of the pas-sivating layer has a major impact on the impedance during open circuit voltage. Since the graphite particles form conducting paths inside the passivating layer the resistance can be lowered by orders of magnitudes with magnesium graphite powder electrodes. Uniform stripping and plating of magnesium ions on the magnesium electrode can be identified as the main influence on lowering magnesium cell overpotentials. While magne-sium powder and magnesium graphite powder electrodes had a uniform stripping and plating, magnesium foil anodes showed a punctual stripping and plating mechanism. Moreover, lower overpotentials, higher specific surface area, and the presence of grain boundaries in the magnesium powder anodes significantly increase the battery lifetime compared to cells using magnesium foil as anode.