Surface-modified microstructured carbon electrodes for lithium-oxygen batteries

  • Oberflächenmodifizierte mikrostrukturierte Kohlenstoffelektroden fĂĽr Lithium-Sauerstoff-Batterien

Wunderlich, Philipp; Simon, Ulrich (Thesis advisor); Sauer, Dirk Uwe (Thesis advisor)

Aachen (2019, 2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019

Abstract

The rechargeable lithium-oxygen battery is an electrochemical energy storage device based on the conversion of lithium metal into lithium oxides and back. In contrast to the intercalation-type cell chemistry of nowadays prevalent Li-ion battery technology, Li-O$_{2}$ cells operate by the deposition of solid discharge products in a porous air electrode. This theoretically enables high-energy batteries with low-cost materials, but practically faces fundamental challenges including self-passivation by discharge products or electrochemical instability of vital cell components. In consequence, the discharge capability of a Li-O$_{2}$ battery is limited and its rechargeability poor, which can be addressed with cathode engineering. This work is an experimental study on air electrode designs in two steps. Firstly, three different electrode microstructures were created out of graphite nanosheets (GNS) as carbon base material. Electrochemical characterization was carried out as battery testing in combination with impedance spectroscopy, while discharge products were analyzed with post mortem electron microscopy and Raman spectroscopy. The impact of the microstructure was discussed on the basis of key performance indicators such as discharge capacity and rate capability. The overall best Li-O$_{2}$ cell performance could be achieved with highly porous GNS-foam electrodes that prove to be superior to dry-pressed GNS discs. Taking the strong interference of electrolyte-related effects into account, a dispersion-based electrode was developed, which works surprisingly well even without having a rigid pore structure. At high depths of discharge, it was discovered that graphite suffers from electrochemically driven exfoliation, which on the one hand inevitably ruins the cyclability of a battery, but on the other hand enables outstanding discharge capacities. Secondly, noble metal coatings were applied to the carbon surface in order to protect it from degradation and also benefit from electrocatalytic effects. A full encasement of carbon particles could not be achieved and partial covers with Au, Ag, Pt or Pd sputter-coatings did not substantially alter the cell performance. Local investigations of the discharge products grown on metal surfaces suggested that coatings actually lower the capacity by electrocatalytically promoting a surface-mediated discharge mechanism. The results put the necessity of solid-state catalysts for Li-O$_{2}$ batteries into question and underline that carbon is the air electrode material of choice.

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