Artificial solid electrolyte interphase protecting metal anodes in metal-sulfur batteries

Kurzfassung

Metal-sulfur batteries represent a promising system for high-energy cells and are therefore the focus of many research groups. However, some problems with the system are known, namely the so-called polysulfide shuttle and the inhomogeneous metal deposition that can lead to dendrites. In order to counteract both, magnesium (Mg) and lithium (Li) metal surfaces are to be coated with an artificial solid electrolyte interphase (SEI) in this thesis. Such a coating prevents the anode from reaction with polysulfides and reduces an excessive electrolyte degradation. In addition, a more homogeneous metal deposition is possible. Ionomer based coatings (e.g. Aquivion) and thermally cyclized poly(acrylonitrile) (PAN) were applied to the metal surface by means of spin or dip coating. Artificial SEI coated anodes resulted in stable cycling performances for over 150 cycles at C/10 in magnesium-sulfur (Mg-S) cells. A better coulombic efficiency as well as a constant capacity gain of around 200% was achieved using an Aquivion-based SEI instead of an uncoated anode. In addition, the capacity of a Mg-S cell was increased by the PAN-based SEI. Other ionomerbased coatings (e.g. Nafion, sulfonated poly(etheretherketone) (SPEEK)) led to higher capacities in the initial cycles. The partial collapse of the artificial SEI, however, led to an adjustment to the values of the Mg-S reference cell with uncoated anode with increasing number of cycles. Electrochemical impedance spectroscopy (EIS) was used in Mg-Mg and Mg-S cells to better understand the interface effects between anode and electrolyte. During polarization, two overall processes were present with the uncoated Mg reference anode. A fast charge transfer process (high-frequency (HF) region) and a slower process (middle-frequency (MF) region) through the formation of an in-situ SEI. Completely coated anodes showed the formation of another process in the low-frequency (LF) region, which was assigned to the artificial SEI. In addition, an adsorption layer formed on the uncoated anode during open circuit voltage (OCV). Similar time constants for the adsorption and the diffusion through the artificial SEI led to an overlay of the impedance response in the LF region. The contribution of sulfur species to interfacial layer formation at the Mg anode was investigated in Mg-S full cells during OCV and the subsequent first ten cycles. During the initialviii OCV phase, two processes were present and could be separated into an HF and MF process in the case of an uncoated anode. With coated anodes, the separation of these processes into an HF and MF process is possible in the subsequent discharge/charge cycle. In order to investigate the influence of the artificial SEI on the cycling performance of Li-S cells, coated anodes along with various cathodes were tested. In conclusion, cells with artificial SEI achieved better long-term stability than cells with uncoated anode. The combination of an aerogel cathode with an artificial SEI based on SPEEK or sulfonated poly(phenylene sulfone) was proved to be particularly promising. Thus, Li-S cells could be cycled for over 400 cycles at C/3 with a high capacity retention.