Coating Techniques and Characterization of Protective Coatings for Electrode Materials in Lithium Ion Batteries -With the Examples of TiO2 on NCM by ALD and Plasma Polymerization of 1.4bis(trifluoromethyl)benzene on Lithium-

Abstract

Today's lithium ion batteries (LIB) are approaching their theoretical limit of 250 Wh/kg. In order to meet the increasing demands for batteries due to the energy transition and the associated electrification of passenger transport, new concepts must be established. One promising possibility is the use of a lithium metal anode (LMA), which, in combination with solid electrolytes in solid-state batteries, has a capacity that is theoretically 10 times larger than the graphite electrodes currently used. The use of an LMA would thus be an important step towards e.g. long-range vehicles, but is associated with a lot of obstacles. The high reactivity of lithium causes unwanted side reactions with the used electrolytes. When liquid electrolytes are used, the high heat generated by dendrites in the event of short circuits and the associated fire hazard are major problems. Safety can be improved by using solid electrolytes, but the high reactivity remains a problem. Unwanted side reactions are not limited to the LMA, but also to the nickel-cobalt-manganese oxide (NCM) particles used at the cathode. Overall, these (electro) chemically induced degradations lead to poorer battery performance or capacity loss culminating in cell failure. In literature it is already reported that thin protective layers are applied to cathode or anode materials. It is now considered proven that protective coatings are unavoidable to ensure the long-term stability of LIBs. In literature, many processes and different materials are used for coatings, while the analysis of the protective layers in order to understand the protective effect often falls short. In the context of this doctoral thesis, protective layers were therefore prepared for the cathode, by using ALD, and for the anode, by using plasma polymerization, suitable analytical methods were used for the investigation, and a guideline for reproduction of the analytics was prepared. In addition to the widely used methods such as X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), further useful complementary methods such as time-of-flight mass spectrometry (ToF-SIMS) and low-energy ion scattering spectroscopy (LEIS) or Raman spectroscopy were applied. The analysis results contribute to the comprehensive elucidation of the protective layers and interfaces. In symmetric Li cells, the electrochemical performance of the polymer protective layer was verified and the mechanical properties were de-termined by nanoindentation. Furthermore, the diffusion coefficients of oxygen ions in the native passivation layer of lithium and the polymer layer were determined by 18O2 experiments. The results in this dissertation, on the one hand, extend the analytical characterization of protective layers on battery materials by using new methods and, on the other hand, introduce with the plasma polymerization a forgotten method for the application of protective layers on LMAs. Without performing a proper analysis of the protective layers, it is difficult to draw conclusions about the causes of the protective effects and misinterpretations can easily occur. The work is an important step towards a systematic characterization of protective coatings in combination with their electrochemical performance to develop new coating concepts with the aim of making predictions about suitable materials.