Protective Coating for the Lithium Metal Anode Prepared by Plasma Polymerization

Abstract

The demand for batteries with higher energy densities for energy storage and electric vehicles motivates research for a stable and reversible lithium metal anode. The native passivation layer on commercial lithium foils does not enable stable cycling in liquid electrolytes due to inhomogeneities in the layer composition and impurities. These inhomogeneities and impurities result in locally varying current densities, which often lead to dendrite growth and ultimately cell failure. Artificial protection layers are one promising option to overcome these issues and enable the reversible operation of lithium metal anodes. In this study, we used plasma polymerization of 1.4 bis(trifluoromethyl)benzene to form an artificial passivation layer on top of plasma-cleaned lithium metal. The layer was characterized with time-of-flight secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and scanning electron microscopy. The mechanical properties of the layer were examined by nanoindentation. Symmetric cell tests showed stable cycling behavior for over 300 h, with overpotentials below 0.1 V at current densities between 0.1 and 1 mA/cm2. 18O2 isotope exchange experiments were used to get an estimation of the diffusion coefficients of oxygen in the native passivation layer of lithium foils at room temperature and for oxygen in the plasma polymer at room temperature. The combination of layer thickness and diffusion coefficient of the plasma polymer is sufficient to protect the lithium metal against oxygen for at least 30 min, which makes it a suitable protective coating.