Protective Coating for the Lithium Metal Anode Prepared by Plasma Polymerization

The demand for batteries with higher energy densitiesfor energystorage and electric vehicles motivates research for a stable andreversible lithium metal anode. The native passivation layer on commerciallithium foils does not enable stable cycling in liquid electrolytesdue to inhomogeneities in the layer composition and impurities. Theseinhomogeneities and impurities result in locally varying current densities,which often lead to dendrite growth and ultimately cell failure. Artificialprotection layers are one promising option to overcome these issuesand enable the reversible operation of lithium metal anodes. In thisstudy, we used plasma polymerization of 1.4 bis(trifluoromethyl)benzeneto form an artificial passivation layer on top of plasma-cleaned lithiummetal. The layer was characterized with time-of-flight secondary ionmass spectrometry, X-ray photoelectron spectroscopy, and scanningelectron microscopy. The mechanical properties of the layer were examinedby nanoindentation. Symmetric cell tests showed stable cycling behaviorfor over 300 h, with overpotentials below 0.1 V at current densitiesbetween 0.1 and 1 mA/cm(2). O-18(2) isotopeexchange experiments were used to get an estimation of the diffusioncoefficients of oxygen in the native passivation layer of lithiumfoils at room temperature and for oxygen in the plasma polymer atroom temperature. The combination of layer thickness and diffusioncoefficient of the plasma polymer is sufficient to protect the lithiummetal against oxygen for at least 30 min, which makes it a suitableprotective coating.