Storage of Lithium Metal: The Role of the Native Passivation Layer for the Anode Interface Resistance in Solid State Batteries

To overcome current challenges of lithium metal anodes (LMAs), which hinder their wide industrial application, the chemical composition of the lithium metal surface is an important factor. Due to its high reactivity and depending on the pre-treatment during processing, lithium is covered with a passivation layer composed of mainly Li2CO3, LiOH, and Li2O, what is mostly neglected in later electrochemical studies. Here, we investigate the effect of storage time and conditions on the surface passivation layer of commercial lithium foils, based on lithium surface characterization with X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry, finding that only sealed pouch bags can prevent lithium surface changes effectively. Otherwise, the passivation layer thickness increases steadily, even in gloveboxes with a low degree of contaminations. Testing the stored lithium foils in solid-state batteries with LLZO as model solid electrolyte, it is demonstrated that the solid electrolyte roughness and the applied pressure have a huge impact on the obtained impedance. While the passivation layer has no major effect on the interface resistance with a rough LLZO pellet and at high pressure, it clearly affects the interface resistance with smoother LLZO surfaces and at lower pressure. Consequently, the lithium passivation layer may hinder the application of the LMA in a solid-state battery what we discuss in depth. By reactivity experiments with model lithium surfaces, we show that water residuals are the main reason for the aging of lithium foil in gloveboxes. Additionally, nitrogen reacts with fresh lithium surfaces and lithium foils with an incomplete or damaged passivation layer. The results demonstrate that storage conditions are important factors for the surface state of lithium metal and consequently for the application as an anode material.