In-Depth Characterization of Lithium-Metal Surfaces with XPS and ToF-SIMS: Toward Better Understanding of the Passivation Layer

Abstract

To significantly increase the energy density of lithium-based batteries, the use of lithium metal as an anode is an option despite all of the associated challenges. Due to its high reactivity, lithium is covered with a passivation layer that may affect cell performance and reproducibility of electrochemical characterization. In most studies, this is ignored and lithium metal is used without considering the passivation layer and carrying out a proper characterization of the surface. Against this background, we systematically characterized various lithium samples with X-ray photoelectron spectroscopy (XPS), time-of-flight secondary-ion mass spectrometry (ToF-SIMS), and complementary energy-dispersive X-ray spectroscopy (EDX), resulting in a complete three-dimensional chemical picture of the surface passivation layer. On all analyzed lithium samples, our measurements indicate a nanometer-thick inorganic passivation layer consisting of an outer lithium hydroxide and carbonate layer and an inner lithium oxide-rich region. The specific thickness and composition of the passivation layer depend on the treatment before use and the storage and transport conditions. Besides, we offer guidelines for experimental design and data interpretation to ensure reliable and comparable experimental conditions and results. Lithium plating through electron beam exposure on electrically contacted samples, the reactivity of freshly formed lithium metal even under ultrahigh-vacuum (UHV) conditions, and the decomposition of lithium compounds by argon sputtering are identified as serious pitfalls for reliable lithium surface characterization.

A Dry‐Processed Al2O3/LiAlO2 Coating for Stabilizing the Cathode/Electrolyte Interface in High‐Ni NCM‐Based All‐Solid‐State Batteries

“…The NCM XPS analysis delivers quantitative element and compound‐specific information with a detection limit of about 1 at%. [ 51 ] The survey spectrum of P‐NCM shown in Figure 4c confirms the presence of Ni, Co, Mn, and O without any additional elements apart from the carbon contaminations at the NCM surface, which is commonly used as a calibration reference for X‐ray photoelectron (XP) spectra. [ 52 ] In addition, the characteristic peak of Al 2p is detected on the surface of Alu‐NCM and Alu‐NCM600, which proves the existence of Al species on the surface of the coated NCM with Al 2 O 3 (Figure 4d).…”

Porosity of Solid Electrolyte Interphases on Alkali Metal Electrodes with Liquid Electrolytes

“…The surface of Li/Na electrodes is expected to be covered by thin (several nanometers) layers of their native oxides, hydroxides, and carbonates. 29 Even though thin layers might affect the electrolyte decomposition by differences in electron transfer, 30 their Pilling–Bedworth ratios (the ratio of molar volume of the Li/Na SEI compounds to the molar volume of the metallic Li/Na, see Supporting Information , part XII, Table S2 ) suggest only partial coverage. As soon as the alkali metal comes in contact with liquid electrolyte, the SEI is formed on its surface, which is not only determined by the nature of the electrode but also by the composition of the liquid electrolyte.…”

A critical discussion on the analysis of buried interfaces in Li solid-state batteries. Ex situ and in situ/operando studies

“…This is the case for Li2CO3 and LiOH, which are commonly found in SEIs and CEIs, and are known to partially convert into Li2O. 166,167 Thus, the presence of Li2O as an inherent and inner component of interphases can also be critically discussed if this information was gathered from Ar + ion etching.…”