The demand for a battery technology that could involve metallic lithium as negative electrode is motivated by the fact that lithium is the lightest and the most electropositive of all metals (E⁰ =-3.04 Volts) ¹. Ddrawbacks of this electrode material are double : first, lithium deposit has a tendency to be dendritic, ending up in short circuiting the battery; secondly, when lithium plates, some electrolyte is co-deposited, resulting in a poor density of the as-deposited metal. This-mossy-like lithium results in a consumption of the electrolyte, leading to a premature death of the battery. Thus, improving the metallic lithium/electrolyte interface quality is important for avoiding the consumption of solvent and/or salt during electrochemical processes.

In this context, the understanding of the chemical proprieties of this interface, commonly named SEI (for Solid Electrolyte Interphase), is crucial in order to improve the quality of lithium plating and stripping and to select the adequate electrolyte for a given battery system.

A particularly well-adapted method for investigating the SEI layer composition is the use of X-ray Photoelectron Spectroscopy (XPS) analyses. This post-mortem surface analysis technique, increasingly used in battery characterization, gives essential information not only on the active material but also on the phenomena occurring during cycling ².

The purpose of this work is to investigate by XPS the SEI layer thickness and chemical nature, and their influence on the battery electrochemical performance. The study is carried out on lithium electrodes stemming from metallic lithium symmetric cells cycled (by lithium electrodeposition and stripping) in different liquid electrolytes (different salts and solvents).

XPS imaging is also used in this work in order to get a chemical mapping of the SEI layer components formed on the metallic lithium electrode surfaces cycled in different conditions. Data processing based on the principal component analysis (PCA) method has been conducted in order to illustrate the SEI layer heterogeneities. The results are compared with energy-dispersive X-ray spectroscopy (EDS) mapping and highlight the XPS imaging benefits and precision to identify chemical compounds distribution.

An example for a lithium electrode cycled in the liquid electrolyte (LiTFSI, Polyethylene Glycol (PEG)) is shown in Figure 1, displaying O 1s (Li₂CO₃) (a), C 1s (Li₂CO₃) (b), N1s (LiTFSI) (c) and F 1s (LiTFSI) (d) XPS core peaks components mapping and in Figure 2, displaying the EDS fluorine- oxygen (a), sulfur-oxygen (b) and fluorine-sulfur (c) overlapped mappings for the same sample.

These results have led to a better knowledge of the redox processes occurring at the top surface of lithium electrodes cycled in different liquid electrolytes.

1. M. Lécuyer, J. Gaubicher, M. Deschamps, B. Lestriez, T. Brousse, D. Guyomard , J. Power Sources, 241, 249–254 (2013)
2. L. Martin, H. Martinez, M. Ulldemolins, B. Pecquenard, and F. Le Cras, Solid State Ionics, 215, 36–44 (2012)