Spatially-Resolved EELS Analysis of Surface Species on Metallic Lithium for the Development of Next Generation Li-Batteries.
Authors: Trudeau Michel L. , Voisard Frédéric , Veillette Rene , Zaghib Karim , Gauvin Raynald 
Published: December 2024

Metallic Li is considered one of the most attractive anode candidates for solid-state battery technology as well as for the next generation Li-ions batteries [1,2] due to its ultrahigh theoretical specific capacity (3860 mAhg−1)[3]. However, several technical issues hinder its application as anode material. The widespread commercial usage of Li metallic sheet will necessitate a much better understanding of its microstructure and chemical nature.
Metallic lithium is highly sensitive to hydrogen, oxygen, nitrogen, and carbon dioxide, which are the major components of wet air, and are likely to form rapidly LiOH, Li2O, Li2O2, Li3N, and Li2CO3 species [4] when exposed to air and/or moisture. It has been shown that this surface passivation layer has a direct impact on the anode interface resistance in solid state batteries [5]. Because of the nature of Li, very few works have looked directly at the surface chemical structure [6]. XPS and SIMS are normally used to study the nature and thickness of this passivation layer. However, these techniques do not allow to directly observe the nature and thickness of this layer. Direct observation, using TEM, would thus be much better.
Because of the very high surface reactivity and the low hardness of Li, it is very difficult to prepare thin sample from a precise Li sheet for direct TEM observations and chemical analysis of the material structure and surface chemical nature. In this presentation we will show the direct HRTEM study of the surface chemical nature of metallic Li sheets. We have used a Hitachi air-protection cryogenic FIB holder to prepare thin Li samples and study the surface chemistry using Spatially-Resolved EELS (SR-EELS). The thin samples were prepared from different Li sheets using FIB (NB5000 from Hitachi) with the sample temperature at ~ -90 °C. The samples were transferred under vacuum using the same holder into a Hitachi HF-3300 cold FEG E-TEM for EELS analysis at low temperature without air contact.
SR-EELS was done using a high-resolution GIF system (Gatan Quantum ER) for the rapid acquisition of the Li-K-edge, O-K-edge and C-K-edge respectively. Using this technique, it is possible to directly observe the different chemical variation in the passivation layer and its variation from sample to sample.
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