Observation of the Surface Layer of Lithium Metal using In Situ Spectroscopy

TL;DR

This study uses in situ spectroscopy to reveal that lithium metal surfaces rapidly develop oxide and carbonate layers, complicating efforts to prepare pristine lithium surfaces for battery applications.

Contribution

It provides real-time insights into the formation and growth of surface layers on lithium metal, highlighting challenges in achieving clean lithium interfaces.

Findings

Surface layers of lithium are a few tens of nanometers thick after cleaning.

Exposure to air causes rapid growth of surface layers at about 24 nm/min.

Pristine lithium surfaces are difficult to prepare, affecting battery interface quality.

Abstract

We have investigated the surface of lithium metal using x-ray photoemission spectroscopy and optical spectroscopic ellipsometry. Even if we prepare the surface of lithium metal rigorously by chemical cleaning and mechanical polishing inside a glovebox, both spectroscopic investigations show the existence of a few tens of nanometer-thick surface layers, consisting of lithium oxides and lithium carbonates. When lithium metal is exposed to room air (~50% moisture), in situ real-time monitoring of optical spectra indicates that the surface layer grows at a rate of approximately 24 nm/min, presumably driven by an interface-controlled process. Our results hint that surface-layer-free lithium metals are formidable to achieve by a simple cleaning/polishing method, suggesting that the initial interface between lithium metal electrodes and solid-state electrolytes in fabricated lithium metal batteries can differ from an ideal lithium/electrolyte contact.