  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.
