CHEMO-STRUCTURAL EVOLUTION OF LITHIUM-RICH SURFACES UNDER DRIVEN CONDITIONS

Abstract

Lithium-rich materials are an important material class with unique properties that enable far-ranging applications in energy storage, chemical synthesis, quantum materials, and advanced manufacturing. Fundamental knowledge of Li-rich materials’ surfaces is in urgent need but remains scant due to significant challenges of high reactivity, instability, and heterogeneity. In this dissertation, I bridged this knowledge gap and extracted nanoscale and atomistic details of Li-rich LixCoO2 (001) and LixAg (111) surfaces and their evolutions under driven conditions. Applied driving forces include oxygen and lithium chemical potentials (μ) and electric potential (ϕ), the chemo-structural surface evolution is tracked in situ with scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS).On cathodes, I investigated the chemo-structural response of LiCoO2 (001) from the meso-to-atomic scale under varied μ_(O_2 ). Under low μ_(O_2 ), surface roughening, initial (√3×√3) R30⁰ reconstruction of LiCoO2 (001), coexisting Li0.8CoO2 (001) and CoO (111) phases, with local Li0.8CoO2 (001) -(√7×√7) R 19.1⁰ reconstruction on a kinetically arrested surface are observed. Under high μ_(O_2 ), the surface realizes a Li1.0CoO2.0 stoichiometry with atomically flat (001) terraces of micron widths decorated with Co3O4 islands, whose formation is suppressed by high μ_(O_2 ). On anodes, under a μ_Li on Ag (111), I investigated LixAg (111) surface alloy formation, revealed new surface phases, including the LixAg (111) solid solution wetting layer, local Li1Ag1 (111) - (√3×1), LixAg herringbone phase, body-centered cubic (bcc) Li (001) islands. Mechanisms of initial LixAg alloy formation, lithiophilicity, metastability of Li/LixAg (111) interface, and Stranski-Krastanov growth mode of Li islands on Ag (111) are discussed. Electric field-driven Li growth and dissolution demonstrate facile Li transport in LixAg alloys and substitutional Li-Ag alloying. I also investigated the chemo-structural response of LixAg and Li surface phases under varied μ_(O_2 ). Initial oxidation of single-layer Li islands occurs elusively at the island perimeter and atomic vacancies, while multi-layer Li islands show self-limiting oxidation to suboxide LixOy islands. Further oxidation driven by a strong local electric potential reveals facile Li transfer from the LixAg substrate and LixOy islands, in accordance with the Cabrera-Mott oxidation model.