Decoupling Li out-diffusion and surface diffusion in the lithiation-assisted epitaxial growth of lithium tungstate

Lithiation-assisted epitaxy offers a flexible and robust approach for synthesizing high-quality Li-containing materials and interfaces with precise control. In this study, we use lithium tungstate (${\mathrm{Li}}_{x}\mathrm{W}{\mathrm{O}}_{3+x/2}$, where $x=0$ to 2) as a model system to investigate the intertwined effects of Li out-diffusion-induced compositional changes and surface-diffusion-induced morphological changes. By systematically varying synthesis and processing conditions, we uncover their impact on lithium tungstate film formation. Comprehensive characterizations, including X-ray diffraction, atomic force microscopy, X-ray photoemission spectroscopy, and time-of-flight secondary ion mass spectrometry, reveal that low-temperature growth ($<{300}^{\ensuremath{\circ}}\phantom{\rule{0.16em}{0ex}}\mathrm{C}$) followed by high-temperature annealing yields continuous lithium tungstate films with significantly reduced surface roughness. In contrast, high-temperature deposition ($\ensuremath{\ge}{300}^{\ensuremath{\circ}}\phantom{\rule{0.16em}{0ex}}\mathrm{C}$) accelerates surface diffusion and Li out-diffusion, leading to island formation. Furthermore, in situ scanning transmission electron microscopy demonstrates the beam sensitivity of ${\mathrm{Li}}_{2}\mathrm{W}{\mathrm{O}}_{4}$ and reveals a phase transition from ${\mathrm{Li}}_{2}\mathrm{W}{\mathrm{O}}_{4}$ to $\mathrm{LiW}{\mathrm{O}}_{3.5}$ under prolonged electron beam exposure. These findings deepen our understanding of how to control the composition and morphology of Li-containing films, providing valuable insights for the design and integration of energy materials.