Lithium-ion surface diffusion and Tantalum redox phenomena in RF sputtered LLZTO thin films: Insights from surface chemistry by XPS

Garnet-type Li 7 La 3 Zr 2-x Ta x O 12 (LLZTO) is a leading electrolyte for all-solid-state Li-ion batteries. Its ionic conductivity is highly sensitive to oxidation states and composition. This study uses X-ray Photoelectron Spectroscopy (XPS) to elucidate surface chemistry, particularly the oxidation states, and detect contaminants and defects in LLZTO thin films. Peak fitting of LLZTO thin film spectra deposited by RF sputtering at 5 mTorr (5-UA) and 25 mTorr (25-UA) and annealed at 500 °C (5-500) and 800 °C (5-800) is performed. XRD results show amorphous structures for 5-UA, 25-UA, and 5-500, with cubic-phase crystallization for 5-800. XPS analysis revealed Li surface enrichment (from 36.2 to 62.5 at.%) with increasing pressure due to reduced re-sputtering and shorter mean-free paths, while annealing enhanced Li diffusion (from 38 to 44 at.%) following Arrhenius-type behavior. Increased oxygen-vacancy (O V ) content (4.6-6.9 at.%) under all modified conditions correlated with decreased Ta 5+ (12.6 to 4.7 at.%) and increased lower oxidation state species, suggesting O V -mediated Ta redox. Stable La 3+ and Zr 4+ binding energies (±0.1 eV) contrasted with Li 1 s and Ta 4 f shifts of up to 1.1 eV, reflecting modified electronic screening and Madelung potentials. These findings provide a mechanistic framework for LLZTO defect chemistry and electronic structure, with potential implications in electrochemical performance.