Epitaxial Thin Films as a Model System for Li-Ion Conductivity in Li₄Ti₅O₁₂

Using an epitaxial thin-film model system deposited by pulsed laser deposition (PLD), we study the Li-ion conductivity in Li4Ti5O12, a common anode material for Li-ion batteries. Epitaxy, phase purity, and film composition across the film thickness are verified employing out-of-plane and in-plane X-ray diffraction, transmission electron microscopy, time-of-flight mass spectrometry, and elastic recoil detection analysis. We find that epitaxial Li4Ti5O12 behaves like an ideal ionic conductor that is well described by a parallel RC equivalent circuit, with an ionic conductivity of 2.5 × 10–5 S/cm at 230 °C and an activation energy of 0.79 eV in the measured temperature range of 205 to 350 °C. Differently, in a co-deposited polycrystalline Li4Ti5O12 thin film with an average in-plane grain size of <10 nm, a more complex behavior with contributions from two distinct processes is observed. Ultimately, epitaxial Li4Ti5O12 thin films can be grown by PLD and reveal suitable transport properties for further implementation as zero-strain and grain boundary free anodes in future solid-state microbattery designs.