Thermodynamics as a Driving Factor of LiCoO₂ Grain Growth on Nanocrystalline Ta-LLZO Thin Films for All-Solid-State Batteries

All-solid-state batteries primarily focus on macrocrystalline solid electrolyte/cathode interfaces, and little is explored on the growth and stability of nanograined Li-garnet and cathode ones. In this work, a thin (∼500 nm) film of LiCoO2 (LCO) has been grown on top of the polycrystalline layer of Ta-doped Li7La3Zr2O12 (Ta-LLZO) solid electrolyte using the pulsed laser deposition (PLD) technique. Scanning transmission electron microscopy, electron diffraction, and electron tomography demonstrated that the LCO film is formed by columnar elements with the shape of inverted cones. The film appears to be highly textured, with the (003) LCO crystal planes parallel to the LCO/Ta-LLZO interface and with internal pores shaped by the {104} and {102} planes. According to density functional theory (DFT) calculations, this specific microstructure is governed by a competition between free energies of the corresponding crystal planes, which in turn depends on the oxygen and lithium chemical potentials during the deposition, indicating that thermodynamics plays an important role in the resulting LCO microstructure even under nonequilibrium PLD conditions. Based on the thermodynamic estimates, the experimental conditions within the LCO stability domain are proposed for the preferential {104} LCO orientation, which is considered favorable for enhanced Li diffusion in the positive electrode layers of all-solid-state batteries.