Effect of crystal structure on the electrochemical performance of LLTO perovskite-type solid electrolytes for lithium-ion batteries

The demand for batteries for energy storage for mobility and stationary energy storage system has increased significantly in recent years and sustained growth is expected due to the insertion of renewable energies, the increase in electric vehicles and energy efficiency. Lithium-ion batteries have high gravimetric and volumetric energy density. However, organic liquid electrolytes may react exothermically and raise safety and environmental concerns. Solid-state batteries have emerged as an alternative to conventional lithium-ion batteries, due to their greater thermal stability, which can provide them with high safety and service life. However, there are still major challenges to overcome, such as low conductivity of lithium ions in solid electrolytes, scalable synthesis methods, and wide windows of electrochemical stability.
Among the different families of solid electrolytes, perovskite ceramic oxides such as Li3xLa2/3-xTiO₃ (LLTO) exhibit high grain ionic conductivities (> 1 mS/cm), but their total conductivities are two orders of magnitude lower. This work seeks to increase the total ionic conductivity by adding dopants such as Zr4+, Ta5+ and V5+ in the structure synthesized by the sol-gel method. The materials were characterized by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, electrochemical impedance, and galvanostatic charge-discharge curves.
All materials have a perovskite phase with a crystalline structure and space group P4/mm, and the dopants modify the TiO6 octahedral geometry, leading to changes in Ti–O bond distances and local structural distortion. Furthermore, the materials have vibrational modes typical of the ABO3 perovskite phase, low porosity, and relative densities greater than 95%, the 1% Zr-doped sample exhibited the highest total ionic conductivity (0.1 mS/cm), which can be associated with the more pronounced TiO6 octahedral distortion observed from the Ti–O bond lengths, and in the discharge-charge curves, the cells exhibited a coulombic efficiency of 90% after 10 cycles in lithium-ion batteries.