Stability of Li7La3Zr2O12 solid electrolyte versus conventional liquid electrolytes based on LiClO4 and LiPF6

Currently, lithium all-solid-state batteries are in great demand. Lithium-ion conducting liquid electrolytes LiPF₆ and LiClO₄, dissolved in organic solvents (mixture of carbonates), are considered promising buffer layers for cathode | solid electrolyte interface modification. However, their stability in contact with solid electrolytes should be investigated. Thus, the study of the stability of solid electrolytes based on Li₇La₃Zr₂O₁₂ (LLZ) in contact with organic solvents, as well as liquid electrolytes, is the aim of the presented work. The formation of an intermediate phase was observed by scanning electron microscopy on the ceramic surface after exposure of the solid electrolyte in LiClO₄ and LiPF₆, dissolved in a mixture of carbonates. According to X-ray diffraction analysis and Raman spectroscopy, LiF and LaOCl are present on the surface of Al-doped LLZ after its exposure to LiPF₆ and LiClO₄ liquid electrolytes, respectively. It was established that an increase in the holding time leads to a decrease in the conductivity of the solid electrolyte (from 1.4·10⁻⁴ to 3.9·10⁻⁵ S cm⁻¹ after exposure to LiClO₄ liquid electrolytes), which is especially noticeable for LiPF₆ liquid electrolyte (to 2.1·10⁻⁸ S cm⁻¹). Despite the decrease in the total resistance of the electrochemical cells modified by LiClO₄ liquid electrolyte, the resistance at the modified cathode | solid electrolyte interface increased over time (from 3 to 180 kΩ cm²) and after galvanostatic cycling (up to tens of MΩ cm²). Thus, the modification of the cathode | LLZ interface cannot be performed by such liquid electrolytes as LiClO₄ and LiPF₆ dissolved in the mixture of carbonates due to their interaction with solid electrolytes based on LLZ and the formation of a low-conductivity layer. The search for lithium-ion conducting liquid electrolytes that are stable in contact with solid electrolytes with a garnet-like structure for cathode | solid electrolyte interface optimization in all-solid-state batteries is an important task for further research.