The solubility mechanism of silica in a fluoride-chloride melt has been determined in situ using Raman spectroscopy. The spectroscopy data revealed that the silica solubility process involved Si-O bond breakage and Si-F bond formation. The process results in the formation of silicate complexes, fluorine-bearing silicate complexes, and silicon tetrafluoride in the melt. Mass spectrometry of the vapor phase over the KF-KCl-K2SiF6 and KF-KCl-K2SiF6-SiO2 melts and differential scanning calorimetry coupled with thermal gravimetric analysis of these melts were performed to verify the silica solubility mechanism.

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The glass-ceramic composite electrolytes based on tetragonal LiLaZrO (t-LLZ) and cubic Al-doped LiLaZrO (c-LLZ) with the LiPO glass additive have been prepared. The electrical conductivity and microstructure of the t-LLZ/LiPO and c-LLZ/LiPO composites have been investigated. The phase evolution of electrolytes has been studied using XRD, SEM, and Raman spectroscopy. It was indicated that the impurities formation depends on the composition of the composite. The phase composition of the solid electrolytes determines their thermal properties, which have been studied by the DSC method. The relative density of the obtained composite electrolytes was established to be higher than one of the sintered t-LLZ and c-LLZ. The Li conductivity of the t-LLZ-based composites gradually increased from 4.6 × 10 S cm (undoped t-LLZ) to 2.5 × 10 S cm (t-LLZ/5 wt % LiPO composite) at 25 °C. The highest total conductivity of the c-LLZ/LiPO composites has been achieved by introducing 1 wt % additive (0.11 mS cm at room temperature), whereas further doping resulted in the impurities formation.