Composite solid-state electrolyte, method of preparing the same, and lithium battery comprising the solid-state electrolyte

A method of preparing a composite solid-state electrolyte comprising a first solid-state electrolyte and a second solid-state electrolyte,

• wherein the first solid-state electrolyte comprises a cubic garnet phase and a pyrochlore phase, and the second solid-state electrolyte comprises a glass phase, and
• wherein a volume of the first solid-state electrolyte is greater than a volume of the second solid-state electrolyte, based on a total volume of the composite solid-state electrolyte, the method comprising:
• mixing a first solid-state electrolyte precursor comprising an amorphous phase and a second solid-state electrolyte precursor comprising a glass phase to prepare a composite solid-state electrolyte-forming composition;
• and
• heat-treating the composite solid-state electrolyte-forming composition to prepare the composite solid-state electrolyte.