the present invention relates to the preparation of essentially silica - free glasses which can be melted at low temperatures and which have compositions within the li 2 o -- al 2 o 3 and / or f -- b 2 o 3 system . the glasses exhibit very high lithium ion mobility which , resulting in low electrical resistivity , has led to their consideration as solid membrane electrolyte - separators in such devices as the lithium metal - sulfur battery .