Method for preparing amorphous lithium borosilicate

The present invention provides a vapour deposition method for preparing an amorphous lithium borosilicate or doped lithium borosilicate compound, the method comprising: providing a vapour source of each component element of the compound, wherein the vapour sources comprise at least a source of lithium, a source of oxygen, a source of boron, and a source of silicon, and, optionally, a source of at least one dopant element; delivering a flow of said lithium, said oxygen, said boron and said silicon, and, optionally, said dopant element; and co-depositing the component elements from the vapour sources onto a substrate wherein the component elements react on the substrate to form the amorphous compound; wherein the amorphous lithium borosilicate or doped lithium borosilicate compound has a lithium content in the range 40-65 atomic %, based on the combined atomic percentages of lithium, boron and silicon.