A vapour deposition method for preparing a crystalline lithium-containing transition metal oxide 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, and a source or sources of one or more transition metals; wherein the source or sources of one or more transition metals comprises a source of each of one or more of scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, lutetium and hafnium; the method further comprising the steps of: heating a substrate to between 150°C and 450°C; and co-depositing each component element from its respective vapour source onto the heated substrate at a controlled rate of deposition wherein the component elements react on the substrate to form the crystalline compound, wherein either: the vapour source of oxygen comprises an ozone source; or the vapour source of oxygen comprises a plasma source, and oxygen provided by the source of oxygen is delivered to the heated substrate as plasma-phase oxygen comprising a flux of oxygen atoms, radicals, and ions, for co-deposition and reaction with the other component elements. 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, and a source or sources of one or more transition metals; wherein the source or sources of one or more transition metals comprises a source of each of one or more of scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, lutetium and hafnium; the method further comprising the steps of: heating a substrate to between 150°C and 450°C; and co-depositing each component element from its respective vapour source onto the heated substrate at a controlled rate of deposition wherein the component elements react on the substrate to form the crystalline compound, wherein either: the vapour source of oxygen comprises an ozone source; or the vapour source of oxygen comprises a plasma source, and oxygen provided by the source of oxygen is delivered to the heated substrate as plasma-phase oxygen comprising a flux of oxygen atoms, radicals, and ions, for co-deposition and reaction with the other component elements. heating a substrate to between 150°C and 450°C; and co-depositing each component element from its respective vapour source onto the heated substrate at a controlled rate of deposition wherein the component elements react on the substrate to form the crystalline compound, wherein either: the vapour source of oxygen comprises an ozone source; or the vapour source of oxygen comprises a plasma source, and oxygen provided by the source of oxygen is delivered to the heated substrate as plasma-phase oxygen comprising a flux of oxygen atoms, radicals, and ions, for co-deposition and reaction with the other component elements. the vapour source of oxygen comprises an ozone source; or the vapour source of oxygen comprises a plasma source, and oxygen provided by the source of oxygen is delivered to the heated substrate as plasma-phase oxygen comprising a flux of oxygen atoms, radicals, and ions, for co-deposition and reaction with the other component elements.