Methods of forming composite material films

A method of forming a composite material film, the method comprising:

• providing a mixture comprising polyimide or a polyimide precursor, the mixture further comprising silicon particles and graphite particles;
• and
• pyrolysing the mixture to convert the polyimide or the polyimide precursor into one or more carbon phases to form the composite material film such that the one or more carbon phases comprises hard carbon that is 10% to 25% by weight of the composite material film and holds together the pyrolysed film, and the silicon particles are between 50% and 90% by weight of the composite material film distributed throughout the one or more carbon phases, wherein after pyrolysing the mixture, the mixture forms a self-supported composite structure.