Nanocrystalline semiconductor films are constituted by a network of mesoscopic oxide particles, such as TiO2, ZnO, Nb2O5 which are interconnected to allow for electronic conduction to take place. The pores between the particles are filled with a semiconducting or a conducting medium, such as a hole transmitter or an electrolyte, forming a junction of extremely large contact area. The salient features of such mesoporous films are: i) an extremely large internal surface area, the roughness factors being in excess of 1000 for a film thickness of 8 microns; ii) the ease of charge carrier percolation across the nanoparticle network making this huge surface electronically addressable; iii) a very rapid and highly efficient interfacial charge transfer between the oxide and redox active species anchored to the particle surface. and iv) the rapid intercalation and release of Li+ ions into such films.