An X-ray photoelectron spectroscopy study of the electrochemical behaviour of iron molybdate thin films in lithium and sodium cells

Iron molybdate thin films are studied here as a possible electrode material for future Li and Na microbatteries working in a lower potential range than currently used systems. Monoclinic Fe2(MoO4)3 thin films are successfully deposited using radio frequency (RF) sputtering and an annealing treatment. The electrochemical behaviour of the obtained electrodes against Li and Na is then studied in a coin cell configuration with liquid electrolytes. The redox processes ruling the insertion/deinsertion of Li+ and Na+ are investigated by means of XPS (X-ray Photoelectron Spectroscopy). The results highlight a different behaviour depending on the alkali, with a better redox reversibility for sodium at the end of the first charge. For subsequent cycles however, improved capacity retention is evidenced for cycling versus lithium as compared to sodium which was attributed to the properties of the SEI layer.