Electrochemical Synthesis of Iron Phosphides as Anode Materials for Lithium Secondary Batteries

We report the electrochemical synthesis of iron phosphide films used as anodes of lithium secondary batteries. The phosphorus content in the film was increased to a level almost equal to that of Fe (corresponding to a FePy composition with 0.85 < y < 0.91) through the selective electrochemical dissolution of Fe and Fe-rich phase from the electroplated film. The as-deposited film exhibited a dense structure characterised by numerous submicron-sized nodules in contact with each other, and maintained its overall mechanical integrity even after the selective dissolution. The as-deposited and the selectively etched films with high P content were tested as anodes of lithium secondary batteries. The performance of the selectively etched films far exceeded that of the as-prepared films. In particular, a 500-nm-thick film maintained a reversible discharge capacity greater than 420 mA h g−1 after 30 cycles. No specific physical damage was observed in the film after the cycling tests.