Origin of electrochemical reactivity enhancement of post-annealed LiFePO4 thin films: Preparation of heterosite-type FePO4

We reported the dependence of the LiFePO4 thin films growth by Pulsed Laser Deposition (PLD) on their electrochemical properties. Whether the crystallized films are directly grown within the PLD chambers or post annealed, they present different textures and levels of surface carbon contamination, with the post-annealing films being the most contaminated. The post-annealed thermal treatment induces a drastic increase in the film electrochemical reactivity vs. Li+ (capacity 50 times higher). We show through combined cyclic voltametry and Raman spectroscopy measurements that the enhanced electrochemical activity lies in the films textural evolution. By purposely acting on the LiFePO4 film texturation we succeeded for the first time in preparing, through chemical oxidation, fully delithiated heterosite-type FePO4 thin films. Furthermore using three-electrode electrochemical impedance spectroscopy measurements on such films, we evidenced a noticeable decrease in the charge transfer resistance upon the de-insertion. Finally to account for the specific electrochemical/chemical reactivity of LiFePO4 thin films, a simple planar model is proposed.