Identifying the Electrochemical Processes in LiFeSO4F Cathodes for Lithium Ion Batteries

Abstract The electrochemical performance of tavorite LiFeSO 4 F can be considerably improved by coating the material with a conducting polymer (poly(3,4‐ethylenedioxythiophene); PEDOT). Herein, the mechanisms behind the improved performance are studied systematically by careful electrochemical analysis. It is shown that the PEDOT coating improves the surface reaction kinetics for the Li‐ion insertion into LiFeSO 4 F. For such coated materials no kinetic limitations remain, and a transition from solid state to solution‐based diffusion control was observed at 0.6 mA cm −2 ( circa C/2). Additionally, the quantity of PEDOT is optimized to balance the weight added by the polymer and the improved electrochemical function. Post mortem analysis shows excellent stability for the LiFeSO 4 F‐PEDOT composite, and maintaining the electronic wiring is the most important factor for stable electrochemical cycling of LiFeSO 4 F. The insights and the methodology used to determine the rate‐controlling steps are readily transferable to other ion‐insertion‐based electrodes, and the findings are important for the development of improved battery electrodes.