A Comparison Study on the Constructed Polypyrrole Coating and Polypyrrole-Derived Carbon Coating for the Electrochemical Performance of the Iron Difluoride Cathode

FeF₂ is considered a high-capacity conversion cathode for next-generation lithium-ion batteries due to its high energy density. However, FeF₂ struggles with poor electronic conductivity, slow reaction kinetics, and serious interfacial side reactions, leading to rapid cell degradation during cycling. In this study, a vapor self-polymerization method is employed to fabricate a polypyrrole (PPy) coating on FeF₂, which acts as an artificial cathode electrolyte interface with improved electronic and ionic conductivities. Additionally, the PPy-derived carbon coating is also prepared, and a systematic comparison study of these two coatings is conducted. The PPy-derived carbon coating has higher electronic conductivity than the PPy coating. However, the merging of the carbon coating, followed by the agglomeration of particles during the carbonization process, hinders the transport of lithium ions. Consequently, the PPy-coated FeF₂ positive electrode exhibits better cycling and capacity retention performance than the PPy-derived carbon coating, owing to the fast diffusion kinetics and low voltage hysteresis, in which the discharge capacity of 189.2 mAh g^-1 over 100 cycles is achieved. This work provides a feasible technology to fabricate a conductive polymer coating on metal fluorides and offers a perspective on the selection of coating materials for battery applications.