Critical thickness of a surface-functionalized coating for enhanced lithium storage: a case study of nanoscale polypyrrole-coated FeS2 as a cathode for Li-ion batteries

The encapsulation or coating of conductive materials is an effective strategy to increase the electrochemical ion-storage performance of some promising electrode materials such as transition metal oxides and sulfides, which are low-cost and have high capacity, but their practical applications are hindered by their intrinsically low conductivity and large volume changes during cycling; however, to date, the effect of the thickness of conductive layers on the ion-storage performance has been rarely studied. In this study, taking nanoscale polypyrrole (PPY)-coated FeS2 as an example, the effect of the critical thickness of the conductive PPY coating on the lithium-ion storage performance of (PPY)-coated FeS2 as a cathode of rechargeable lithium-ion batteries (LIBs) was investigated. Via a facile vapor-phase polymerization method, uniform PPY coatings with the thickness of 1–18 nm on microsized FeS2 particles were prepared. It was found that the critical thickness of PPY was 5 nm, at which the PPY-coated FeS2 cathode exhibited remarkablely superior high-rate capability (808, 583, 543, 511, and 489 mA h g−1 at 0.1, 1, 2, 5 and 10 A g−1, respectively) and long-term stability (504 mA h g−1 at 1.0 A g−1 after 500 cycles) as compared to those with other coating thicknesses owing to the acheivement of optimal electrical conductivity and ion diffusion efficiency. Thus, this study provides an insight into the critical thickess of a surface-functionalized coating of active materials and opens a new avenue for the futher enhancement of the performance of energy storage deivces.