Mechanistic insights into high-rate Li/Na storage: natural pyrite-derived FeS₂/graphene anodes with...

The exploitation of earth-abundant and cost-effective electrode materials is crucial for large-scale energy storage applications. Herein, we report a facile synthesis strategy to fabricate FeS₂/graphene heterostructure composite electrodes derived from natural pyrite ore via a scalable ball milling method. The obtained FeS₂/graphene electrode delivers superior electrochemical performance in both lithium-ion and sodium-ion batteries, all demonstrating an intriguing "negative fading" phenomenon, in which the reversible capacity increases over extended cycling. Among them, the FeS₂-G5 electrode-prepared via 5 h of ball milling-delivers the most pronounced performance, with the capacity rising to approximately 620 mAh g^-1 after 1173 cycles at a high current density of 1 A g^-1 in sodium-ion batteries. Systematic investigations employing in-situ electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were conducted to elucidate the underlying mechanisms of "negative fading" and capacity degradation. Moreover, full cells constructed using Na₃V₂(PO₄)₃ cathodes paired with FeS₂-G5 anodes demonstrate the reversible capacity of 237 mAh g^-1 after 200 cycles and a promising high initial energy density of 136 wh kg^-1_{(totalactive material mass)} at a current density of 100 mA g^-1, highlighting the practical viability of natural FeS₂-based electrodes for large-scale energy storage applications.