Configuring Optimal FeS2@Carbon Nanoreactor Anodes: Toward Insights into Pyrite Phase Change/Failure Mechanism in Rechargeable Ni-Fe Cells.

Pyrite FeS2 has long been a research focus as the alternative anode of rechargeable Ni-Fe cells owing to its eye-catching merits of great earth-abundance, attractive electrical conductivity and output capacity. However, its further progress is impeded by unsatisfactory cyclic behaviors due to still "ill-defined" phase changes. To gain insights into the pyrite working principles/failure factors, we herein design a core-shell hybrid of FeS2@carbon nanoreactor, an optimal anode configuration approaching the practical usage state. The resultant electrodes exhibit a Max. specific capacity of ~272.89 mAh g-1 (at ~0.81 A g-1), remarkably improved cyclic longevity/stability (beyond ~80% capacity retention after 103 cycles) and superior rate capability (~146.18 mAh g-1 is remained at ~20.01 A g-1) in contrast to bare FeS2 counterparts. The as-built Ni-Fe full cells can also output impressive specific energy/power densities of ~87.38 Wh kg-1/∼11.54 kW kg-1. Moreover, a refreshed redox reaction working mechanism of "FeS2OH ↔FeS2↔Fe0(in pyrite domains)" is re-defined based on real-time electrode characterizations at distinct operation stages. In a total cyclic period, the configured pyrite-based anodes would stepwise undergo three critical stages nominally named "retention", "phase transition/coexistence" and "degradation", each of which is closely related to variations on anodic compositions/structures. Combined with optimal electrode configurations and in-depth clarifications on inherent phase conversions, this focus study may guide us to maximize the utilization efficiency of pyrite for all other aqueous electrochemical devices.