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 a 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 redefined 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.