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.