Improved cycling stability of the capping agent-free nanocrystalline FeS2 cathode via an upper cut-off voltage control

Transition metal chalcogenides such as FeS2 are promising electrode materials for energy storage. However, poor rate performance and low cycling stability hinder the practical application of FeS2 cathode in secondary batteries. In this study, highly pure pyrite FeS2 nanocrystals (NCs) with octahedral shape and 200–300 nm size have been synthesized via a facile and environmentally benign approach based on a surfactant-free aqueous reaction. Combined with a compatible ether electrolyte, the prepared FeS2 NCs, despite their dimension far beyond the quantum confined regime, could achieve high utilization and reversibility as a cathode active material due to the well-defined crystal structure and the uncapped rough surfaces. Furthermore, we find that the last charging voltage step of FeS2 only contributes a minor capacity but caused severe capacity fading due to the formation of soluble polysulfides. By suppressing this step through setting a proper upper cut-off voltage, the cycle life of the Li/FeS2 cell is dramatically improved. The Li/FeS2 cell running over a voltage window of 1.0–2.4 V at 1C delivers an initial capacity of 486.1 mA h g−1, slightly lower than that running over 1.0–3.0 V (561.1 mA h g−1), but outperforms the latter substantially after 500 cycles (367 mA h g−1 vs 315 mA h g−1), corresponding to a capacity decay rate as low as 0.048% per cycle. Our results provide a meaningful approach for the development of not only the advanced FeS2 material for long-life rechargeable batteries, but also other transition metal chalcogenide nanomaterials for a variety of potential applications.