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.