Pyrite-based mixtures as composite electrodes for lithium-sulfur batteries

Pyrite-based composite materials (Li/S) were prepared by high-energy ball milling of FeS₂ with one or more metal sulfides, Li₂S, Cu₂S, and CuS, in various molar ratios and under different conditions. Prolonged grinding resulted in the formation of mixed compounds rather than single-phase lithiated chalcopyrite. The composite materials were primarily investigated via XPS, XRD, and TOF-SIMS characterization methods, and performance of the materials as composite cathodes was evaluated via analysis of dQ/dV curves. Evidence of main active phases in the materials was found, including, Cu-doped FeS₂ (pyrite) and/or nonstoichiometric chalcopyrite (Cu_xFe_yS_{2 − z}). TOF-SIMS measurements exhibited further evidence of formation of partially lithiated pyrite mixtures. Lithium cells comprising mixed pyrite cathodes demonstrate superior electrochemical performance, namely, 98–100% coulombic efficiency and 300–320 mAh g⁻¹ (per gram of active material) after 150 cycles over a voltage range of 1.2–2.8 V at 0.1 C-rate, whereas pristine FeS₂/Li cells exhibit only 220 mAh g⁻¹ at similar operating conditions. Lowering of cut-off discharge voltage from 1.2 to 1.0 V of the FeS₂/Li cells with FeS₂-rich mixed cathodes is accompanied by about 15% capacity enhancement (350–450 mAh g⁻¹ after 200 cycles). The cells with mixed pyrite composite cathodes exhibit an increase in maximum pulse power capability from 5 to 15 W/g and C-rate from 0.3 to 2.5, which points towards higher electrical conductivity of the active material as compared to the pristine pyrite.