Remarkably Improved Electrode Performance of Bulk MnS by Forming a Solid Solution with FeS - Understanding the Li Storage Mechanism

Transition metal compounds based on conversion reactions are promising electrode materials for lithium-ion batteries due to their higher lithium storage capacity compared with currently available commercial battery electrodes. Most of the studies on these materials in the literature focus on transition metal oxides and fluorides, and not much work on transition metal sulphides has been reported, partially due to their relatively poor electrochemical performance. Here, synthesis and characterization of a series of solid solution FexMn1-xS (x = 0.2, 0.5, 0.8) monosulphide compounds is reported. Interestingly, hexagonal FeS and cubic MnS can form a solid solution of FexMn1-xS (x < 0.57). It is demonstrated that the lithium storage voltage can be tuned by changing the Fe concentration in the FexMn1-xS matrix; meanwhile, the discharge-charge coulombic efficiency and cycle stability of FexMn1-xS are greatly enhanced in comparison with that of pure MnS. A half cell using Fe0.5Mn0.5S as electrode material achieves a high first cycle coulombic efficiency of 78.0% and a high reversible capacity of ca. 477 mAh g(-1) after 35 cycles, while for pure MnS the first cycle coulombic efficiency is only 45.9% and the capacity rapidly fades to approximate to 200 mAh g(-1) after 15 cycles. Although the solid solution state of Fe0.5Mn0.5S cannot be retained during conversion reaction as indicated by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and transmission electron microscopy (TEM), the initial discharge polarization, which has been considered as one of the major hurdles for conversion reaction, can be significantly reduced by this type of material design. In addition, the size and distribution of the nucleated nanophases might also be altered by the initial solid solution state of Fe0.5Mn0.5S, contributing to the improved electrochemical performance reported here.