Transition Metal Monochalcogenides as Anodes for Metal-Ion Batteries

As the most promising energy storage system (ESS), rechargeable alkali metal-ion batteries have been attracting great attention. The selection of suitable electrode material is a fundamental step in the development of metal-ion batteries to achieve enhanced performance. In the present study, we have explored the feasibility of monolayers of phosphorene analogs, namely, group IV monochalcogenides. The monochalcogenide layers form structures belonging to the same orthorhombic crystal system and possess similar hinge-like structures like phosphorene. Just this unique hinge-like structure and the isolation of 2D forms will introduce strong anisotropic properties endowing them with fascinating properties, such as tunable bandgaps, high carrier mobility, and predominantly anisotropic and optical properties. We present two-dimensional (2D) sheets of germanium and tin sulfides and tellurides as promising high-capacity and stable materials for energy storage. GeS, GeTe, SnS, and SnTe with layered structures are prepared via a simple liquid-phase exfoliation approach. As-synthesized 2D nanosheets can effectively increase the electrolyte-electrode interface area and facilitate metal ion transport. As a result, GeS, GeTe, SnS, and SnTe nanosheets deliver a high areal capacity of 1.76 mAh cm^-2,1.05 mAh cm^-2, 2.66 mAh cm^-2, 1.22 mAh cm^-2  respectively as anodes in lithium-ion batteries (LIBs). Further analysis of these monochalcogenides in sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs) suggest that layered monochalcogenides can be potential anodes in other metal-ion batteries. Besides, monochalcogenides encapsulated in porous carbon fibers as anode materials are fabricated by a facile single-nozzle electrospinning technique followed by heat treatment. A solution of polyacrylonitrile (PAN) containing monochalcogenide is utilized to prepare hybrid precursor fibers. The resulted porous hybrid fibers composed of compact carbon shell and monochalcogenide-embedded honeycomb-like carbon core are formed due to the thermal decomposition of PAN. The high performances of electrodes are attributed to the high electric conductivity and structural stability of the porous carbon fibers with unique structure, which not only buffers the volume change of monochalcogenides with the internal space but also acts as high-efficient transport pathways for ions and electrons. Furthermore, the compact carbon shell can promote the formation of stable solid electrolyte interphase on the fiber surface. Investigations on the structural and compositional development of the fibers are conducted via Raman spectroscopy, Fourier-transform infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. All of these characteristics suggest that transition metal monochalcogenides are promising candidates for designing negative electrodes in metal-ion batteries. Hence, the synthesis of these materials through an efficient deposition technique is highly desirable for application in future energy technology.