Structural characterization of an amorphous VS and its lithiation/delithiation behavior studied by solid-state NMR spectroscopy.

Vanadium sulfide (VS) is one of the promising positive electrode materials for next-generation rechargeable lithium-ion batteries because of its high theoretical capacity (1196 mA h g). Crystalline VS has a unique structure, in which the Peierls-distorted one-dimensional chains of V-V bonds along the axis are loosely connected to each other through van der Waals interactions. In this study, an amorphous VS is prepared by mechanical milling of the crystalline material, and its lithiation/delithiation behavior is investigated by solid-state nuclear magnetic resonance (NMR) spectroscopy. The amorphous VS shows a chain structure similar to that of crystalline VS. The amorphous host structure is found to change drastically during the lithiation process to form LiVS: the V ions become tetrahedrally coordinated by S ions, in which the valence states of V and S ions simultaneously change from V to V and S to S, respectively. When the Li insertion proceeds further, the valence state of V ions is reduced. After the 1 cycle, the amorphous VS recovers to the chain-like structure although it is highly disordered. No conversion to elemental V is observed, and a high capacity of 700 mA h g is reversibly delivered between 1.5 and 2.6 V.