Structural characterization of an amorphous VS(4) and its lithiation/delithiation behavior studied by solid-state NMR spectroscopy

Vanadium sulfide (VS(4)) 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(−1)). Crystalline VS(4) has a unique structure, in which the Peierls-distorted one-dimensional chains of V–V bonds along the c axis are loosely connected to each other through van der Waals interactions. In this study, an amorphous VS(4) 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(4) shows a chain structure similar to that of crystalline VS(4). The amorphous host structure is found to change drastically during the lithiation process to form Li(3)VS(4): the V ions become tetrahedrally coordinated by S ions, in which the valence states of V and S ions simultaneously change from V(4+) to V(5+) and S(−) to S(2−), respectively. When the Li insertion proceeds further, the valence state of V ions is reduced. After the 1(st) cycle, the amorphous VS(4) 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(−1) is reversibly delivered between 1.5 and 2.6 V.