Reversible lithium insertion and conversion process of amorphous VS4 revealed by operando electrochemical NMR spectroscopy

Due to the high theoretical capacity, VS4 is a promising electrode material for next-generation rechargeable batteries. In this study, the lithium insertion and conversion process of amorphous VS4 was investigated using operando electrochemical nuclear magnetic resonance (NMR) spectroscopy. Amorphous VS4 has a chain-like structure similar to that of crystalline VS4. The chain structure was drastically changed to the [VS4]3− tetrahedral structure by lithium insertion up to the Li3VS4 composition. The lithium insertion into the [VS4]3−-based structure proceeded further up to the Li6VS4 composition, with charge compensation by the reduction of the V valency. Finally, the conversion reaction from amorphous Li6VS4 to metallic V and 4Li2S was observed. The structural reversibility of amorphous VS4 was confirmed after the delithiation. It is worth mentioning that the delithiation process from the conversion products was different from the lithiation, resulting in a relatively large voltage hysteresis. Broadly, this study demonstrates that the operando electrochemical NMR technique is a useful tool for investigating the complex reaction system of non-crystalline battery materials.