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

Abstract

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.