Recently, 2D layered transition metal dichalcogenides (TMDs) have attracted great scientific interest in electrochemical energy storage research. Vanadium disulfide (VS2) as an important family member of TMDs, is a promising electrode material for lithium-ion cells because of its remarkable electrical conductivity and high Li+ diffusion rate, but its electrochemical reaction mechanism is still poorly understood. Herein, we have prepared VS2 nanosheets as the electrode and systematically investigated its structural and chemical evolution during the electrochemical processes by employing both in situ and ex situ x-ray spectroscopy. The VS2 undergoes intercalation and conversion reactions in sequence during discharge and this process is found to be partially reversible during the subsequent charge. The decreased reversibility of the conversion reaction over extended cycles could be mainly responsible for the capacity fading of the VS2 electrode. In addition, the hybridization strength between S and V shows a strong dependence on the states of charge, as directly illustrated by the intensity change of the V-S hybridized states and pure V states. We have also found that the solid electrolyte interphase on the electrode surface is dynamically evolved during cycling, which may be a universal phenomenon for the conversion-based electrodes. This study is expected to be beneficial for the further development of high-performance VS2-based electrodes.

With the increasing interest in transition metal chalcogenides, sulfide minerals containing the disulfide unit (S22-) have gained intensive attention for potential applications in energy storage devices, such as lithium-ion batteries (LIBs). Vanadium tetrasulfide (VS4) possesses a unique linear-chain structure with a Peierls distortion and shows great promise for application in LIBs. However, its electrochemical reaction mechanism is still controversial, mainly due to the amorphous nature of the intermediates and final products. Here, by applying multiple X-ray spectroscopies, we reveal that VS4 undergoes lithium intercalation and conversion reactions sequentially during the first discharge process, which are partially reversible in the subsequent charge process. However, an anomalous intercalation/conversion mixed reaction mechanism is dominant for the second cycle, mainly owing to the amorphization of the VS4 electrode during the first cycle. In addition, the sulfur atoms are also involved in the redox reaction during cycling, with the anionic contribution of S22- ↔ 2S2- transformation. Furthermore, we find that the formation process of the solid electrolyte interphase is highly dynamic during the discharge and charge processes. The present study provides deeper insights into the complex reaction mechanism of VS4. This knowledge can accelerate the development of high-performance VS4-based electrode materials for LIBs.