The use of “water-in-salt” electrolyte (WISE) (i. e., a highly concentrated aqueous solution) in rechargeable batteries has received increasing attention due to the significantly expanded electrochemical window compared to the limited voltage of conventional aqueous electrolytes. It enables the use of more positive/negative electrode material couples in aqueous batteries, resulting in an enhanced output voltage. However, one of the challenges is to identify promising anode materials for the “water-in-salt” Li-ion batteries (WIS-LIBs). Herein we for the first time demonstrate that MoS₃, an amorphous chain-like structured transitional metal trichalcogenide, is promising as anode in the WIS-LIBs. In this work, hollow MoS₃ nanospheres were synthesized via a scalable room-temperature acid precipitation method. When applied in WIS-LIBs, the prepared MoS₃ achieved a high specific capacity of 127 mAh/g at the current density of 0.1 A/g and good stability over 1000 cycles. During operation, MoS₃ underwent irreversible conversion to Li₂MoO₄ (with H₂S and H₂ evolution) during the initial Li ion uptake, and was then converted gradually to a more stable and reversible Li_xMoO_y (2≤y≤4)) phase along cycling. Amorphous Li-deficient Li_x-mMoO_y/MoO_z was formed upon delithiation. Nevertheless, MoS₃ outperformed MoO₃ in WIS-LIBs, which could be accredited to its initial one-dimensional molecular structure and the amorphous nature of the delithiated product facilitating charge transport. These results demonstrated a novel routine for synthesizing metal sulfides with hollow structures using a template-based method and push forward the development of metal sulfides for aqueous energy storage applications.

Prevention of mechanical and finally electrochemical failures of lithium batteries is a critical aspect to be considered during their design and performance, especially for those with high specific capacities. Internal failure is observed as one of the most serious factors, including loss of electrode materials, structure deformation and dendrite growth. It usually incubates from atomic/molecular level and progressively aggravates along with lithiation. Understanding the internal failure is of great importance for developing solutions of failure prevention and advanced anode materials. In this research, different internal failure processes of anode materials for lithium batteries are discussed. The progress on observation technologies of the anode failure is further summarized in order to understand their mechanisms of internal failure. On top of them, this review aims to summarize innovative methods to investigate the anode failure mechanisms and to gain new insights to develop advanced and stable anodes for lithium batteries.