Water as an Additive in Lithium-Sulfur Batteries Electrolytes—Mechanism Elucidation via Operando FTIR and XPS Analysis

Additives in the electrolytes of Li-S batteries aim to increase overall capacity, improve ion conductivity, enhance cyclability, and mitigate the shuttle effect, which is one of the major issues of this system. Here, the use of water as an additive in the commonly used electrolyte, 1.0 M LiTFSI/1.0% (w/w) LiNO₃ and a 1:1 mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) was investigated. Electrochemical tests determined 1600 ppm as the optimal water concentration, significantly reducing the shuttle effect. Post-mortem X-ray photoelectron spectroscopy (XPS) analysis focused on the lithium metal anode revealed the formation of Li₂O layers in dry electrolyte and LiOH in wet electrolyte. Better capacity was observed in wet electrolyte, which can be attributed to the superior ionic conductivity of LiOH at the electrode/electrolyte interface, surpassing that of Li₂O by 12 times. Finally, operando Fourier-transform infrared spectroscopy (FTIR) experiments provided real-time insights into electrolyte degradation and solid electrolyte formation (SEI) formation, elucidating the activity mechanisms of H₂0 and Li₂CO₃ with cycling. These results could aid future advancements in Li-S battery technology, offering possibilities to mitigate its challenges with inexpensive and scalable additives.