Understanding Degradation Mechanisms in Water-In-Salt Electrolyte. Part 2: Impact of the Electrochemical Parameters on the Cycling Behavior of LiFePO4 versus TiS2

Water-in-salt electrolytes (WISE) have emerged as a promising route for the development of safe and high-voltage aqueous lithium-ion batteries, owing to their expanded electrochemical stability window (ESW) and reduced safety issue. Despite this potential, the long-term cycling performance of WISE-based cells remains hindered by multiple degradation phenomena all related to water, including hydrogen and oxygen evolution reactions, carbon corrosion, and interfacial instabilities. In this study, the electrochemical behavior and degradation mechanisms of full cells based on LiFePO₄ and TiS₂ electrodes are systematically investigated in 21 m LiTFSI electrolyte. Gas evolution is quantified using online electrochemical mass spectrometry (OEMS), while surface chemistry and morphology were analyzed via X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). The results demonstrate that both electrode materials undergo significant parasitic reactions—even within the ESW—leading to the formation of a LiF-rich but unstable solid electrolyte interphase and progressive accumulation of salt decomposition products. The degradation is further influenced by electrochemical parameters such as C-rate, electrode balancing, and voltage window.