Understanding How Solid Electrolyte Interphases and Diluents Contribute to Expanding the Electrochemical Stability Window of Aqueous Electrolytes in Li-Ion Batteries

With the increasing adoption of intermittent renewable energy sources, the rapid growth of electric vehicles, and the rising demand for portable and wearable consumer electronics, the need for reliable, safe, and cost-effective electrochemical energy storage devices has never been greater. Aqueous electrolyte lithium-ion batteries have emerged as a promising candidate, offering a competitive alternative to commercial organic electrolyte systems. They are inherently safer, cheaper to manufacture, and benefit from a higher ionic conductivity. However, their widespread commercial adoption has been limited by a key drawback: the narrow electrochemical stability window (ESW) dictated by the decomposition of water when driven to extreme potentials. Recent research has shown that this limitation can be significantly mitigated through the use of highly concentrated electrolytes — specifically Water-in-Salt Electrolytes (WiSEs) 1 — and by incorporating diluents (additives) in ternary eutectic mixtures, such as urea, N-methylurea, and hydroxyurea. 2,3 These electrolyte formulations modify the bulk solvation structure of water, facilitating electrode passivation, and altering the kinetics and mass transport of both ions and water. 4 Together, these effects suppress the decomposition of water at both the anode and cathode. With careful tuning, the ESW of aqueous electrolytes can be to values exceeding 3.5 V, giving rise to aqueous electrolyte Li-ion batteries that can cycle in LiMn 2 O 4 (LMO) vs Li 4 Ti 5 O 12 (LTO) full cells. Here, we bring together surface-sensitive techniques — including depth-resolved X-ray Photoelectron Spectroscopy (XPS) and soft X-ray Absorption Spectroscopy (XAS) — with operando methods including Differential Electrochemical Mass Spectrometry (DEMS). These approaches provide direct insight into the mechanisms behind the suppressed water breakdown at extreme potentials. Additionally, Classical Molecular Dynamics simulations, alongside experimental techniques such as Nuclear Magnetic Resonance (NMR) and Raman spectroscopy, are used to probe the solvation environment of WiSE systems. These methods help to clarify how salt and diluent degradation pathways contribute to the formation of cathode/solid-electrolyte interphases (C/SEIs), which play a crucial role in extending the ESW. Building on recent literature, this work deepens our understanding of how WiSEs and eutectic diluents reshape the electrochemical behaviour of aqueous systems. It also introduces new experimental data that highlight the role of specific functionalities in diluents that encourage particular solvation motifs and ion–dipole interactions which give rise to water O-H bond stabilisation across a broader potential range. We thereby propose new candidate diluents and electrolyte designs that could further expand the ESW of aqueous lithium-ion batteries— enabling higher-capacity electrode materials and energy densities comparable to today’s commercial lithium-ion cells, all while offering superior safety and sustainability. Bibliography Suo, L., Borodin, O., Gao, T., Olguin, M., Ho, J., Fan, X., Luo, C., Wang, C., and Xu, K., Science, 2015, 350 , 938–943.“Water-in-salt” electrolyte enables high-voltage aqueous lithium-ion chemistries. Yang, C., Chen, J., Qian, T., Li, T., Li, S., Jin, Y., Zhang, B., Xu, H., Zhou, J., Chen, L., Xu, K., and Wang, C., Nat. Energy, 2021, 6 , 176–185. Aqueous electrolyte design for super-stable 2.5 V LiMn₂O₄ || Li₄Ti₅O₁₂ pouch cells. Lin, R., Ke, C., Chen, J., Liu, S., and Wang, J., Joule, 2022, 6 , 399–417. Asymmetric donor–acceptor molecule-regulated core–shell–solvation electrolyte for high-voltage aqueous batteries. Phelan, C. M. E., Björklund, E., Singh, J., Fraser, M., Didwal, P. N., Rees, G. J., Ruff, Z., Ferrer, P., Grinter, D. C., Grey, C. P., and Weatherup, R. S., Chem. Mater., 2024, 36 , 3334–3344. Role of Salt Concentration in Stabilizing Charged Ni-Rich Cathode Interfaces in Li-Ion Batteries.