A comprehensive study of highly concentrated Lithium‑ion aqueous electrolytes: from structural characterizations to electrochemical properties

Due to their scarce and expensive components, lithium‑ion batteries raise safety and cost concerns. Aiming for less toxic and cheaper batteries, aqueous electrolytes have emerged as a good alternative. To overcome their narrow electrochemical stability window, highly concentrated electrolytes have shown promising properties in reducing water reactivity. However, the lithium salts most commonly used in these electrolytes contain fluorinated organic anions, which have greater environmental and cost impacts when used in large amounts. Some low‑cost and more environmentally‑friendly salts started to emerge in aqueous electrolytes for battery and supercapacitor applications. In the present work, a comparative study between a low‑cost and highly concentrated lithium nitrate (LiNO₃) based electrolyte and an electrolyte using lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt as a reference was conducted. Electrochemical measurements reveal that LiNO₃‑based electrolytes have a higher ionic conductivity but a narrower electrochemical stability window than LiTFSI‑based ones. Infrared spectroscopy, small‑angle X‑ray scattering and wide‑angle X‑ray scattering characterisations, coupled with self‑diffusion and viscosity measurements, shed light on the lack of structure formation in LiNO₃‑based electrolytes compared to LiTFSI‑based ones which have well‑organised domains. It demonstrates that LiNO₃ used alone in an aqueous electrolyte does not disrupt the network of water molecules and, therefore, does not reduce their reactivity. This study also proposes a systematic approach to evaluate salts or electrolyte formulations for application in low-cost aqueous batteries. By combining various characterisation techniques, it provides a framework to determine whether a given composition meets the essential criteria for widening the electrochemical stability window of aqueous electrolytes.