Boron-Stabilized Anisotropic Water-in-Polymer Salt Electrolyte with an Exceptionally Low Salt Concentration by Hofmeister Effect for Aqueous Lithium-Ion Batteries

Water-based electrolytes provide safe, reliable, and cost-effective energy storage solutions; however, their application in aqueous lithium-ion batteries is hindered by low energy density and short cycling life due to the limited electrochemical stability window. While high lithium salt concentrations can mitigate some of these issues, they often lead to increased solvent viscosity and higher costs, limiting commercialization. In this study, a boron-stabilized anisotropic polyvinyl alcohol (PVA) hydrogel electrolyte, referred to as BaP, is proposed to address the challenges related to high lithium salt (LiTFSI) concentrations. Due to the Hofmeister effect, the BaP water-in-polymer electrolyte can retain a high concentration of lithium salt even when low concentrations of lithium salt are used. Briefly, the BaP promotes the salting-in phenomenon of Li ions, while the TFSI ions induce salting-out, allowing BaP to synergistically achieve high lithium salt concentrations. Due to these unique characteristics, the BaP hydrogel exhibits a wide electrochemical stability window similar to that of highly concentrated electrolytes, enabling stable operation in a LiMn₂O₄||Li₄Ti₅O₁₂ full cell by suppressing hydrogen evolution. Moreover, the biodegradability of BaP contributes to the development of a more environmentally friendly battery system.